Artificial sand, also called crushed sand or mechanical sand (m sand), refers to rocks, mine tailings or industrial waste granules with a particle size of less than 4.75 mm. It is processed by mechanical crushing and sieving.
In China, the artificial sand was mainly used in the construction of hydropower systems. For example, the Three Gorges Project and the Yellow River Xiaolangdi Project used artificial sand to prepare concrete. Due to the remote environment of the hydropower project and the high quality of sand and gravel, the projects have taken the materials locally.
Many Indian states have decreed the use of crushed sand in infrastructure construction because of its high compressive strength and cohesion and the adverse environmental effects of river sand mining, which will greatly boost the demand for artificial sand.
There are both natural and human factors in the increasing demand for artificial sand. The former is that the natural sand is about to run out, while human factors include people's requirements for environmental protection and the need for high-quality concrete.
With the development of infrastructure, the natural sand resources formed by hundreds of thousands of years in many countries and regions have been almost exhausted, which has affected the further development of construction projects.
Driven by huge interests, natural sand has been indiscriminately mining, which changes the river course, affects the safety of river embankments, destroys the living environment of fish and contaminates the groundwater. The crushed stone sand is an important alternative resource to change this phenomenon.
In the process of mining river sand, it often produces a large amount of tailings which is not used reasonably. Especially in small mines, the tailings are piled up at random, occupying land and polluting the environment.
Besides, in urban planning and construction, a large amount of construction waste is generated, which actually can be crushed by the crushers to produce the artificial sand and aggregates for promoting resource utilization.
With the rapid development of concrete technology, the comprehensive performance of high-performance concrete and high-strength structural concrete has higher requirements on the quality of aggregates, requiring it with stable quality, good gradation and shape, while less and less natural sand meets the requirements. Therefore, people are turning their focus on artificial sand.
Artificial sand and mixed sand are mainly used in building construction, municipal construction, transportation, and other projects whose concrete strength grade is below C60. When meeting the corresponding technical requirements, they can also be used for concrete projects such as ports and water conservancy.
The vibrating feeder feeds the stone evenly into the coarse crushing machines for the primary crushing of the stone. The crushed stone from the coarse crushing equipment is transported through the belt conveyor to the fine crushing machines for the secondary crushing.
The primary crushing equipment directly processes supplied materials from the stone material factory and is the foremost processing plant. The jaw crusher is the ideal choice for primary crushing.
The secondary crushing equipment is responsible for the middle and fine crushing in the stone crushing process. The secondary crushing equipment mainly includes impact crushers and cone crushers.
The crushed material is conveyed by the belt conveyor to the crushed sand making machine for fine crushing. The finely crushed material is then screened through the vibrating screen for coarse sand, medium sand, fine sand and other specifications.
VSI sand making machine is called high cost-effective sand making equipment with low initial investment cost and good use effect. It is high in operation efficiency with the PLST crushing cavity.
The HVI sand making machine manufactured by Fote is more advanced in technology. Its butt ends and lubrication scheme are modular in design, and the peripheral guard plate can be turned up and down to extend its life cycle. It has the effects of sand making and shaping at the same time.
For those with strict requirements on the powder content of sand, a sand washing machine can be installed behind the sand manufacturing machine, and the sewage discharged from the sand washing machine can be recycled by the fine sand recovery plant.
The following video shows clearly the difference between crushed sand and river sand from the aspects of source, wastage, setting time, shape, silt content and so on. It is worth collecting and sharing with friends.
The table below shows the mix ratio of M15 M55 and M55 M55 concrete designed for artificial sand and river sand. The results show that in the same Concrete Mix Design, the strength of concrete by artificial sand is higher.
As the demand for aggregates from China, Africa, Latin America, Europe,etc. will continue to rise, and the price of artificial sand will also increase, the outlook for the global aggregate market for sand and gravel is worth looking forward to.
According to the Global Sandstone Aggregate report, by 2020, the demand for aggregates of sand and gravel in the Asia-Pacific region will increase the fastest in the world, and the growth rate of India, Vietnam, Malaysia, Indonesia and other regions will be among the best.
Due to the economic transformation, the demand for sand and gravel aggregates in China will slow down. However, the report predicts that by 2020, China will still be the world's largest aggregate demander, accounting for almost half of the world's total aggregate.
The Global Sandstone Aggregate report shows that the reserves of natural sand are gradually decreasing globally, and in some developing countries where the demand for aggregates and sand is huge, illegal sand mining has happened frequently. According to the report, by 2020, the reserves of natural sand resources in some countries will be quickly depleted.
The depletion of natural sand resources will lead to a sharp increase in the price of aggregates and artificial sand, especially in areas where the demand for aggregates is huge. It can be seen that the profit margin of the artificial sand is very large.
The m sand manufacturing machine produced by the supplier Fote is suitable for m sand making production line of high hard and abrasion-resistant materials such as limestone, quartz stone, granite, river pebble, basalt, cement, various ores, glass raw materials, mechanism building stone, gold slag, etc.
The limestone, widely distributed in nature, is easy to obtain. It is the main raw material for manufacturing cement, lime and calcium carbide. After being processed, limestone is used in large quantities as building materials. Generally, simple processing process is through the limestone sand production line.
Quartz is a hard, wear-resistant, chemically stable silicate mineral mainly composed of silicon dioxide. It is an important industrial mineral raw material generally used in glass, building materials, ceramics, high-tech electronics industry, etc. after crushing and sand making.
The granite has the characteristics of hard texture, high strength, abrasion resistance and weathering resistance. It has the title of "The King of Rocks" and is a good building material. It is rich in resources with low mining costs but high product value. Granite is a popular choice when the product needs to be weather-resistant or durable.
The river pebble resources are rich with low collection costs and high application value. After making sand from river pebble, it is widely used in water conservancy and hydropower, expressways, high-speed railways, passenger dedicated lines, bridges, airport runways, municipal engineering and high-rise buildings in the engineering field.
Basalt is the best material used for repairing roads, railways and airport runways. With the characteristics of strong compressive resistance, low crushing value and strong corrosion-resistance, it is the best cornerstone for the development of railway transport and road transport.
As a professional manufacturer of mechanical sand production equipment, Fote Heavy Machinery has rich experience in the design of mechanical sand production equipment, a perfect service system and professional after-sales team.
As a leading mining machinery manufacturer and exporter in China, we are always here to provide you with high quality products and better services. Welcome to contact us through one of the following ways or visit our company and factories.
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Silica sand low in iron is much in demand for glass, ceramic and pottery use, and for many of these applications clean, white sand is desired. Impurities such as clay slime, iron stain, and heavy minerals including iron oxides, garnet, chromite, zircon, and other accessory minerals must not be present. Chromium, for example, must not be present, even in extremely small amounts, in order for the sand to be acceptable to certain markets. Feldspars and mica are also objectionable. Generally, iron content must be reduced to 0.030% Fe2O3 or less.
Silica sand for making glass, pottery and ceramics must meet rigid specifications and generally standard washing schemes are inadequate for meeting these requirements. Sand for the glass industry must contain not more than 0.03% Fe2O3. Concentrating tables will remove free iron particles but iron stained and middling particles escape gravity methods. Flotation has been very successfully applied in the industry for making very low iron glass sand suitable even for optical requirements.Sub-A Flotation Machines are extensively used in this industry for they give the selectivity desired and are constructed to withstand the corrosive pulp conditions normally encountered (acid circuits) and also the abrasive action of the coarse, granular, slime free washed sand.
The flowsheet illustrates the more common methods of sand beneficiation. Silica may be obtained from sandstone, dry sand deposits and wet sand deposits. Special materials handling methods are applicable in each case.
The silica bearing sandstone must be mined or quarried much in the manner for handling hard rock. The mined ore is reduced by a Jaw Crusher to about 1 size for the average small tonnage operation. For larger scale operations two-stage crushing is advisable.
The crushed ore is reduced to natural sand grain size by Rod Milling. Generally, one pass treatment through the Rod Mill is sufficient. Grinding is done wet at dilutions in excess of normal grinding practice. A Spiral Screen fitted to the mill discharge removes the plus 20 mesh oversize which either goes to waste or is conveyed back to the mill feed for retreatment.
Sand from such deposits is generally loaded into trucks and transported dry to the mill receiving bin. It is then fed on to a vibrating screen with sufficient water to wash the sand through the 20 mesh stainless screen cloth. Water sprays further wash the oversize which goes to waste or for other use. The minus 20 mesh is the product going to further treatment.
The sand and water slurry for one of the three fore-mentioned methods is classified or dewatered. This may be conveniently done by cyclones or by mechanical dewatering classifiers such as the drag, screw, or rake classifiers.
From classification the sand, at 70 to 75% solids, is introduced into a Attrition Scrubber for removal of surface stain from the sand grains. This is done by actual rubbing of the wet sand grains, one against another, in an intensely agitated high density pulp. Most of the work is done among the sand grains not against the rotating propellers.
For this service rubber covered turbine type propellers of special design and pitch are used. Peripheral speed is relatively low, but it is necessary to introduce sufficient power to keep the entire mass in violent movement without any lost motion or splash. The degree of surface filming and iron oxide stain will determine the retention time required in the Scrubber.
The scrubbed sand from the Attrition Machine is diluted with water to 25-30% solids and pumped to a second set of cyclones for further desliming and removal of slimes released in the scrubber. In some cases the sand at this point is down to the required iron oxide specifications by scrubbing only. In this case, the cyclone or classifier sand product becomes final product.
Deslimed sand containing mica, feldspar, and iron bearing heavy minerals can be successfully cleaned to specifications by Sub-A Flotation. Generally this is done in an acid pulp circuit. Conditioning with H2SO4 and iron promoting reagents is most effective at high density, 70-75% solids. To minimize conditioning and assure proper reagentizing a two-stage Heavy Duty Open Conditioner with Rubber Covered Turbine Propellers is used. This unit has two tanks and mechanisms driven from one motor.
The conditioned pulp is diluted with water to 25-30% solids and fed to a Sub-A Flotation Machine especially designed for handling the abrasive, slime free sand. Acid proof construction in most cases is necessary as the pulps may be corrosive from the presence of sulfuric acid. A pH of 2.5-3.0 is common. Wood construction with molded rubber and 304 or 316 stainless steel are the usual materials of construction. In the flotation step the impurity minerals are floated off in a froth product which is diverted to waste. The clean, contaminent-free silica sand discharges from the end of the machine.
The flotation tailing product at 25 to 30% solids contains the clean silica sand. A SRL Pump delivers it to a Dewatering Classifier for final dewatering. A mechanical classifier is generally preferable for this step as the sand can be dewatered down to 15 to 20% moisture content for belt conveying to stock pile or drainage bins. In some cases the sand is pumped directly to drainage bins but in such cases it would be preferable to place a cyclone in the circuit to eliminate the bulk of the water. Sand filters of top feed or horizontal pan design may also be used for more complete water removal on a continuous basis.
Dry grinding to minus 100 or minus 200 mesh is done in Mills with silica or ceramic lining and using flint pebbles or high density ceramic or porcelain balls. This avoids any iron contamination from the grinding media.
In some cases it may be necessary to place high intensity magnetic separators in the circuit ahead of the grinding mill to remove last traces of iron which may escape removal in the wet treatment scrubbing and flotation steps. Iron scale and foreign iron particles are also removed by the magnetic separator.
In general most silica sands can be beneficiated to acceptable specifications by the flowsheet illustrated. Reagent cost for flotation is low, being in the order of 5 to 10 cents per ton of sand treated. If feldspars and mica must also be removed, reagent costs may approach a maximum of 50 cents per ton.
Laboratory test work is advisable to determine the exact treatment steps necessary. Often, attrition scrubbing and desliming will produce very low iron silica sand suitable for the glass trade. Complete batch and pilot plant test facilities are available to test your sand and determine the exact size of equipment required and the most economical reagent combinations.
Silica sand for making glass, pottery and ceramics must meet rigid specifications and generally standard washing schemes are inadequate for meeting these requirements. Sand for the glass industry must contain not more than 0.03% Fe2O3. Concentrating tables will remove free iron particles but iron stained and middling particles escape gravity methods. Flotation has been very successfully applied in the industry for making very low iron glass sand suitable even for optical requirements.
Sub-A Flotation Machines are extensively used in this industry for they give the selectivity desired and are constructed to withstand the corrosive pulp conditions normally encountered (acid circuits) and also the abrasive action of the coarse, granular, slime free washed sand.
The flowsheet illustrated is typical for production of glasssand by flotation. Generally large tonnages are treated, forexample, 30 to 60 tons per hour. Most sand deposits can be handled by means of a dredge and the sand pumped to the treatment plant. Sandstone deposits are also being treated and may require elaborate mining methods, aerial tramways, crushers, and wet grinding. Rod Mills with grate discharges serve for wet grinding to reduce the crushed sandstone to the particle size before the sand grains were cementedtogether in the deposit. Rod milling is replacing the older conventional grinding systems such as edge runner wet mills or Chilean type mills.
Silica sand pumped from the pit is passed over a screen, either stationary, revolving or vibrating type, to remove tramp oversize. The screen undersize is washed and dewatered generally in a spiral type classifier. Sometimes cone, centrifugal and rake type classifiers may also be used for this service. To clean the sand grains it may be necessary to thoroughly scrub the sand in a heavy-duty sand scrubber similar to the Heavy-duty Agitator used for foundry sand scrubbing. This unit is placed ahead of the washing and dewatering step when required. The overflow from the classifier containing the excess water and slimes is considered a waste product. Thickening of the wastes for water reclamation and tailings disposal in some areas may be necessary.
The washed and dewatered sand from the spiral-type classifier is conveyed to a storage bin ahead of the flotation section. It is very important to provide a steady feed to flotation as dilution, reagents and time control determines the efficiency of the process.
Feeding wet sand out of a storage bin at a uniform rate presents a materials handling problem. In some cases the sand can be uniformly fed by means of a belt or vibrating-type feeder. Vibrators on the storage bin may also be necessary to insure uniform movement of the sand to the feeder. In some cases the wet sand is removed from the bin by hydraulic means and pumped to a spiral-type classifier for further dewatering before being conveyed to the next step in the flowsheet.
Conditioning of the sand with reagents is the most critical step in the process. Generally, for greater efficiency, it is necessary to condition at maximum density. It is for this reason the sand must be delivered to the agitators or conditioners with a minimum amount of moisture. High density conditioning at 70 to 75% solids is usually necessary for efficient reagentizing of the impurity minerals so they will float readily when introduced into the flotation machine.
The Heavy-duty Duplex Open-type Conditioner previously developed for phosphate, feldspar, ilmenite, and other non-metallic mineral flotation is ideal for this application. A duplex unit is necessary to provide the proper contact time. Circular wood tanks are used to withstand the acid pulp conditions and the conditioner shafts and propellers are rubber covered for both the abrasive and corrosive action of the sand and reagents.
Reagents are added to the conditioners, part to the first and the balance to the second tank of the duplex unit, generally for flotation of impurities from silica sand. These reagents are fuel oil, sulphuric acid, pine oil, and a petroleum sulfonate. This is on the basis that the impurities are primarily oxides. If iron is present in sulphide form, then a xanthate reagent is necessary to properly activate and float it. The pulp is usually regulated with sulfuric acid to give a pH of 2.5-3.0 for best results through flotation.
A low reagent cost is necessary because of the low value of the clean sand product. It is also necessary to select a combination of reagents which will float a minimum amount of sand in the impurity product. It is desirable to keep the weight recovery in the clean sand product over 95%. Fatty acid reagents and some of the amines have a tendency to float too much of the sand along with the impurities and are therefore usually avoided.
After proper reagentizing at 70 to 75% solids the pulp is diluted to 25 to 30% solids and introduced into the flotation machine for removal of impurities in the froth product. Thepulp is acid, pH 2 .5 to 3.0 and the sand, being granular and slime free, is rapid settling so a definite handling problem is encountered through flotation.
The Sub-A Flotation Machine has been very successful for silica sand flotation because it will efficiently handle the fast settling sand and move it along from cell to cell positively. Aeration, agitation and selectivity due to the quiet upper zone can be carefully regulated to produce the desired separation. The machine is constructed with a wood tank and molded rubber wearing parts to withstand the corrosive action of the acid pulp. Molded rubber conical-type impellers are preferred for this service when handling a coarse, granular, abrasive sand.
Flotation contact time for removal of impurities is usually short. A 4, and preferably a 6 cell, machine is advisable. Cell to cell pulp level control is also desirable. A 6 cell No. 24 (43 x 43) Sub-A Flotation Machine in most cases is adequate for handling 25 to 30 tons of sand per hour. If the impurities are in sulphide form a standard machine with steel tank and molded rubber parts is adequate provided the pulp is not acid. Otherwise acid proof construction is essential.
The flotation tailing product is the clean sand discharging from the end of the flotation machine at 25 to 30% solids and must be dewatered before further processing. Dewatering can be accomplished in a dewatering classifier and then sent to storage or drying. Top feed or horizontal vacuum filters are often used to remove moisture ahead of the dryer. Dry grinding of the sand to meet market requirements for ceramic and pottery use is also a part of the flowsheet in certain cases.
This particular sand was all minus 20 mesh with only a trace minus 200 mesh and 70% plus 65 mesh. Iron impurity was present as oxide and stained silica grains. The plant which was installed as a result of this test work is consistently making over a 95% weight recovery and a product with not over 0.02% Fe2O3 which at times goes as low as 0.01% Fe2O3.
Si02, minimum..99.8 per cent Al2O3, maximum..0.1 percent Fe2O3, maximum..0.02 per cent CaO + MgO, maximum.0.1 percent For certain markets, a maximum of 0.030 per cent Fe2O3 is acceptable.
Natural silica-sand deposits generally contain impurityminerals such as clay, mica, and iron oxide and heavy iron minerals which are not sufficiently removed by washing and gravity concentration. Flotation is often used to remove these impurity minerals to meet market specifications.
Anionic-type reagents, such as fatty acids, are used to float some impurities in alkaline pulp. Cationic-type reagents such as amines or amine acetates are also used with inhibitors such as sulphuric or hydrofluoric acids to float certain impurity minerals and depress the silica.
Before we get into the advantages and disadvantages of manufactured sand, what exactly is manufactured sand? Manufactured sand is sand produced by crushing rocks, quarry stones or larger aggregates pieces into sand-sized particles. Natural sand, on the other hand is the naturally formed sand extracted from river beds.
The produced sand is then sieved and washed to remove fine particles and impurities, and tested for various quality aspects before it is deemed fit as a construction aggregate. The size specification for manufactured sand is that it should pass completely through a 3/8 inch sieve.
Workability issues: Manufactured sand can be of a coarser and angular texture than natural sand, which is smooth and rounded due to natural gradation. This can lead to more water and cement requirement to achieve the expected workability, leading to increased costs.
Larger proportion of micro fines: Manufactured sand can contain larger amounts of micro fine particles than natural sand, owing to its production process. This again can affect the strength and workability of the screed or concrete.
Manufactured sand can be as an economic and more eco-friendly alternative to natural sand. But, the key is to ensure the sand is procured from a reliable source and that it has been adequately processed and tested to meet the required quality specifications.
The raw cement ingredients needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, mill scale and bauxite. The ore rocks are quarried and crushed to smaller pieces of about 6 inches. Secondary crushers or hammer mills then reduce them to even smaller size of 3 inches. After that, the ingredients are prepared for pyroprocessing.
The crushed raw ingredients are made ready for the cement making process in the kiln by combining them with additives and grinding them to ensure a fine homogenous mixture. The composition of cement is proportioned here depending on the desired properties of the cement. Generally, limestone is 80% and remaining 20% is the clay. In the cement plant, the raw mix is dried (moisture content reduced to less than 1%); heavy wheel type rollers and rotating tables blend the raw mix and then the roller crushes it to a fine powder to be stored in silos and fed to the kiln.
A pre-heating chamber consists of a series of cyclones that utilizes the hot gases produced from the kiln in order to reduce energy consumption and make the cement making process more environment-friendly. The raw materials are passed through here and turned into oxides to be burned in the kiln.
The kiln phase is the principal stage of the cement production process. Here, clinker is produced from the raw mix through a series of chemical reactions between calcium and silicon dioxide compounds. Though the process is complex, the events of the clinker production can be written in the following sequence:
The kiln is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes. By the time the raw-mix reaches the lower part of the kiln, clinker forms and comes out of the kiln in marble-sized nodules.
After exiting the kiln, the clinker is rapidly cooled down from 2000C to 100C-200C by passing air over it. At this stage, different additives are combined with the clinker to be ground in order to produce the final product, cement. Gypsum, added to and ground with clinker, regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid and dodecyl-benzene sulphonate.
The heat produced by the clinker is circulated back to the kiln to save energy. The last stage of making cement is the final grinding process. In the cement plant, there are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred to these rotating drums and ground into such a fine powder that each pound of it contains 150 billion grains. This powder is the final product, cement.
Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains or ships, and only a small amount is packed for customers who need small quantities.
Please note that the information in Civiltoday.com is designed to provide general information on the topics presented. The information provided should not be used as a substitute for professional services.
RHA generated contains high percentage of silica (> 90%), hence it is a suitable cementitious material (Lim et al., 2018) and a feasible precursor for geopolymerisation (Suksiripattanapong, et al., 2017).
Rice husks are the hard protective coverings of rice grains which are separated from the grains during milling process. Rice husk is an abundantly available waste material in all rice producing countries, and it contains about 30%50% of organic carbon. In the course of a typical milling process, the husks are removed from the raw grain to reveal whole brown rice which upon further milling to remove the bran layer will yield white rice. Current rice production in the world is estimated to be 700 million tons. Rice husk constitutes about 20% of the weight of rice and its composition is as follows: cellulose (50%), lignin (25%30%), silica (15%20%), and moisture (10%15%). Bulk density of rice husk is low and lies in the range 90150kg/m3.
Sources of rice husk ash (RHA) will be in the rice growing regions of the world, as for example China, India, and the far-East countries. RHA is the product of incineration of rice husk. Most of the evaporable components of rice husk are slowly lost during burning and the primary residues are the silicates. The characteristics of the ash are dependent on (1) composition of the rice husks, (2) burning temperature, and (3) burning time. Every 100kg of husks burnt in a boiler for example will yield about 25kg of RHA. In certain areas, rice husk is used as a fuel for parboiling paddy in rice mills, whereas in some places it is field-burnt as a local fuel. However, the combustion of rice husks in such cases is far from complete and the partial burning also contributes to air pollution. The calorific value of rice husks is about 50% of that of coal, and assuming that husks have about 8%10% of moisture content and zero bran, the calorific value is estimated to be 15MJ/kg. Under controlled burning conditions, the volatile organic matter in the rice husk consisting of cellulose and lignin are removed and the residual ash is predominantly amorphous silica with a (microporous) cellular structure (Fig. 13.1). Due to its highly microporous structure, specific surface area of RHA as determined by the BrunauerEmmettTeller (BET) nitrogen adsorption method can range from 20 to as high as 270m2/g, while that of silica fume, for example is in the range of 1823m2/g.
The chemical composition of RHA is significantly dependent on combustion conditions, and the burning temperature must be controlled to keep silica in an amorphous state. The ash obtained from uncontrolled combustion (as in open-field burning or in industrial furnaces at temperatures greater than 700C800C) will contain significant amounts of cristobalite and tridymite which are nonreactive silica minerals. In order to develop pozzolanic activity, such ashes will be required to be ground to a very fine particle size which is likely to make their use financially unviable. Under controlled combustion (burning temperatures in the range of 500C700C for a period of about 1hour), amorphous silica is the major constituent of ash whose reactivity is attributed to the presence of this form of silica and to its very large surface area resulting from the microporous structure of ash particles. Although reactivity of a pozzolanic material improves upon increasing its fineness, Mehta and Monteiro (1997) reckon that grinding RHA to a high degree of fineness is not advisable since this material derives its pozzolanic activity from the internal surface area of its microporous particles which is already very high. When obtained from controlled combustion, the specific surface (as measured using nitrogen adsorption) of RHA can be as high as 50,000m2/kg even though the particle size may be in the range of 1075m, which is large when compared to that of silica fume for example. The average composition of well-burnt RHA is 90% amorphous silica, 5% carbon, and 2% K2O.
The applications of RHA include its use as a pozzolan in the construction industry, as a filler, additive, abrasive agent, oil adsorbent, sweeping component, and as a suspension agent for porcelain enamels. In the construction industry, RHA can be used as a partial replacement for cement. According to Chandrasekhar et al. (2006), each application requires specific properties such as reactivity for cement and concrete, chemical purity for synthesizing advanced materials, whiteness, and proper particle size for filler applications and high surface area and porosity for use as an adsorbent and catalyst. If used as a supplementary cementitious material in concrete, for example, RHA particles may have a high water demand due to their porous microstructure. This can be controlled by intergrinding the RHA particles with clinker during the process of cement manufacture so as to breakdown the porous structure and thereby reduce water demand. If intergrinding is not possible, then RHA may be used by blending it with cement at site. RHA in the blended cement will fix free lime released by clinker silicates during their hydration. The amorphous silica in the RHA can react with Ca(OH)2 in the secondary hydration reaction to form a kind of C-S-H gel, which has a floc-like morphology with a porous structure and large specific surface. The formation of the additional C-S-H contributes to both strength development and enhanced durability of concrete since in the secondary hydration reaction the free lime is converted into C-S-H gel which is insoluble in water. Fig. 13.2 presents a schematic illustration of the hydration mechanism proposed by Hwang and Chandra (1997) of cement paste containing RHA. RHA in blended cement is known to contribute to concrete strength from as early as 13 days of maturing. In addition to its contribution to strength, even at relatively small replacement dosages of 10% by weight of cement, RHA can produce a strong transition zone and very low permeability in hardened concrete in addition to significant reduction of bleeding in fresh concrete. Since Portland cement (PC) is typically the most expensive constituent of concrete, replacement of a part of it with RHA offers improved concrete affordability, particularly for developing countries.
RHA is grayish-black in color due to unburned carbon. At burning temperatures of 550800C, amorphous silica is formed, while crystalline silica is produced at higher temperatures. The specific gravity of RHA varies from 2.11 to 2.27; it is highly porous and light weight, with a very high specific surface area. Table11.13 shows the physical properties of RHA reported by several researchers. Fig.11.2 shows images of RHA as received and after burning at 700C for 6h (Della etal., 2002). Typically, RHA is used in the form of ground RHA, having typical particle sizes generally less than 10 m; natural RHA (NRHA) has larger sizes of approximately 100 m.
Rice husk ash (RHA) is an abundantly available and renewable agriculture by-product from rice milling in the rice-producing countries. It has the highest proportion of silica content among all plant residues (Siddique, 2008; Xu, Lo, & Memon, 2012; Yalin & Sevin, 2001). A rice mill turns the paddy plant into 78% rice, 20% rice husk and 2% is lost in the process (Ash, 2010). The rice husk contains about 50% cellulose, 2530% lignin and 1520% silica (Ismail & Waliuddin, 1996). Hence, after the combustion, one-fifth to one-quarter of the rice husk will change into ash.
Rice husk is difficult to ignite and does not burn easily with an open flame, unless air is blown through the husk. Also, it has a high average calorific value of 3410kcal/kg. Therefore, it is a good, renewable energy source. Rice husk can be used as an alternative energy source, i.e. as the fuel in the boiler of a rice-milling kiln to generate electricity where the heating value of the husk ranges from 12.6MJ/kg (Xu etal., 2012) to 13.3416.20MJ/kg (Mansaray & Ghaly, 1997) to 15.7MJ/kg, of which 18.8% is carbon, 62.8% is volatile materials, and 9.3% is moisture content (Ekasilp, Soponronnarit, & Therdyothin, 1995; Thorburn, 1982), and even up to 17MJ/kg (Ferraro, Nanni, Vempati, & Matta, 2010).
The end product of RH in the boiler is RHA, which for the most part will end up as waste since it has little or no commercial value. Its disposal also evokes environmental problems because RHA does not biodegrade easily (Beagle, 1978) and it generates pollution, which has caused health problems to the inhabitants. In Uruguay, RHA was thrown into the river and brought about great contamination and ecological concern (Sensale, 2006).
CEB made of clay, calcium hydroxide (Ca(OH)2) and fine-grind uncontrolled burnt RHA can reach a maximum dry compression strength up to 20.7MPa, with the best proportion of lime and RHA 1: (Muntohar, 2011). In another experiment (Riza, 2011), the best result in compressive strength was attained by a sample with a ratio of RHA and lime equal to 0.25:0.75 in the fourteenth day, with 3.62MPa, and its twenty-eighth-day strength was 3.48MPa. Overall trends indicate that increasing RHA proportion in a mix ratio will consequently reduce the strength.
Initially, the raw BRHA was sieved using a sieve size of 150m. The purpose of sieving the BRHA is to ensure the homogeneity of the BRHA size before the grinding process. Then, the BRHA passing through the 150m sieve was taken and ground for four different grinding periods. Each BRHA ground for four different grinding periods was designated as shown in Table 14.3. The BRHA was ground by a laboratory mill grinder, as shown in Fig. 14.4. The drum, balls, and rods used in this study were made from steel. Each drum contained one size of steel rods and four different sizes of steel balls. The milling speed used in this procedure was 60rev/min. During the grinding process, each drum contained 500g BRHA. The same amount of BRHA was placed in each drum for the following grinding process to ensure the consistency of the ground BRHA property. Fig. 14.5 shows the nanosized ground black RHA.
Rice husk ash-derived aerogels, namely Hamzel, GEAT 0.125 and GEAT CDZ, waterglass-synthesized AeroVa, and TEOS-produced Enova Aerogels IC3100 and IC3110 were supplied from Maerotech Sdn Bhd, Green Earth Aerogels Technologies, JIOS Aerogel, and Cabot Corporation, respectively. All the materials were used as-obtained with no further treatment.
A preliminary investigation was conducted to obtain and reaffirm the relevant physical properties of the aerogels in order to recognize their suitability for plasma spraying. It was also of prime importance to have a microscopic visual of the microstructural features of the aerogels in order to compare with the ones residing inside the single-layered aerogel-based thermally sprayed coatings yet to be developed.
The evaluation of which type of aerogel powder was suitable for which plasma spraying processes was deduced by analyzing the physical properties obtained and comparing them with the requisites of plasma spray feedstock found in the notable textbook by Pawlowski . Furthermore, the granule size distribution and flowability are two essential characteristics to be looked into when considering powder injection, APS in this case, whereas in the case of SPS, the flowability is not of prime importance . These factors were taken into consideration to carry out the evaluation.
The ultrasonic pulse velocity method used to examine the homogeneity and quality of concrete is based on propagation of high-frequency sound wave through the material. The speed of ultrasonic wave depends on density of concrete. On inclusion of CBA as sand replacement, at early age, the porosity of concrete and time to pass through the concrete by the high-frequency sound wave increases. In other words the pulse velocity through concrete decreases on inclusion of CBA as sand replacement. Pulse velocity through concrete made with CBA as sand replacement decreases almost linearly with increase in CBA content. However, with age, concrete micro-structure becomes dense due to filling of pores with extra CSH gel formed by pozzolanic action of CBA and the pulse velocity increases. The published literature also shows decrease in margin of pulse velocity values of concrete incorporating CBA and normal concrete with increase in cement content. The experimental study conducted by Singh and Siddique (2015a), indicates that at 28 days, pulse velocity values through concrete containing CBA are almost comparable to that of control concrete containing 479kg/m3 cement content. The pulse velocity data presented in Table 1.7 show that according to BIS: 13311-92 and Neville (2012), the quality of concrete incorporating CBA as sand replacement can be graded as good. Higher values of pulse velocity indicate good quality of concrete in terms of density, homogeneity, uniformity and the increase in gel/space ratio due to continued hydration process with age (Singh and Siddique, 2015a, Singh, 2015). Experimental study carried out by Rafieizonooz et al., (2016) also shows that at early age, pulse velocity decreases marginally on use of 25%100% CBA as replacement of sand in concrete made with 20% FA as replacement of cement. This study also validates that with age, pulse velocity through concrete containing CBA increases at a faster rate than that through control concrete. The use of CBA as sand replacement in mortar also shows linear decrease in the pulse velocity with increase in CBA content (Topcu and Bilir, 2010).
The regression analysis shows strong relation between compressive strength and pulse velocity values. Concrete being a heterogeneous material, it is believed that coefficient of determination more than 0.7 indicates the strong relationship and more than 0.9 shows excellent relation between independent and dependent variables (Yang et al., 2015; Al-Amoudi et al., 2009; Maiulaitis and Malaikien, 2009; Ghrieb et al., 2014).
Using low alkaline concrete and adding pozzolanic by-products such as rice husk ash, blast furnace slag, or fly ashes to Portland cement (Gutirrez et al, 2005; Agopyan et al., 2005; Savastano et al., 2005a). Results show that the use of ternary blends containing slag/metakaolin and silica fume are effective in preventing degradation (Mohr et al, 2007). But in some cases the low alkalinity is not enough to prevent lignin from being decomposed (John et al., 2005). Other authors reported that fast carbonation can induce lower alkalinity (Agopyan et al., 2005). These results are confirmed by others that used artificial carbonation in order to obtain CaCO3 from Ca(OH)2 leading to increased strength and reduced water absorption (Tonoli, 2010a). The use of cement-based polymers can contribute to increased durability (Pimentel, 2006). DAlmeida (2009) used blends where 50% of Portland cement was replaced by metakaolin to produce a matrix totally free of calcium hydroxide in order to prevent migration of calcium hydroxide to the fiber lumen, middle lamella and cell walls and thus avoid embrittlement behavior.
Ash exhibiting a marked pozzolanic character can be obtained by burning rice husk within certain temperature ranges. Rice husk ash contains >80% silica,85 with a range of relatively high surface areas reported.64,8587 Crystalline silica, such as quartz and cristobalite, can be present in large amounts depending on the burning conditions.67
The pozzolanic activity of rice husk ash depends on the firing temperature and the retention period. It has been noted that controlled burning between 550C and 700C for 1 h converts silica into an amorphous phase.86
Ash exhibiting a marked pozzolanic character can be obtained by burning rice husk within certain temperature ranges. Rice husk ash contains >80 per cent silica103 (Table 10.14) and its BET specific surface area is as high as 5060 m2/g91 or over 152 m2/g.103 Crystalline silica, such as quartz and cristobalite, can be present in large amounts depending on the burning conditions.93
The pozzolanic activity of rice husk ash depends on the firing temperature and the retention period. The sensitivity to burning conditions is the primary reason which prevents the widespread use of this material as pozzolana.104 The amorphous fraction of rice husk contains silica in polymeric form only.93
Finally, even if the distinction between the previous industrial SCMs is somewhat artificial, we can distinguish SCMs coming from the waste treatment industry. Among them, the two main ones in terms of quantity and efficiency are: rice husk ashes and municipal solid waste incineration fly ash.
Rice husk ash (RHA) is a highly reactive pozzolan (Malhotra and Mehta, 1996) obtained when rice husks are calcinated below the crystallization temperature at 780C (Yu et al., 1999). RHA-based concrete has high strength and high durability performance (Anwar et al., 2000; Sousa Coutinho, 2003; Zain et al., 2011). Since each ton of rice generates 40kg of RHA (Zerbino et al., 2011), this means that the annual world rice production of almost 600 Mt can generate almost 20 Mt of RHA. Usually after calcination, the ashes are ground using a ball mill; however, Zerbino et al. (2011) mentioned that also unground RHA can be used to replace 15% of Portland cement with similar mechanical and durability properties. The use of ashes obtained from the calcination of other vegetable species as pozzolans in concrete has already been reported by several authors (Elinwa and Mahmood, 2002; Elinwa and Ejeh, 2004; Akram et al., 2009). But as rice is the principal production in many developing countries where the cement needs are drastically increasing, it is probably the most promising vegetable ash. Note that 20 Mt is similar to GBFS and can absolutely not be compared to the 4000 Mt of cement that will be needed in 2050.
Municipal solid waste (MSW) ash is the by-product of the combustion of municipal solid waste during incineration. Two widely used processes of incinerating MSW are the refuse-derived fuel process and the mass-burning process. The refuse-derived fuel process consists of first separating metals and glass from the MSW. The MSW is then shredded and incinerated, and the generated heat is recovered to produce electricity. The mass-burning process consists of burning the MSW as it is received in the plant without waste separation or shredding. Major portions of the MSW are then transformed physically and chemically as a result of incineration. The by-product of the incineration process is ash. Ash is typically 130% by wet weight and 515% by volume of the wet MSW, depending on the nature of the incineration plant in various countries. Two types of ashes are produced as a result of the incineration process: bottom ash and fly ash. Out of the total MSW ash, bottom ash constitutes 7580% of the total combined ash stream and is difficult to use as SCM (Li et al., 2012). But MSW fly ash is used as SCM in cement (Siddique, 2010). It is a grey to black amorphous, glass-like material, which contains high levels of several toxic metals such as lead and cadmium and organic compounds (such as dioxins). These toxic metals should be encapsulated in the cement matrix (Ubbriaco and Calabrese, 1998; Qian et al., 2008). The quality of MSW ash depends greatly on (i) the nature of the waste; (ii) type of combustion unit; and (iii) nature of the air pollution control device. Major elements in MSW ash are silica, calcium and iron. In addition, the ash has high chlorine, sodium and potassium contents. The main difficulties of using this source of SCM are its variability in chemical composition and its potential toxicity.
S. P. Dunuweera, R. M. G. Rajapakse, "Cement Types, Composition, Uses and Advantages of Nanocement, Environmental Impact on Cement Production, and Possible Solutions", Advances in Materials Science and Engineering, vol. 2018, Article ID 4158682, 11 pages, 2018. https://doi.org/10.1155/2018/4158682
We first discuss cement production and special nomenclature used by cement industrialists in expressing the composition of their cement products. We reveal different types of cement products, their compositions, properties, and typical uses. Wherever possible, we tend to give reasons as to why a particular cement type is more suitable for a given purpose than other types. Cement manufacturing processes are associated with emissions of large quantities of greenhouse gases and environmental pollutants. We give below quantitative and qualitative analyses of environmental impact of cement manufacturing. Controlling pollution is a mandatory legal and social requirement pertinent to any industry. As cement industry is one of the biggest CO2 emitters, it is appropriate to discuss different ways and means of CO2 capture, which will be done next. Finally, we give an account of production of nanocement and advantages associated with nanocement. Nanofillers such as nanotitania, nanosilica, and nanoalumina can be produced in large industrial scale via top-down approach of reducing size of naturally available bulk raw materials to those in the nanorange of 1nm100nm. We mention the preparation of nanotitania and nanosilica from Sri Lankan mineral sands and quartz deposits, respectively, for the use as additives in cement products to improve performance and reduce the amount and cost of cement production and consequent environmental impacts. As of now, mineral sands and other treasures of minerals are exported without much value addition. Simple chemical modifications or physical treatments would add enormous value to these natural materials. Sri Lanka is gifted with highly pure quartz and graphite from which silica and graphite nanoparticles, respectively, can be prepared by simple size reduction processes. These can be used as additives in cements. Separation of constituents of mineral sands is already an ongoing process.
This paper is an extended version of the Conference Paper published in the Proceedings of the 28th International Symposium on Transport Phenomena, 2224 September 2017, Peradeniya, Sri Lanka . As described in it, cement is a powdery substance made with calcined lime and clay as major ingredients. Clay used provides silica, alumina, and iron oxide, while calcined lime basically provides calcium oxide. In cement manufacturing, raw materials of cement are obtained by blasting rock quarries by boring the rock and setting off explosives . These fragmented rocks are then transported to the plant and stored separately in silos. They are then delivered, separately, through chutes to crushes where they are then crushed or pounded to chunks of 1/2 inchsized particles . Depending on the type of cement being produced, required proportions of the crushed clay, lime stones, and any other required materials are then mixed by a process known as prehomogenization and milled in a vertical steel mill by grinding the material with the pressure exerted through three conical rollers that roll over a turning milling table. Additionally, horizontal mills inside which the material is pulverized by means of steel balls are also used. It is then homogenized again and calcined, at 1400C, in rotary kilns for the raw material to be transformed to a clinker, which is a small, dark grey nodule 3-4cm in diameter. The clinker is discharged from the lower end of the kiln while it is red-hot, cooled by various steps, ground and mixed with small amounts of gypsum and limestone, and very finely ground to produce cement .
In the calcination process, in the kiln, at high temperatures, the above oxides react forming more complex compounds . For instance, reaction between CaCO3, Al3(SiO3)2, and Fe2O3 would give a complex mixture of alite, (CaO)3SiO2; belite, (CaO)2SiO2; tricalcium aluminate, Ca3(Al2O3); and ferrite phase tetracalcium aluminoferrite, Ca4Al2O3Fe2O3 with the evolution of CO2 gas in the Portland cement clinker . However, there can be many other minor components also since natural clay also contains Na, K, and so on. In the chemical analysis of cement, its elemental composition is analyzed (e.g., Ca, Si, Al, Mg, Fe, Na, K, and S). Then, the composition is calculated in terms of their oxides and is generally expressed as wt.% of oxides. For simplicity, if we assume that the clinker contains the above four main oxides, they can be simply represented by the Bogue formulae where CaO, Al2O3, Fe2O3, and SiO2 are denoted as C, A, F, and S, respectively . In this notation, alite (tricalcium silicate) [(CaO)3SiO2], belite (dicalcium silicate) [(CaO)2SiO2], celite (tricalcium aluminate) [Ca3Al2O6=3CaOAl2O3], and brownmillerite (tetracalcium aluminoferrite) [Ca4Al2Fe2O10=4CaOAl2O3Fe2O3] are represented by C3S, C2S, C3A, and C3AF, respectively. If we analyze the elemental composition of Ca, Al, Fe, and Si, usually from X-ray fluorescence spectroscopy, then we express them as wt.% of their respective oxides. For example, if the experimentally determined clinker composition is CaO=65.6%, SiO2=21.5%, Al2O3=5.2%, and Fe2O3=2.8%, then Bogue calculations would give C3S=64.7%, C2S=12.9%, C3A=9.0%, and C4AF=8.5%, respectively . However, cement contains water (H2O), sulphate (SO3), sodium oxide (Na2O), potassium oxide (K2O), gypsum (CaSO42H2O), which are denoted as H, S, N, K, and CSH2, respectively. Note that gypsum (calcium sulphate dihydrate) is considered as CaOSO32H2O and hence its notation is CSH2. As such, approximate composition of the cement clinker is different from the above values and is depicted in Table 1.
There are several different types of cements of which Portland cement, Siliceous (ASTM C618 Class F) Fly Ash, Calcareous (ASTM C618 Class C) Fly Ash, slag cement, and silica fume are the major types [9, 10]. They differ from their chemical composition. Table 2 gives the compositions of the above cement types in terms of SiO2, Al2O3, Fe2O3, CaO, MgO, and SO3, and the remaining can be other materials such as Na2O and K2O. Note that SO3 stands for oxide of S, where S is derived from gypsum (CaSO42H2O). Given in Table 2 are also important physical properties such as specific surface area (surface area per unit mass, SSA) and specific gravity (SG) of these different types of cements [11, 12].
General use of the Portland cement, Siliceous (ASTM C618 Class F) Fly Ash, Calcareous (ASTM C618 Class C) Fly Ash, slag cement, and silica fume in concrete is as primary binder, cement replacement, cement replacement, cement replacement, and property enhancer, respectively .
There are over ten different types of cements that are used in construction purposes, and they differ by their composition and are manufactured for different uses. These are rapid-hardening cement (RHC), quick-setting cement (QSC), low-heat cement (LHC), sulphate-resisting cement (SRC), blast furnace slag cement (BFSC), high-alumina cement (HAC), white cement (WC), coloured cement (CC), pozzolanic cement (PzC), air-entraining cement (AEC), and hydrophobic cement (HpC). RHC has increased the lime content compared to the Portland cement (PC) [17, 18]. Purpose of having high lime content is to attain high strength in early days. It is used in concrete when formwork is to be removed early. Since hardening of cement is due to the formation of CaCO3 by absorbing atmospheric CO2 by CaO, increased CaO results in increased CaCO3 formation even at the early stage to result in rapid hardening .
QSC is produced by adding a small percentage of aluminium sulphate as an accelerator and reducing the amount of gypsum used with fine grinding. This cement is used when the work is to be completed very quickly as in static and running waters. LHC has reduced the amount of C3A, which is used to produce massive concrete constructions like gravity dams. LHC has compressive strength to heat of the hydration ratio of at least 7 at the age of 13 weeks. The usual wt. ratio of CaO to SiO2 is between 0.8 and 1.5, but Al2O3 wt.% is less than 10% . This is prepared by grinding the CaO, SiO2, and Al2O3 materials, melting the mixture, quenching the melt, and grinding the quenched matter to have mainly amorphous material of the above composition. Alumina is a hydratable material and reduced alumina gives reduced hydration to produce less heat of hydration. This is important in the construction of large structures to avoid possible thermal cracking during concrete setting .
Sulphate attack on concrete is a chemical breakdown mechanism, where sulphate reacts with C3A and/or Ca(OH)2 components of the hardened cement forming ettringite, which is hexacalcium aluminate trisulphate hydrate [(CaO)6(Al2O3) (SO3) 32H2O=C6ASH32]. Sulphate ions can react with C3A and/or Ca(OH)2 in hardened concrete in the presence of water forming gypsum. These newly formed ettringite and gypsum crystals occupy empty spaces of concrete, and as they grow, they tend to damage the paste by cracking. The most important parameters determining the sulphate attack are C3A, C3S/C2A ratio, and C4AF. It has been reported that the addition of pozzolonic admixtures such as fly ash reduces the C3A content of cement  when sulphate is present in water and soil used; in places like canal linings, culverts, retaining walls, and siphons, it is important to use SRC. SRC is prepared by maintaining C3A content below 6%.
BFSC is prepared by grinding the clinkers with 60% slag. BFSC resembles properties of the Portland cement and is used for works in which economic considerations are predominant. HAC is obtained by melting a mixture of bauxite and lime and grinding the mixture with the clinker. Since it contains high alumina content, it is rapid-hardening cement with initial and final setting times of about 3.5h and 5h, respectively . HAC is used in works where concrete is subjected to high temperatures, frost, and acidic conditions. WC is prepared from raw materials free from iron oxides and oxides of other transition metals such as Cr, Mn, Cu, V, and Ti. The colouring effect takes the order Cr2O3>Mn2O3>Fe2O3>V2O3>CuO>Ti2O3. As such, the amounts of these transition metal ions, particularly Cr3+, Mn3+, and Fe3+, should be minimized to form white cement. Usually, Cr2O3, Mn2O3, and Fe2O3 are kept below 0.003%, 0.03%, and 0.35%, respectively, in the clinker . Cheap quarried raw materials usually contain Cr, Mn, and Fe. For example, lime stones and clays usually contain 0.31% and 515% Fe2O3. Keeping Fe2O3 below 0.5% is desirable to make WC, and as such, kaolin and sand are used instead of other clays in making WC. The abrasiveness of sand particles with size <45m also ensures less wearing of chrome-steel grinding mill used to grind raw materials, which would otherwise contaminate the mixture with Fe and Cr. Usually, sand is ground separately using ceramic grinding media to avoid chromium contamination. WC is costly and hence used in aesthetic applications such as precast curtain wall and facing panels and terrazzo surface. Contrary to WC, CC is prepared by deliberately adding mineral pigments to cement. CCs are widely used in decorative works on floors. Iron oxides are used to get red, yellow, and black base colours, and several mixed colours such as browns-terracotta-tuscany-sepia-beach. Standard green and blue pigments are chrome oxide and cobalt aluminium oxide, respectively. TiO2 is the usual white pigment. PzC is prepared by grinding the pozzolanic clinker with the Portland cement . It is used in marine structures, sewage works, and for laying concrete under water such as in bridges, piers, and dams.
AEC is produced by adding air-entraining agents that are surfactants such as alkali salts of wood resins, synthetic detergents of the alkyl-aryl sulphonate type, calcium lignosulphate derived from the sulphite process in paper making, and calcium salts of glues and other proteins obtained in the treatment of animal hides, animal and vegetable fats, oil and their acids, wetting agents, aluminium powder, and hydrogen peroxide, during the grinding of the clinker . They are added in 0.0250.1% in either solid or liquid form. At the time of mixing, AEC produces tough, tiny, discrete noncoalescing air bubbles of 10500m in diameter in the body of the concrete. These bubbles can compress to some extent, and hence, they can absorb stress created by freezing.
HpC is prepared by adding water-repellent chemicals . They are prepared particularly for use in high-rainfall regions to prevent water absorption during storage. Particles of HpC are coated with nonpolar substances, usually by adsorbing oleic acid, stearic acid, and so on, to cement particles [27, 28]. When adsorbed, these surfactant molecules self-assemble by coordinating with surface cations through their carboxylic acid groups thereby allowing the nonpolar hydrocarbon chain to extend from the particles. When a water drop falls on them, they are stuck on hydrocarbon chains and stay as spherical particles as does by the lotus leaf. The cement particles are then not wetted, and water drops roll off when slightly slanted. These hydrophobic coatings prevent the attacks by chloride and sulphate ions, and hence, they resist to deterioration of concretes by these ions .
Measured data of the European cement kiln emissions show that cement industry contributes substantially to environmental pollution. Table 3 lists main environmental pollutants emitted by the European cement kilns in tonnes per year.
TOC/VOC, PCCD, and PCDF indicate total organic compounds including volatile organic compounds, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans, respectively. It has been reported that toxic metals such as Hg, Cd, Tl, As, Sb, Pb, Cr, Co, Cu, Mn, Ni, and V are also emitted in considerable amounts. For example, masses of Hg, (Cd, Tl), and (As, Sb, Pb, Cr, Co, Cu, Mn, Ni, V) emitted in kg per year are 01311, 01564, and 09200, respectively . In addition to material pollutants, noise emission is also associated with almost all the processes involved in cement manufacturing. These environmental impacts contribute to abiotic depletion, global warming, acidification, and marine ecotoxicity .
Cement is produced by utilizing an extensive amount of raw materials treated and reacted at extreme conditions such as high temperatures. The high-temperature processes are called pyroprocessing processes where raw materials are heated at high temperatures for solid-state reactions to take place, which utilize fuel sources such as coal, fuel oil, natural gas, tires, hazardous wastes, petroleum coke, and basically anything combustible . Some cement manufacturing plants utilize the organic waste generated in other industries such as rubber processing industries. As such, cement industry contributes to a significant extent of anthropogenic carbon dioxide emissions, which is in the range of 57% of total anthropogenic carbon dioxide emissions . In the clinker burning process, in order to produce 1 tonne of clinkers, 1.52 tonnes of raw materials are used on average. The balance of 0.52 tonne of raw materials is converted mainly to carbon dioxide by the processes such as CaCO3 CaO+CO2. This is a serious global environmental problem since increase in carbon dioxide in the atmosphere has direct consequences on global warming. In addition to CO2, other key polluting substances emitted to air by the cement industry include dust, other carbon oxides such as carbon monoxide (CO), nitrogen oxides (NOxs), sulphur oxides (SOxs), polychlorinated dibenzo-p-dioxins, dibenzofurans, total organic carbon, metals, hydrogen chloride, and hydrogen fluoride, which are serious health-hazardous substances and some are hilariously odorous . However, the type and amount of air pollution caused by the cement industry depend on various parameters, such as inputs (the raw materials and fuels used) and the type of process used in the industry.
As for water pollution, the contribution from cement industry may be insignificant through the storage and handling of fuels that may contribute to soil and groundwater contaminations . In order to reduce the amount of raw materials, particularly in the manufacturing of specialized cement types as described above, supplementary cementitious materials such as coal fly ash, slag, and natural pozzolans such as rice husk ash and volcanic ashes are used. This will not only reduce the waste materials generated for landfilling but also the cost of cement production .
However, cement is an essential material for human survival nowadays. As such, there is no alternative, but the production of cement is mandatory. At the same time, controlling pollution created by cement industry is also very important. In the next section, we discuss ways and means of controlling pollution resulting from cement industries.
The air pollution occurs in the excavation activities, dumps, tips, conveyer belts, crushing mills, and kilns of cement industry. Minimizing air pollution is a mandatory legislative requirement, which also contributes to minimizing wastage and survival of the industry . Dust particles emitted at sites other than kilns can be captured using a hood or other partial enclosure and transported through a series of ducts to the collectors. The dust collected can be fed to the kiln provided that it is not too alkaline not exceeding 0.6% as per the Na2O (N) content. However, if the alkalinity is higher than this value, then the dust must be either discarded or pretreated before feeding to the kiln. Flexible pulse jet filters, electrostatic precipitators, wet scrubbers, and baghouse method can be used to collect dust from flue gas . The US Environmental Protection Agency has reviewed the available and emerging technologies for reducing greenhouse gas emissions from Portland cement industry. The primary greenhouse gas emitted in the cement industry is carbon dioxide, but in lower quantities, NOxs and SOxs are also emitted as detailed in Table 3 .
This involves separation and capture of carbon dioxide from the flue gas, pressurization, and transportation via pipelines, injection, and long-term storage. In regard to this, several processes have been developed as detailed below.
This involves capture of CO2 from flue gas and conversion to carbonates. This utilizes a scrubber containing high-pH water with calcium, magnesium, sodium, hydroxide, and chloride as the scrubbing liquid. CO2 captured by this water is converted to CaCO3 and MgCO3, which are precipitated out of the solution. The precipitates can be filtered, washed, and dried for reuse as feed material for the kiln to make blended cement. Water used may be seawater or reject brine. Capture efficiency of over 90% has been reported in a 10-MW coal-fired pilot plant. It is interesting to note that when captured carbon is reused, the overall carbon footprint becomes negative since the carbon emissions avoided from the cement manufacturing process could be greater than those of carbon emissions from the power plant [39, 40].
In the oxy-combustion process, fuel is burnt with pure or nearly pure oxygen instead of air. Since there is no nitrogen gas, the fuel consumption is reduced due to the fact that there is no need to heat and burn nitrogen gas. Since air contains nearly 79% nitrogen gas and any combusted nitrogen comes as NOx in flue gas, the volume of flue gas and NOx in it is significantly reduced when pure oxygen is used for combustion [39, 41]. This process should utilize an air separation process to separate out nitrogen gas, which can be used for other processes such as for inflating vehicle tires. Nitrogen-removed air basically contains majority of oxygen, and it can be used for the oxy-combustion process. When oxy-combustion is used, the resulting kiln exhaust contains over 80% CO2 gas, which can be recovered by the Calera process. There are several technical issues as laid down in  that have to be tackled before implementing this process in cement industry.
When flue gas is passed through a column containing monoethanolamine, CO2 gas is selectively absorbed. High-pressure, low-temperature conditions favour the absorption. When CO2-rich MEA solution is subjected to low-pressure, high-temperature conditions, it releases absorbed CO2 which can be converted to some other product like CaCO3 or MgCO3 and the solvent recovered can be reused. One of the problems with this method is that acidic gases such as SOx and NOx present in the flue gas can react with MEA. Therefore, levels of these gases must be kept below 0.001% prior to absorption by MEA. Instead of regular amines, hindered amines can also be used. Hindered amines have special functional to prevent degradation of the amine .
Flue gas contains SOx, which could be separated using limestone-based compounds. They are then converted to slurries to use as CO2 absorbents. This way, both SOx and CO2 can be removed from flue gas .
Cryogenics is the science that addresses the production and effects of very low temperatures. In the cryogenic separation, all other gases except CO2 and N2 have to be removed prior to subjecting to low-temperature conditions. The triple point for CO2 is 256.68C and 7.4atm, and when these conditions are maintained, CO2 will condense while N2 will remain as a gas. N2 gas is then escaped through an outlet at the top of the chamber, and the dense liquid is taken from the bottom of the chamber. Refrigeration under pressure is an alternative method to cryogenic distillation but utilizes even harsh conditions such as higher pressures and lower temperatures. Cryogenic methods have distinct advantages over other separation methods. Since CO2 is separated as a liquid, it can be transported via pipelines for sequestration. Also, the recovery and purity of CO2 is very high (CO2 purity after distillation can exceed 99.95%) .
Suitable membranes can be used to separate or adsorb CO2 in the kiln exhaust gas. Poly(methoxyethoxy)ethanol phosphazene (MEEP) hollow fibre membranes are excellent CO2 separation and storing membranes, where (methoxyethoxy)ethanol groups attached P have strong interactions with CO2 . One such example is given in Figure 1.
Polymer blends with required properties such as strong interaction with CO2 can be used as CO2-selective membranes. For example, cross-linked thin-film composite of poly(vinylalcohol) (PVA)/polyvinylpyrrolidone (PVP) blend membranes doped with suitable amine carriers are excellent CO2-selective membranes as reported by Mondal and Mandal. The CO2 permeability of this membrane is 1396 Barrer at 2.8atm and 100C . Combination of grafting and cross-linking is an advanced technique capable of suppressing plasticization. In this respect, Achoundong et al. developed cellulose acetate (CA) membranes and grafted vinyltrimethoxysilane (VTMS) to OH groups, which due to subsequent condensation of hydrolyzed methoxy groups on the silane form cross-linked polymer networks. The modified membranes have an order of magnitude higher CO2 permeability than neat cellulose acetate membranes .
Polymers of intrinsic microporosity (PIMs), thermally rearranged polymers (TRPs), polyimides, and polyurethanes are advanced polymers with high selectivity for CO2 and hence are suitable membranes for CO2 separation. PIMs are ladder polymers with high free volume and high selectivity for CO2. These ladder polymer backbones can be prepared by polycondensation reaction of tetrahydroxy monomers containing spiro- or contorted centres with tetrafluoromonomers. One such example is the PIM-1 prepared by the polycondensation reaction of commercial monomers such as 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spiro-bisindane with tetrafluorophthalonitrileo. Chemical structures of the monomers are given in Figure 2.
These polymers have high CO2 solubility and spirocentres, such as thianthrene , 9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene , ethanoanthracene , and pyrazine , and could be incorporated in PIM membranes for adjusting the gas permeation properties.
Thermally rearranging polymers (TRPs) are prepared by a thermal postmembrane conversion process of functionalized polyimides. They have uniform cavities with tailored free-volume elements with well-connected morphology in the amorphous state . For example, thermal rearrangement of poly(hydroxyimide)s is shown in Figure 3.
Separation of CO2 from a gas mixture by selective adsorption involves both thermodynamics (adsorption) and kinetics (diffusion selectivity), and designing adsorbents for CO2 in the presence of gases such as CH4 and N2 is challenging since all three gases have similar kinetic diameters of 3.30, 3.76, and 23.64, respectively . In this sense, sorbents such as zeolites and metal-organic frameworks (MOFs) stand out as adsorbents of CO2.
Zeolites are microporous aluminosilicate minerals such as analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite. Figure 4 shows the microporous molecular structure of zeolite, ZSM-5. Synthetic zeolites are prepared by the slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates.
Zeolites are added to the Portland cement as a pozzolan and water reservoir to reduce chloride permeability and to improve workability. Siriwardane et al. have studied competitive gas adsorption properties of zeolites 13X, 4A, 5A, UOP-WE-G 592, and UOP APG-II with gas mixtures containing CO2 and found that all of them have good CO2 adsorption capacities down to ppm levels from a gas mixture containing 15% CO2, 3% O2, and 83% N2 . Zeolites are microporous and aluminosilicate minerals commonly used as commercial adsorbents. Zeolite has been used for trapping CO2 from ice or air . A zeolite trap was also used as an alternative to a cryogenic trap for collecting CO2 from oxidation of organic carbon. The selective gas absorption and desorption characteristics of zeolite as a function of temperature will be useful for simplifying the system for trapping CO2 and transferring the gas to a graphitization reactor .
Metal organic frameworks (MOFs) are yet another good sorbents for CO2. Their structures are composed of metal-containing nodes linked by organic ligand bridges, which are assembled through strong coordination bonds (Figure 5) .
Compared to other CO2 sorbents such as zeolites and activated carbon, MOFs have higher pore volume and surface area and hence have higher CO2 sorption capacity [60, 61]. Adsorption is the process of entrapping atoms or molecules which are incident on a surface; hence, the adsorption capacity of a material concerned increases with respect to its surface area. In 3D nature, the maximum surface area would be obtained by a structure that is highly porous such that molecules and atoms can access internal surfaces of the materials. It clearly suggests that the highly porous metal organic frameworks (MOFs) should have to have an excellent ability of entrapping CO2. Generally, the most successful MOFs demonstrate extremely high BET surface areas of 4,0007,000m2g1 with many also possessing coordinatively unsaturated metal sites . In order to gain high efficiency of CO2 entrapment inside the MOF and for the development of higher performance MOFs, the interior part of MOFs should be designed to have coordinative porosity, hydrophobicity, defects and embedded nanoscale metal catalysts, unsaturated metallic suitable sites, specific heteroatoms, and other building unit interactions . Some examples of MOFs capable of CO2 sorption are NiII2NiIII(3-OH)(pba)3(2,6-ndc)1.5 (MCF-19; pba=4-(pyridin-4-yl)benzoate, 2,6-ndc=2,6-naphthalenedicarboxylate), Zn4O(bdc)3 (MOF-5 or IRMOF-1, bdc=1,4-benzenedicarboxylate), Zn4O(btb)2 (MOF-177, btb=benzene-1,3,5-tribenzoate), and Zn4O(bte)14/9(bpdc)6/9 (MOF-210, bte=4,4,4-(benzene-1,3,5-triyltris(ethyne-2,1-diyl))tribenzoate, bpdc=biphenyl-4,4-dicarboxylate) .
Nanocement is the cement produced by mechanical activation of nuclear cement particles in the size range 2-3m by coating with 10 to 100nm-thick membranes of modifier materials. More than 65% of mineral supplements such as sand, ash, slag, and tuff and polymer additives are used as modifier materials . The development of modern cement and concrete industry seeks for the improvement of the durability of the materials by the addition of required amount of nanoparticles, or nano-based structure of cement-based materials can be improved. Frequently used nanoparticles are nanosilica, nanoclay, and carbon nanotubes [68, 69]. Improvement of durability of the materials is approached through alteration of the physicochemical properties of the binder. In addition, usages of nanoclay and carbon nanotubes can decrease the transport property, optimize microstructure, and decrease the volume instability of cement-based materials. The process of nanocement production is shown schematically in Figure 6. It has been reported that nanocement can be used to produce 500800 brands of high-strength concretes and 13001500 brands of heavy-duty concretes . US Patent on Method for producing nano-cement, and nano-cement  deals with the procedure developed to produce nanocement, which involves mechanochemical activation of dispersed grains of the Portland cement in the presence of a polymeric modifier. They used at least 60% by wt. of sodium naphthalenesulfonate and mineral siliceous additive containing at least 30 wt.% SiO2 and gypsum to form nanoshells around cement grins. Capsules of 20100nm thickness are formed around Portland cement grains, which are made of sodium naphthalenesulfonate and structured by calcium cations. Subsequent to mechanochemical activation, the resultant material is ground to a specific surface area of 300900m2kg1. Nanocement improves the technical quality of the Portland cement, reduces cost of production due to the use of 70 wt.% mineral additives, 1.22 times reduction of the fuel cost, and 2-3 times reduction of emission of NOx, SO2, and CO2 per tonne of cement. Nanocement has very high performance; for instance, the deflection strength of nanocement-based concrete and ordinary Portland cement-based concrete at 2-day hardening are around 6.37.1MPa and 2.9MPa with corresponding compressive strengths of 49.354.7MPa and 21.3MPa, respectively. At 28-day hardening, deflection strength improves to 8.28.7MPa and 6.4Mpa, respectively, while compressive strength improves to 77.582.7MPa and 54.4MPa, respectively .
Use of nano-graphite as an additive in cement is also currently under investigation. Use of graphite nanoparticles in cement is expected to not only improve mechanical properties but also improve faster curing time, inhibition of premature failure in concretes, and ability to withstand large external forces produced in earthquakes and explosions . The use of less concrete is also possible which means eventual contribution to the production of less Portland cement and hence reduction of consequent environmental problems associated with Portland cement manufacturing . Other nanofillers used to improve properties of Portland cement include nanotitania (TiO2), carbon nanotubes, nanosilica (SiO2) and nanoalumina (Al2O3), nanohematite/iron oxide (iii) (Fe2O3), nano-magnetite/iron oxide (ii) (Fe3O4), nano-ZnO2, nano-ZrO2, nano-Cu2O3, nano-CuO, nano-CaCO3, as nanotubes or fibres (carbon nanotubes and carbon nanofibers, and nano-clay).
According to the FeldmanSereda model, the cement paste consists principally of gel pores, capillary pores, and an interlayer of water. In the concrete, there is an interfacial transition zone between the cement paste and the aggregates, which establishes a weak link in the concrete, basically the site at which the first cracks occur. Hence, it is significant to generate crack-free concrete with the possible incorporation of nanosilica to pursue . Chen et al. demonstrated that TiO2 is an inert and stable compound during the cement hydration process, in which the total porosity of the cement pastes decreased, so that the pore size distribution is also changed. Normally, the acceleration of the hydration rate and the change in the microstructure also affected the physical and mechanical properties of the cement-based materials. The nano-TiO2 role is to work as a catalyst in the cement hydration reactions. Water absorption and capillary absorption show a significant decrease when TiO2 nanoparticles are included in the concrete, as the nanoparticles represent as nanofillers and thereby improve the concretes resistance to water permeability. Moreover, TiO2 nanoparticles can progress the filler effect, and also the great pozzolanic action of fine particles substantially rises the quantity of strengthening gel formed . Nanomontmorillonite (NM) is the most common member of the smectite clay family, which is sometimes referred to as nanoclay. This kind of clay belongs to a general mineral group of clays, which have particles with a sheet-like structure in which the dimensions in two directions far exceed the thickness.
We have investigated the production of these nanoparticles from both top-down and bottom-up approaches. Top-down approach is more industrially viable since large quantities of bulk materials found naturally can be used to produce corresponding nanomaterials through particle size reduction. The top-down approach relies on reducing the size of bulk materials to the size of the nanorange of 1100nm. This can be done by crushing bulk materials to make powders, sieving to different fractions, further crushing of large size fractions, and finally milling to obtain sizes in the nanorange. Sri Lanka is gifted with very high-purity quartz, which contains almost 100% SiO2. This quartz can be used to obtain nanosized SiO2 particles. Our ongoing research in collaboration with the Sri Lanka Industrial Technology Institute (ITI) is very successful, and we are able to produce 50nm size SiO2 nanoparticles in large quantities by this top-down approach. We have also attempted converting ilmenite obtained from Sri Lanka Mineral Sand Corporation to produce nanotitania with great success. Birgisson et al.  summarized the key breakthroughs in concrete technology most probable to result from the usage of nanotechnology. Basically, it has shown the development of high-performance cement and concrete materials as measured by their mechanical and durability properties, development of sustainable concrete materials and structures through engineering for different adverse environments, reducing energy consumption during cement production and enhancing safety, improvement of intelligent concrete materials through the integration of nanotechnology-based self-sensing and self-powered materials and cyber infrastructure technologies, advancement of novel concrete materials through nanotechnology-based innovative processing of cement and cement paste, and also development of fundamental multiscale model(s) for concrete through advanced characterization and modeling of concrete at the nano-, micro-, meso-, and macroscale . The frost resistance of concrete comprising nano-Al2O3 is better than that comprising the same amount of nano-SiO2. The compressive strength of normal concrete containing nano-SiO2 is higher than that of the same amount of nano-Al2O3. The frost resistance of the concrete mixtures can be improved significantly by adding either nano-Al2O3 or nano-SiO2. These nanomaterials not only promote the pozzolanic reaction, but they also act as fillers, thereby improving the pore structure of the concrete and densifying the microstructure of the cement paste. The frost resistance of the concrete containing nano-Al2O3 is better than that containing the same amount of nano-SiO2.
Nanoparticles have a large surface area to volume ratio than their bulk counterparts, and due to their small size, they can fill in small cavities of the cement matrix, densifying the structure to result in improved strength and faster chemical reactions such as hydration reactions associated with cement setting. Further, the material requirement can be reduced drastically thus saving fast depleting natural resources and energy requirements for cement manufacturing and reducing associated adverse environmental consequences.
Basically, different types of cements and their chemical composition and applications in the current engineering and chemical world have been discussed in detail. Different types of enhancing materials and fillers developed using nanotechnology for the productive and effective cement manufacturing have been mentioned with the chemical background. The mechanical defects when concrete is concerned and possible solutions that can be given through chemistry and nanotechnology have been deliberated in detail. In addition, CO2-entrapping chemical compounds such as zeolites and metal organic framework and their contribution in making durability of the cement manufacturing have been illustrated with their chemistry. Environment effects of cement manufacturing and how to control the pollution of the environment when manufacturing processes that are being executed have been discussed using several standard processes, including the Calera process, oxy-combustion process, and monoethanolamide (MEA) process. Currently, the applications of nanoscience and nanotechnology have been gaining popularity in different fields of science and technology. The potential of nanotechnology to progress the performance of concrete and to lead to the development of novel, sustainable, advanced cement-based composites, and smart materials with unique mechanical, thermal, and electrical properties is promising, and many novel opportunities are expected to arise in the future. So finally, the newest trend of making nanocement and its development towards current developing and updating world is described in advance.
Copyright 2018 S. P. Dunuweera and R. M. G. Rajapakse. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Glass, an inorganic solid material that is usually transparent or translucent as well as hard, brittle, and impervious to the natural elements. Glass has been made into practical and decorative objects since ancient times, and it is still very important in applications as disparate as building construction, housewares, and telecommunications. It is made by cooling molten ingredients such as silica sand with sufficient rapidity to prevent the formation of visible crystals.
A brief treatment of glass follows. Glass is treated in detail in a number of articles. Stained glass and the aesthetic aspects of glass design are described in stained glass and glassware. The composition, properties, and industrial production of glass are covered in industrial glass. The physical and atomic characteristics of glass are treated in amorphous solid.
The varieties of glass differ widely in chemical composition and in physical qualities. Most varieties, however, have certain qualities in common. They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass mixtures are fused with certain metallic oxides; they are, when cold, poor conductors both of electricity and of heat; most types are easily fractured by a blow or shock and show a conchoidal fracture; and they are but slightly affected by ordinary solvents but are readily attacked by hydrofluoric acid.
Commercial glasses may be divided into sodalimesilica glasses and special glasses, most of the tonnage produced being of the former class. Such glasses are made from three main materialssand (silicon dioxide, or SiO2), limestone (calcium carbonate, or CaCO3), and sodium carbonate (Na2CO3). Fused silica itself is an excellent glass, but, as the melting point of sand (crystalline silica) is above 1,700 C (3,092 F) and as it is very expensive to attain such high temperatures, its uses are restricted to those in which its superior propertieschemical inertness and the ability to withstand sudden changes of temperatureare so important that the cost is justified. Nevertheless, the production of fused silica glass is quite a large industry; it is manufactured in various qualities, and, when intended for optical purposes, the raw material used is rock crystal rather than quartz sand.
To reduce the melting point of silica, it is necessary to add a flux; this is the purpose of the sodium carbonate (soda ash), which makes available the fluxing agent sodium oxide. By adding about 25 percent of the sodium oxide to silica, the melting point is reduced from 1,723 to 850 C (3,133 to 1,562 F). But such glasses are easily soluble in water (their solutions are called water glass). The addition of lime (calcium oxide, or CaO), supplied by the limestone, renders the glass insoluble again, but too much makes a glass prone to devitrificationi.e., the precipitation of crystalline phases in certain ranges of temperature. The optimum composition is about 75 percent silica, 10 percent lime, and 15 percent soda, but even this is too liable to devitrification during certain mechanical forming operations to be satisfactory.
In making sheet glass it is customary to use 6 percent of lime and 4 percent of magnesia (magnesium oxide, or MgO), and in bottle glass about 2 percent alumina (aluminum oxide, or Al2O3) is often present. Other materials are also added, some being put in to assist in refining the glass (i.e., to remove the bubbles left behind in the melting process), while others are added to improve its colour. For example, sand always contains iron as an impurity, and, although the material used for making bottles is specially selected for its low iron content, the small traces of impurity still impart an undesirable green colour to the container; by the use of selenium and cobalt oxide together with traces of arsenic trioxide and sodium nitrate, it is possible to neutralize the green colour and produce a so-called white (decolourized) glass.
Glasses of very different, and often much more expensive, compositions are made when special physical and chemical properties are necessary. For example, in optical glasses, a wide range of compositions is required to obtain the variety of refractive index and dispersion needed if the lens designer is to produce multicomponent lenses that are free from the various faults associated with a single lens, such as chromatic aberration. High-purity, ultratransparent oxide glasses have been developed for use in fibre-optic telecommunications systems, in which messages are transmitted as light pulses over glass fibres.
When ordinary glass is subjected to a sudden change of temperature, stresses are produced in it that render it liable to fracture; by reducing its coefficient of thermal expansion, however, it is possible to make it much less susceptible to thermal shock. The glass with the lowest expansion coefficient is fused silica. Another well-known example is the borosilicate glass used for making domestic cookware, which has an expansion coefficient only one-third that of the typical sodalimesilica glass. In order to effect this reduction, much of the sodium oxide added as a flux is replaced by boric oxide (B2O3) and some of the lime by alumina. Another familiar special glass is the lead crystal glass used in the manufacture of superior tableware; by using lead monoxide (PbO) as a flux, it is possible to obtain a glass with a high refractive index and, consequently, the desired sparkle and brilliance.
The agents used to colour glass are generally metallic oxides. The same oxide may produce different colours with different glass mixtures, and different oxides of the same metal may produce different colours. The purple-blue of cobalt, the chrome green or yellow of chromium, the dichroic canary colour of uranium, and the violet of manganese are constant. Ferrous oxide produces an olive green or a pale blue according to the glass with which it is mixed. Ferric oxide gives a yellow colour but requires an oxidizing agent to prevent reduction to the ferrous state. Lead gives a pale yellow colour. Silver oxide gives a permanent yellow stain. Finely divided vegetable charcoal added to a sodalime glass gives a yellow colour. Selenites and selenates give a pale pink or pinkish yellow. Tellurium appears to give a pale pink tint. Nickel with a potashlead glass gives a violet colour, and a brown colour with a sodalime glass. Copper gives a peacock blue, which becomes green if the proportion of the copper oxide is increased.
An important class of materials is the chalcogenide glasses, which are selenides, containing thallium, arsenic, tellurium, and antimony in various proportions. They behave as amorphous semiconductors. Their photoconductive properties are also valuable.
Certain metallic glasses have magnetic properties; their characteristics of ease of manufacture, magnetic softness, and high electrical resistivity make them useful in the magnetic cores of electrical power transformers.
Many different useful and decorative articles have been made from glass over the centuries. The history of glass as a creative art has been determined partly by technical advances in its manufacture and decoration and partly by the history of taste and fashion.
Glass was first made in the ancient world, but its earliest origins are obscure. Egyptian glass beads are the earliest glass objects known, dating from about 2500 bce. Later in Egyptian civilization, a type of glass characterized by feathery or zigzag patterns of coloured threads on the surface of the glass vessel was made.
The real origins of modern glass were in Alexandria during the Ptolemaic period and, later, in ancient Rome. Alexandrian craftsmen perfected a technique known as mosaic glass in which slices of glass canes of different colours were cut crossways to make different decorative patterns. Millefiori glass, for which the canes are cut in such a way as to produce designs reminiscent of flower shapes, is a type of mosaic glass.
Glassblowing was probably developed during the 1st century bce by glassmakers in Syria. With this technique the possibilities of shaping glass into desired forms were endless. Glass could be blown into a mold or shaped completely free-form. The Romans perfected cameo glass, in which the design has been produced by cutting away a layer of glass to leave the design in relief.
The next major developments in the history of glass came during the 15th century in Venice. As early as the 13th century the Venetian island of Murano had become the centre for glassmaking. At first, Venetian glassmakers made use of many of the ancient and medieval decorative techniques to produce richly coloured and ornamental pieces having motifs characteristic of the Italian Renaissance.
Later they developed a clear glass similar to crystal, called cristallo, which was to form the basis for a thriving export trade and spread throughout Europe. Simple blown glasses of this type were much in demand in the 16th century. Such glass lent itself to decoration by the engraving of delicate designs; used from the early 16th century, the technique remained popular well into the 18th century throughout Europe. Diamond-point engraving was practiced in particular in the Netherlands and in Germany.
Late in the 17th century Bohemia became an important glass-producing area, and it remained important until early in the 20th century. By the 17th century England was making glass in the Venetian tradition that was notable for its simplicity. The glassmaker George Ravenscroft discovered about 1675 that the addition of lead oxide to Venetian-type glass produced a solid, heavier glass. Lead crystal, as it was known, thereafter became a favourite type of glass for fine tableware.
Enameling came into fashion in the middle of the 18th century in England, leading to the development of the type of glass sometimes called Bristol glass. In the 18th century glass cutting came into fashion. As this technique was perfected, great richness of effect became possible. Eventually, by the end of the 18th century, when the technique was further developed in Ireland, the whole surface of glass was being deeply cut to reflect light. This English and Irish cut lead crystal was imitated in Europe and in the United States and has remained popular to the present day. Waterford crystal is an important example of this type.
The Art Nouveau period saw some important changes. The Favrile glass invented by Louis Comfort Tiffany, with its flowing shapes derived from naturalistic forms and its lustrous surface, was much admired and particularly influenced glassmakers in central Europe. The French glassmaker mile Gall and the firm of Daum Frres were also important designers in the Art Nouveau epoch.
Ren Lalique, one of the leaders of French glass art, made glass characterized by relief decoration. The Steuben Glass Company of New York produced clear glass objects, often with engraved or incised designs.
Adjustment and control of moisture levels in solid materials through drying is a critical process in the manufacture of many types of chemical products. As a unit operation, drying solid materials is one of the most common and important in the chemical process industries (CPI), since it is used in practically every plant and facility that manufactures or handles solid materials, in the form of powders and granules.
The effectiveness of drying processes can have a large impact on product quality and process efficiency in the CPI. For example, in the pharmaceutical industry, where drying normally occurs as a batch process, drying is a key manufacturing step. The drying process can impact subsequent manufacturing steps, including tableting or encapsulation and can influence critical quality attributes of the final dosage form.
Apart from the obvious requirement of drying solids for a subsequent operation, drying may also be carried out to improve handling characteristics, as in bulk powder filling and other operations involving powder flow; and to stabilize moisture-sensitive materials, such as pharmaceuticals.
Drying may be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. It is a complicated process that involves simultaneous heat and mass transfer, accompanied by physicochemical transformations. Drying occurs as a result of the vaporization of liquid by supplying heat to wet feedstock, granules, filter cakes and so on. Based on the mechanism of heat transfer that is employed, drying is categorized into direct (convection), indirect or contact (conduction), radiant (radiation) and dielectric or microwave (radio frequency) drying.
Heat transfer and mass transfer are critical aspects in drying processes. Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor into the surrounding gas. The drying rate is determined by the set of factors that affect heat and mass transfer. Solids drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two zones are demarcated by a break point called the critical moisture content.
In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1), section AB represents the constant-rate period. In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it. This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation.
At the end of the constant rate period, (point B, Figure 1), a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, segment BC, is called the first falling-rate period. As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature. This section, represented by segment CD in Figure 1 is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, or as a result of capillary forces, or through a cycle of vaporization and condensation, or by osmotic effects.
The capacity of the air (gas) stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle. The two elements essential to this process are inlet air temperature and air flowrate. The higher the temperature of the drying air, the greater its vapor holding capacity. Since the temperature of the wet granules in a hot gas depends on the rate of evaporation, the key to analyzing the drying process is psychrometry, defined as the study of the relationships between the material and energy balances of water vapor and air mixture.
There are a number of approaches to determine the end of the drying process. The most common one is to construct a drying curve by taking samples during different stages of drying cycle against the drying time and establish a drying curve. When the drying is complete, the product temperature will start to increase, indicating the completion of drying at a specific, desired product-moisture content. Karl Fischer titration and loss on drying (LOD) moisture analyzers are also routinely used in batch processes. The water vapor sorption isotherms are measured using a gravimetric moisture-sorption apparatus with vacuum-drying capability.
For measuring moisture content in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee, jute, tobacco, rice and concrete, electrical-resistance-type meters are used. This type of instrument operates on the principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Dielectric moisture meters are also used. They rely on surface contact with a flat plate electrode that does not penetrate the product.
For measuring moisture content in paper rolls or stacks of paper, advanced methods include the use of the radio frequency (RF) capacitance method. This type of instrument measures the loss, or change, in RF dielectric constant, which is affected by the presence or absence of moisture.
Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-adiabatic dryers involve situations where a dryer does not use heated air or other gases to provide the energy required for the drying process
Non-adiabatic dryers (contact dryers) involve an indirect method of removal of a liquid phase from the solid material through the application of heat, such that the heat-transfer medium is separated from the product to be dried by a metal wall. Heat transfer to the product is predominantly by conduction through the metal wall and the impeller. Therefore, these units are also called conductive dryers.
Although more than 85% of the industrial dryers are of the convective type, contact dryers offer higher thermal efficiency and have economic and environmental advantages over convective dryers. Table 1 compares direct and indirect dryers, while Table 2 shows the classification of dryers based on various criteria.
Tray dryers. This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type. This type is most widely used in the food and pharmaceutical industries. The chief advantage of tray dryers, apart from their low initial cost, is their versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Drying times are typically long (usually 12 to 48 h).
Vacuum dryers. Vacuum dryers offer low-temperature drying of thermolabile materials or the recovery of solvents from a bed. Heat is usually supplied by passing steam or hot water through hollow shelves. Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the solid material remains at the boiling point of the wetting substance. Drying times are typically long (usually 12 to 48 h).
Fluidized-bed dryers. A gas-fluidized bed may have the appearance of a boiling liquid. It has bubbles, which rise and appear to burst. The bubbles result in vigorous mixing. A preheated stream of air enters from the bottom of the product container holding the product to be dried and fluidizes it. The resultant mixture of solids and gas behave like a liquid, and thus the solids are said to be fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying, granulation or coating purposes. In the process of fluidization, intense mixing occurs between the solids and air, resulting in uniform conditions of temperature, composition and particle size distribution throughout the bed.
Freeze dryers. Freeze-drying is an extreme form of vacuum drying in which the water or other solvent is frozen and drying takes place by subliming the solid phase. Freeze-drying is extensively used in two situations: (1) when high rates of decomposition occur during normal drying; and (2) with substances that can be dried at higher temperatures, and that are thereby changed in some way.
Microwave vacuum dryers. High-frequency radio waves with frequencies from 300 to 30,000 MHz are utilized in microwave drying (2,450 MHz is used in batch microwave processes). Combined microwave-convective drying has been used for a range of applications at both laboratory and industrial scales. The bulk heating effect of microwave radiation causes the solvent to vaporize in the pores of the material. Mass transfer is predominantly due to a pressure gradient established within the sample. The temperature of the solvent component is elevated above the air temperature by the microwave heat input, but at a low level, such that convective and evaporative cooling effects keep the equilibrium temperature below saturation. Such a drying regime is of particular interest for drying temperature-sensitive materials. Microwave-convective processing typically facilitates a 50% reduction in drying time, compared to vacuum drying.
Continuous dryers are mainly used in chemical and food industries, due to the large volume of product that needs to be processed. Most common are continuous fluid-bed dryers and spray dryers. There are other dryers, depending on the product, that can be used in certain industries for example, rotary dryers, drum dryers, kiln dryers, flash dryers, tunnel dryers and so on. Spray dryers are the most widely used in chemical, dairy, agrochemical, ceramic and pharmaceutical industries.
Spray dryer. The spray-drying process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, removal and collection of the dry particles (Figure 2). Atomization may be achieved by means of single-fluid or two-fluid nozzles, or by spinning-disk atomizers. The flow of the drying gas may be concurrent or countercurrent with respect to the movement of droplets. Good mixing of droplets and gas occurs, and the heat- and mass-transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give rise to very high evaporation rates. The residence time of a droplet in the dryer is only a few seconds (530 s). Since the material is at wet-bulb temperature for much of this time, high gas temperatures of 1,508 to 2,008C may be used, even with thermolabile materials. For these reasons, it is possible to dry complex vegetable extracts, such as coffee or digitalis, milk products, and other labile materials without significant loss of potency or flavor. The capital and running costs of spray dryers are high, but if the scale is sufficiently large, they may provide the cheapest method.
With increasing concern about environmental degradation, it is desirable to decrease energy consumption in all sectors. Drying has been reported to account for anywhere from 12 to 20% of the energy consumption in the industrial sector. Drying processes are one of the most energy-intensive unit operations in the CPI.
One measure of efficiency is the ratio of the minimum quantity of heat that will remove the required water to the energy actually provided for the process. Sensible heat can also be added to the minimum, as this added heat in the material often cannot be economically recovered. Other newer technologies have been developed, such as sonic drying, superheated steam, heat-pump-assisted drying and others.
Drying is an essential unit operation used in various process industries. The mechanism of drying is well understood as a two-stage process and depends on the drying medium and the moisture content of the product being dried.
Batch dryers are common in chemical and pharmaceutical industries, while continuous dryers are routinely used where large production is required. Since the cost of drying is a significant portion of the cost of manufacturing a product, improving efficiency or finding alternative drying routes is essential.
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6. Raghavan, G.S.V., Rennie, T.J., Sunjka, P.S., Orsat, V., Phaphuangwittayakul, W. and Terdtoon, P., Overview of new techniques for drying biological materials, with emphasis on energy aspects, Brazilian Journal of Chemical Engineering, 22(2), pp. 195201, 2005.
Dilip M. Parikh is president of the pharmaceutical technology development and consulting group DPharma Group Inc. (Ellicott City, MD 21042; Email: [email protected]). As an industrial pharmacist, Parikh has more than 35 years of experience in product development, manufacturing, plant operations and process engineering at various major pharmaceutical companies in Canada and the U.S. Prior to staring DPharma Group, he held the position of vice president of operations and technology at Synthon Pharmaceuticals in North Carolina and vice president and general manager at Atlantic Pharmaceuticals Services in Maryland. He is the editor of Handbook of Pharmaceutical Granulation 3rd ed. He has authored several book chapters and articles on various pharmaceutical technologies, including quality by design, process assessment and contract manufacturing. He has been an invited speaker at scientific conferences worldwide on solid-dosage technologies development and manufacturing.
The main function of traditional proppants is to provide and maintain conductive fractures during well production where proppants should meet closure stress requirement and show resistance to diagenesis under downhole conditions. Many different proppants have been developed in the oil & gas industry, with various types, sizes, shapes, and applications. While most proppants are simply made of silica or ceramics, advanced proppants like ultra-lightweight proppant is also desirable since it reduces proppant settling and requires low viscosity fluids to transport. Additionally, multifunctional proppants may be used as a crude way to detect hydraulic fracture geometry or as matrices to slowly release downhole chemical additives, besides their basic function of maintaining conductive hydraulic fractures. Different from the conventional approach where proppant is pumped downhole in frac fluids, a revolutionary way to generate in-situ spherical proppants has been reported recently. This paper presents a comprehensive review of over 100 papers published in the past several decades on the subject. The objectives of this review study are to provide an overview of current proppant technologies, including different types, compositions, and shapes of proppants, new technologies to pump and organize proppants downhole such as channel fracturing, and also in-situ proppant generation. Finally, the paper sheds light on the current challenges and emphasizes needs for new proppant development for unconventional resources.
Irem Zeynep Yildirim, Monica Prezzi, "Chemical, Mineralogical, and Morphological Properties of Steel Slag", Advances in Civil Engineering, vol. 2011, Article ID 463638, 13 pages, 2011. https://doi.org/10.1155/2011/463638
Steel slag is a byproduct of the steelmaking and steel refining processes. This paper provides an overview of the different types of steel slag that are generated from basic-oxygen-furnace (BOF) steelmaking, electric-arc-furnace (EAF) steelmaking, and ladle-furnace steel refining processes. The mineralogical and morphological properties of BOF and electric-arc-furnace-ladle [EAF(L)] slag samples generated from two steel plants in Indiana were determined through X-Ray Diffraction (XRD) analyses and Scanning Electron Microscopy (SEM) studies. The XRD patterns of both BOF and EAF(L) slag samples were very complex, with several overlapping peaks resulting from the many minerals present in these samples. The XRD analyses indicated the presence of free MgO and CaO in both the BOF and EAF(L) slag samples. SEM micrographs showed that the majority of the sand-size steel slag particles had subangular to angular shapes. Very rough surface textures with distinct crystal structures were observed on the sand-size particles of BOF and EAF(L) slag samples under SEM. The characteristics of the steel slag samples considered in this study are discussed in the context of a detailed review of steel slag properties.
The steelmaking industries in the US generate 1015 million tons of steel slag every year. Approximately 15 to 40% of the steel slag output is initially stockpiled in the steel plants and, eventually, sent to slag disposal sites. Utilization of steel slag in civil engineering applications can alleviate the need for their disposal and reduce the use of natural resources. A better understanding of the properties of steel slag is required for large volumes of this material to be utilized in a technically sound manner in civil engineering applications.
Knowledge of the chemical, mineralogical, and morphological properties of steel slags is essential because their cementitious and mechanical properties, which play a key role in their utilization, are closely linked to these properties. As an example, the frictional properties of steel slag are influenced by its morphology and mineralogy. Similarly, the volumetric stability of steel slag is a function of its chemistry and mineralogy. The chemical, mineralogical, and morphological characteristics of steel slag are determined by the processes that generate this material. Therefore, knowledge of the different types of steelmaking and refining operations that produce steel slag as a byproduct is also required. This paper provides an overview of steel slag generation and a literature review on the chemical and mineralogical properties of steel slags. Moreover, the mineralogical and morphological characteristics of steel slag samples generated from two steel plants in Indiana were evaluated through XRD analyses and SEM studies.
The main types of slags that are generated from the iron and steelmaking industries are classified as follow:(i)blast-furnace slag (ironmaking slag),(ii)steel-furnace slag,(a)basic-oxygen-furnace (BOF) slag,(b)electric-arc-furnace (EAF) slag,(c)ladle slag.
Basic-oxygen furnaces, which are located at integrated steel mills in association with a blast furnace, are charged with the molten iron produced in the blast furnace and steel scraps. Typically, the proper basic-oxygen furnace charge consists of approximately 1020% of steel scrap and 8090% of molten iron [1, 3]. The presence of steel scraps in the basic-oxygen furnace charge plays an important role in cooling down the furnace and maintaining the temperature at approximately 1600C1650C for the required chemical reactions to take place.
Figure 2 shows a schematic representation of a basic-oxygen furnace [1, 4]. First, steel scrap is charged to the furnace and, immediately after this charge, a ladle of molten iron (~200 tons) is poured on top of it with the help of a crane. Then an oxygen lance, lowered into the furnace, blows 99% pure oxygen on the charge at supersonic speeds. During the blowing cycle, which lasts approximately 2025 minutes, intense oxidation reactions remove the impurities of the charge. Carbon dissolved in the steel is burned to form carbon monoxide, causing the temperature to rise to 16001700C (the temperature in the furnace is carefully monitored throughout the oxygen blowing period). The scrap is thereby melted, and the carbon content of the molten iron is lowered [1, 3]. In order to remove the unwanted chemical elements of the melt, the furnace is also charged with fluxing agents, such as lime (CaO) or dolomite (MgCa(CO3)2), during the oxygen blowing cycles. The impurities combine with the burnt lime or dolomite forming slag and reducing the amount of undesirable substances in the melt. Samples of the molten metal are collected near the end of the blowing cycle and tested for their chemical composition. Once the desired chemical composition is achieved, the oxygen lance is pulled up from the furnace.
Slag resulting from the steelmaking process floats on top of the molten steel. The basic-oxygen furnace is tilted in one direction in order to tap the steel into ladles. The steel produced in the basic-oxygen furnace can either undergo further refining in a secondary refining unit or be sent directly to a continuous caster where semifinished shapes (blooms, billets, or slabs) are solidified in integrated steel mills. After all the steel is removed from the basic-oxygen furnace, it is tilted again in the opposite direction to pour the liquid slag into ladles. The slag generated from a steelmaking cycle is later processed, and the final product after processing is referred to as basic-oxygen-furnace slag (BOF slag). The chemical reactions occurring during the removal of impurities determine the chemical composition of the basic-oxygen-furnace slag [1, 3, 5].
Electric-arc furnaces (mini mills) use high-power electric arcs, instead of gaseous fuels, to produce the heat necessary to melt recycled steel scrap and to convert it into high quality steel. The electric-arc furnace steelmaking process is not dependent on the production from a blast furnace since the main feed for it is steel scrap with some pig iron. Electric-arc furnaces are equipped with graphite electrodes and resemble giant kettles with a spout or an eccentric notch on one side. The roof of the electric-arc furnaces can pivot and swing to facilitate the loading of raw materials. Steel scraps, either as heavy melt (large slabs and beams) or in shredded form are separated, graded, and sorted into different classes of steel in scrap yards. Scrap baskets are loaded carefully with different types of scrap according to their size and density to ensure that both the melting conditions in the furnace and the chemistry of the finished steel are within the targeted range .
The electric-arc furnace steelmaking process starts with the charging of various types of steel scrap to the furnace using steel scrap baskets. Next, graphite electrodes are lowered into the furnace. Then, an arc is struck, which causes electricity to travel through the electrodes and the metal itself. The electric arc and the resistance of the metal to this flow of electricity generate the heat. As the scrap melts, the electrodes are driven deeper through the layers of scrap. In some steel plants, during this process, oxygen is also injected through a lance to cut the scrap into smaller sizes. As the melting process progresses, a pool of liquid steel is generated at the bottom of the furnace. CaO, in the form of burnt lime or dolomite, is either introduced to the furnace together with the scrap or is blown into the furnace during melting. After several baskets of scraps have melted, the refining metallurgical operations (e.g., decarburization and dephosphorization) are performed. During the steel refining period, oxygen is injected into the molten steel through an oxygen lance. Some iron, together with other impurities in the hot metal, including aluminum, silicon, manganese, phosphorus, and carbon, are oxidized during the oxygen injections. These oxidized components combine with lime (CaO) to form slag. As the steel is refined, carbon powder is also injected through the slag phase floating on the surface of the molten steel, leading to the formation of carbon monoxide. The carbon monoxide gas formed causes the slag to foam, thereby increasing the efficiency of the thermal energy transfer. Once the desired chemical composition of the steel is achieved, the electric-arc furnace is tilted, and the slag and steel are tapped out of the furnace into separate ladles. Steel is poured into a ladle and transferred to a secondary steelmaking station for further refining. The molten slag is carried to a slag-processing unit with ladles or slag pot carriers [13, 5].
In electric-arc furnaces, up to 300 tons of steel can be manufactured per cycle (a cycle takes one to three hours to complete). Initially, the EAF steelmaking process was more expensive than the BOF process and, hence, it was only used for production of high quality steels. However, as the size of the electric-arc furnaces increased over the years, the EAF steelmaking process has become competitive in the production of different grades of steel and has started to dominate the US steel industry with a 55% share of the total steel output in 2006, according to USGS .
After completion of the primary steelmaking operations, steel produced by the BOF or EAF processes can be further refined to obtain the desired chemical composition. These refining processes are called secondary steelmaking operations. Refining processes are common in the production of high-grade steels. The most important functions of secondary refining processes are final desulfurization, degassing of oxygen, nitrogen, and hydrogen, removal of impurities, and final decarburization (done for ultralow carbon steels). Depending on the quality of the desired steel, molten steel produced in the EAF and BOF process goes through some or all of the above mentioned refining processes [1, 2]. Most of the mini mills and integrated steel mills have ladle-furnace refining stations for secondary metallurgical processes. Figure 3 shows a schematic representation of an electric-arc-furnace and a ladle-refining unit associated with it [2, 4].
Ladle furnaces, which look like smaller versions of EAF furnaces, also have three graphite electrodes connected to an arc transformer used to heat the steel. Typically, the bottom of the ladle furnace has a pipeline through which argon gas is injected for stirring and homogenization of the liquid steel in the furnace. By injecting desulfurizing agents (such as Ca, Mg, CaSi, CaC2) through a lance, the sulfur concentration in the steel can be lowered to 0.0002% . The addition of silicon and aluminum during deoxidation forms silica (SiO2) and alumina (Al2O3); these oxides are later absorbed by the slag generated by the refining process. In addition, in order to adjust precisely the chemical composition of the steel to produce different grades of steel, the desired alloys are added to the molten steel through an alloy hopper that is connected to the ladle furnace. Ladle furnaces also function as a storage unit for the steel before the initiation of casting operations. Therefore, ladle furnaces reduce the cost of high-grade steel production and allow flexibility in the steelmaking operations [1, 2].
Both BOF and EAF slags are formed during basic steelmaking operations, as explained above. Therefore, in general, the chemical and mineralogical compositions of BOF and EAF slags are similar. Calcium oxide and iron oxide are the two major chemical constituents of both EAF and BOF slags. Ladle slag is generated during the steel refining processes in which several alloys are added to the ladle furnace to produce different grades of steel. For this reason, the chemical constituents of ladle slag differ from those of BOF and EAF slags. Table 1 provides the chemical composition of basic-oxygen-furnace (BOF), electric-arc-furnace (EAF), and ladle slags from various sources .
The main chemical constituents of the basic-oxygen-furnace slag are CaO, FeO, and SiO2. During the conversion of molten iron into steel, a percentage of the iron (Fe) in the hot metal cannot be recovered into the steel produced. This oxidized iron is observed in the chemical composition of the BOF slag. Depending on the efficiency of the furnace, the iron oxide (FeO/Fe2O3) content of BOF slag can be as high as 38% (refer to Table 1); this is the amount of oxidized iron that cannot be recovered during the conversion of molten iron into steel. The silica (SiO2) content of BOF slag ranges from 7 to 18%. The Al2O3 and MgO contents are in the 0.54% and 0.414% ranges, respectively. The free lime content can be as high as 12%. Large quantities of lime or dolomotic lime are used during the process of conversion from iron to steel and, hence, the CaO content of BOF slag is typically very high (CaO >35%) [1, 8, 12, 23].
EAF slag has a chemical composition similar to that of BOF slag (refer to Table 1). The EAF steelmaking process is essentially a steel scrap recycling process. Therefore, the chemical composition of EAF slag depends significantly on the properties of the recycled steel. Compared to BOF slags, the main chemical constituents of EAF slags can vary widely. Typically, the FeO, CaO, SiO2, Al2O3, and MgO contents of EAF slags are in the 1040%, 2260%, 634%, 314%, and 313% ranges, respectively. Other minor components include other oxidized impurities, such as MgO, MnO, and SO3. EAF slags also contain free CaO and MgO along with other complex minerals and solid solutions of CaO, FeO, and MgO. The FeO content of EAF slags generated from stainless steel production processes can be as low as 2% .
Information on the chemical composition of ladle slags (LS) is limited in the literature. During the steel refining process, different alloys are fed into the ladle furnace in order to obtain the desired steel grade. Hence, the chemical composition of ladle slag is highly dependent on the grade of steel produced. As a result, compared to BOF and EAF slags, the chemical composition of ladle slag is highly variable. Typically, the FeO content of ladle slag is much lower (<10%) than that of EAF and BOF slags. On the other hand, the Al2O3 and CaO contents are typically higher for ladle slags (refer to Table 1).
Crystal formation is a function of both the chemical composition of the melt and its cooling rate. Silica rich blast-furnace slag vitrifies (forms a glassy phase) easily when it is rapidly cooled. Steel slag has a lower silica content than blast-furnace slag and, hence, steel slag seldom vitrifies even when rapidly cooled. Tossavainen et al.  studied the effect of the cooling rate on the mineralogy of BOF, EAF, and ladle slag samples with different proportions of major chemical constituents and showed that ladle slag rapidly cooled using the water granulation technique becomes almost completely amorphous, with the exception of the crystalline phase of periclase (MgO). On the other hand, the rapidly cooled (granulated) BOF and EAF slag samples showed very complex crystalline structures similar to those of slowly cooled BOF and EAF slag samples. Reddy et al.  also identified a very crystalline structure in quenched BOF slag using XRD analysis. These studies indicate that even when rapidly cooled, in general, steel slag tends to crystallize due to its chemical composition.
Several researchers studied the mineralogical composition of steel slags. X-ray diffraction analysis of steel slag samples shows a complex structure with many overlapping peaks reflecting the crystalline phases present in steel slag. These crystalline phases appear to be mainly due to the chemical composition of steel slag and the slow cooling rate applied during processing [1, 2628]. The feed (charge) into the furnaces vary from one steelmaking plant to another, so variations in the chemical constituents of steel slags produced at different steelmaking plants are expected. A variety of mineral phases were identified and reported in the literature for EAF, BOF, and ladle slags. Table 2 presents the minerals identified in steel slags, as reported in the literature [8, 13, 16, 17, 20, 21, 25, 2830].
The common mineral phases present in steel slags include merwinite (3CaOMgO2SiO2), olivine (2MgO2FeOSiO2), -C2S (2CaOSiO2), -C2S, C4AF (4CaOAl2O3FeO3), C2F (2CaOFe2O3), CaO (free lime), MgO, FeO and C3S (3CaOSiO2), and the RO phase (a solid solution of CaO-FeO-MnO-MgO) [21, 24, 31], as can be seen in Table 2. Since BOF and EAF slags both have high iron oxide contents, solid solutions of FeO (wustite) are typically observed as one of the main mineral phases. Ladle slag has a lower FeO content, and polymorphs of C2S are therefore frequently observed as the main phase [19, 24, 27, 29].
Due to the presence of unstable phases in its mineralogy, steel slags can show volumetric instability, caused mainly by the presence of free CaO. In the presence of water, free lime hydrates and forms portlandite (Ca(OH)2). Portlandite has a lower density than CaO and, hence, hydration of free CaO results in volume increase. Ramachandran et al.  studied the hydration mechanism of CaO and proved that when it is immersed in water, compacted CaO can hydrate almost completely in a few days with a volume increase as high as 100%. Their study also demonstrated that hydration of lime by exposure to water vapor causes more expansion than hydration caused by exposure to water due to the effect of temperature. The fact that limes hydrates quickly suggests that the majority of the free lime in steel slag will hydrate in a few days if it is given access to water. However, residual lime can be embedded in small pockets in gravel-size steel slag particles. Figure 4 depicts a gravel-size BOF slag particle with a lime pocket (seen in white). Lime pockets may not hydrate at all if they are not given access to water through the fractures extending to them. If there are fractures in the slag particles extending to these lime pockets, then hydration can progress [8, 12, 33].
Other expansive compounds, such as free MgO, may also be present in steel slag. Unlike CaO, free MgO hydrates at a much slower rate, causing significant volume changes for months or even years. In general, slags generated from modern steelmaking technologies have low MgO content. However, if dolomite (CaMg(CO3)2) is used as a fluxing agent instead of lime, the free MgO content in steel slag increases and, therefore, the possibility of volumetric expansion due to hydration of MgO increases as well [8, 3437].
Another reaction that causes volumetric expansion involves the dicalcium silicate (C2S) phase. The C2S phase is commonly present in all types of steel slags and, in particular, is abundant typically as the main phase in ladle slags. C2S exists in four well-defined polymorphs: , , , and . -C2S is stable at high temperatures (>630C). At temperatures below 500C, -C2S starts transforming into -C2S. This transformation produces volumetric expansion of up to 10%. If the steel slag cooling process is slow, crystals break, resulting in a significant amount of dust. This phase conversion and the associated dusting are typical for ladle slags. For this reason, ladle slags are commonly called self-dusting or falling slags [8, 27].
The chemical composition, mineralogy, and morphology of steel slag particles can influence both the cementitious characteristics and mechanical properties of steel slag. Two different types of steel slag (BOF and EAF ladle slags) generated from Indiana steel plants were considered in this study.
Mittal Steel, Indiana Harbor Works West Plant, which is located in Highland, Indiana, was the source plant for the BOF slag. Multiserv Ltd., Harsco Corporation, which performs slag processing operations at the Mittal Steel Plant, supplied representative samples of BOF slag consisting of particles smaller than 15mm. The Whitesville Steel Mill at Nucor Steel, which is located in Crawfordsville, Indiana, was the source for the EAF ladle (L) slag. The Edward C. Levy Co., which operates at the Whitesville Steel Mill, supplied The EAF(L) slag. This slag is referred to as EAF(L) slag, as it is the ladle slag generated from the refining of the steel from the electric-arc furnace. Edward C. Levy Co. provided representative samples of EAF(L) slag consisting of particles smaller than 9.5mm.
The oxide composition of both the BOF slag and EAF(L) samples was determined by the slag processing companies (Multiserv and Edward C. Levy Co.) using X-ray fluorescence (XRF) analysis. In order to determine the mineralogical phases present in the steel slag samples, X-ray diffraction analyses were carried out on both BOF slag and on EAF(L) slag samples with a Siemens D-500 diffractometer using copper radiation. Representative oven-dried steel slag samples (with both gravel-size and finer particles) were crushed until a powder passing the No. 200 (0.075mm opening) sieve was attained. The powder samples were step-scanned from 5 to 65 ( in 0.02 increments and 1s count time. The X-ray diffraction patterns of the steel slag samples were analyzed by comparing the peaks present in the XRD patterns with those provided in The Joint Committee for Powder Diffraction Standards, Hanawalt System for identification of inorganic compounds (JCPDS). The software program Jade was also used to help identify the minerals present in the samples. Only qualitative analyses were performed due to the presence of overlapping peaks in the XRD patterns and to the complexity of the crystalline phases in the slag samples tested. The main, minor, and probable phases were determined for each slag sample tested.
Steel slag particles were subjected to microscopic examination to characterize their shape, angularity, and surface texture. The examination was performed with a scanning electron microscope (manufactured by ASPEX, Model Personal SEM) and a light microscope (manufactured by Nikon). The shape and surface texture of the gravel-size particles were visible to the naked eye. The medium sand-size particles were examined under the light microscope. Finer sand and silt-size particles were examined under the SEM. To prevent charging of the steel slag particles, they were coated with palladium with the Hummer 6.2 sputtering system. The coated steel slag particles were examined on a two-sided copper tape. The SEM images were captured on both photomicrographs and digital files.
Table 3 gives the oxide composition of the BOF slag samples. The percentages of most of the oxides present in the BOF slag samples tested in this study are within the ranges reported by other researchers [8, 10, 13, 38, 39]. However, the FeO content of the tested BOF slag samples is slightly higher than that of most of the BOF slags reported in the literature.
The XRD patterns of the BOF slag samples were very complex, with several overlapping peaks resulting from the many minerals present in the samples (see Figure 5). BOF slag is cooled slowly in slag pits thereby allowing enough time for formation of well-defined crystals. Several other researchers have reported similar, complex XRD patterns for BOF slag [13, 20, 25].
Table 4 summarizes all of the mineral phases that were identified in the BOF slag samples. The mineral phases identified in the BOF slag samples were determined as major or minor phases depending on the intensity of the peaks, which is an indication of the quantity of the minerals present in the samples. It is important to note that the very complex mineralogical composition of BOF slag, with many overlapping peaks and different solid solutions of oxides (FeO and MgO), makes the identification of the phases very difficult. Therefore, some of the overlapping mineral phases that could not be determined with certainty were identified as probable. The most abundant mineral phase present in BOF slag is portlandite (Ca(OH)2). The presence of this mineral is expected since BOF slag contains 39% lime (CaO), which in the presence of moisture, converts to Ca(OH)2. The other major phases included merwinite (Ca3Mg(SiO4)2), and srebrodolskite (Ca2Fe2O5). The presence of free lime (CaO) and the probable presence of free magnesia (MgO) in the samples are an indication of the potential for volumetric instability of the tested BOF slag.
Figure 6 shows the gravel-size particles of BOF slag. The gravel-size particles of BOF slag had shapes varying from subrounded to subangular. Distinct asperities and edges were visible in subangular, bulky particles. Most of the gravel-size particles had a high sphericity and a solid structure. A heterogeneous porous structure was also observed on the surface of a few particles.
Figures 7(a) and 7(b) are SEM micrographs showing the shape and surface texture of BOF slag particles, respectively. The SEM studies showed that the sand- and silt-size BOF slag particles had subrounded to angular shapes. Distinct asperities and edges were visible in angular, bulky particles. Most of the sand- and silt-size particles examined under the SEM had rough surface textures.
Shi  reported that the CaO, SiO2, Al2O3, MgO, and FeO contents of ladle slag are in the ranges of 3060%, 235%, 535%, 110%, and 0.115%, respectively. The SiO2 content of the EAF(L) slag used in this study was slightly higher than the lower limit of the range reported by Shi . The EAF(L) slag used in this research is cooled very slowly in the pits under ambient atmospheric conditions. These slow cooling conditions allow the formation of various crystalline phases; these are reflected in the very complex XRD patterns shown in Figure 8. Mineral phases with distinct peaks of high intensities and some overlapping peaks of low intensities were detected. Several other researchers have reported similar XRD patterns for EAF(L) slag [13, 20, 28].
Table 6 summarizes all the mineral phases that were identified in the EAF(L) slag samples. As done for BOF slag, the mineral phases identified in the EAF(L) slag samples were determined as major or minor depending on the intensity of the peaks. Some of the overlapping mineral phases that could not be determined with certainty were identified as probable. The two major mineral phases present in the EAF(L) slag samples were portlandite (Ca(OH2)) and mayenite (Ca12Al14O33). The highest peak in the XRD pattern of the EAF(L) slag samples was observed for portlandite (see Table 5). Other minor phases identified were lime (CaO), larnite (Ca2SiO4), uvavorite (Ca3Cr2(SiO4)3), wollastonite (Ca, Fe)SiO3), and periclase (MgO).
Figure 9 shows the gravel-size particles of EAF(L) slag. The gravel-size particles of the EAF(L) slag sample had shapes varying from subrounded to subangular. Both bulky and platy gravel-size particles were observed. Distinct asperities and edges were also visible in subangular, bulky particles. Most of the platy particles had irregular shapes with very low sphericity and sharp edges. Figures 10(a) and 10(b) show the EAF(L) slag sand- and silt-size particles. The EAF(L) slag sand- and silt-size particles had subrounded to subangular shapes. Some very irregularly shaped platy particles were also observed. Most of the EAF(L) slag sand-size particles examined under SEM had extremely rough surface textures with platy, crystalline structures (see Figure 10). Some of the SEM micrographs of the EAF(L) slag sand-size particles indicated the presence of a porous structure.
The mineralogical and morphological properties of BOF and EAF(L) slag samples generated from two steel plants in Indiana were investigated through XRD analyses and SEM studies. The following conclusions were reached.(1)The main mineral phases identified in the BOF slag samples were Portlandite, srebrodolskite, and merwinite. (2)Most of the BOF slag gravel-size particles had a high sphericity and a solid structure. Sand- and silt-size BOF slag particles had subrounded to angular shapes and rough surface textures under SEM.(3)The main mineral phases identified in the EAF(L) slag samples were portlandite, mayenite, and malenterite. (4)Both bulky and platy gravel-size particles with very low sphericity and sharp edges were observed in the EAF(L) slag samples. Sand- and silt-size particles of EAF(L) slag samples showed subrounded to subangular shapes. SEM micrographs showed that the majority of the sand-size particles had extremely rough surface textures with distinct crystal structures.(5)The morphological studies suggest that both the BOF and EAF(L) slag samples tested in this study have favorable frictional characteristics.(6)The complex XRD patterns of the tested BOF and EAF(L) slag samples were a result of their chemical composition and the very slow cooling conditions applied during their processing. The XRD analyses of both the BOF and EAF(L) slag samples indicated the presence of free MgO and CaO. Since these compounds expand when hydrated, the volumetric instability of the tested steel slags needs to be assessed for their use in civil engineering applications.
This work was supported by the Joint Transportation Research Program administered by the Indiana Department of Transportation (INDOT) and Purdue University, Edw. C. Levy Co., and Multiserv Ltd., Harsco Corporation. The contents of this paper reflect the views of the writers, who are responsible for the facts and the accuracy of the data presented herein. The contents neither necessarily reflect the official views or policies of the Indiana Department of Transportation, nor do the contents constitute a standard, specification or regulation. The writers are thankful to John Yzenas of Levy Co., and Nayyar Siddiki of INDOT for their support during this project.
Copyright 2011 Irem Zeynep Yildirim and Monica Prezzi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.