cement grinding with gypsum

joyal-limestone and gypsum grinding plant for cement additive production

Crushed limestone and gypsum are popular cement additives. However, the crushed limestone and gypsum must be pulverized into 200mesh powder before being used to produce cement additives. Joyal offers MTM Trapezium grinding mill and MTW European trapezium grinding machine for limestone grinding and gypsum grinding. Aside from single mill units, Joyal also offers customized solutions, for instance, you can choose trusted spare parts for the whole grinding plant, such as ABB or SIMENS.

Cement additive, mainly composed of limestone and gypsum, offers all the benefits of traditional grinding aids such as increased grinding efficiency and cement flow ability. In addition, in many cases it has been found to improve the strength, particularly the late strength of cement. It allows the cement producer to trade off strength gains for reduced fineness and achieves lower unit power savings compared with other grinding aids. It has also been known to improve the workability of cement mortars and concrete. Joyal has been engaged for decades in mineral material grinding equipment R&D and manufacturing. Our MTM Mill and MTW Mill which adopts advanced European technical innovations can meet your limestone and gypsum processing requirement for cement additive production. Limestone or gypsum raw materials are firstly crushed by jaw crusher to specified size, and then elevated into a hopper. Then vibrating feeder sends evenly and continuously the materials into the grinding chamber for powder processing. The ground limestone or gypsum grains are carried by air from the blower into the classifier for classification. The finer powers are blown into cyclone collector, and the rough grains will be recycled back into the grinding chamber for further grinding, forming a closed circuit. Final products of 200mesh and less in size are poured out through out-put valve.

cement milling

Cement milling is usually carried out using ball mills with two or more separate chambers containing different sizes of grinding media (steel balls).Grinding clinker requires a lot of energy. How easy a particular clinker is to grind ("grindability") is not always easy to predict, but large clusters of belite due to coarse silica in the feed are difficult to grind. Rapid cooling of the clinker is thought to improve grindability due to the presence of microcracks in alite and to the finer crystal size of the flux phases.

Vertical roller mills (VRMs) are the main alternative means of grinding the clinker and are increasingly being used. For a quick introduction to VRMs see, for example, this advertisement on Youtube for an FL Smidth VRM.

Because the cement gets hot due to the heat generated by grinding, gypsum can be partly dehydrated, forming hemihydrate, or plaster of Paris - 2CaSO4.H2O. On further heating, hemihydrate dehydrates further to a form of calcium sulfate known as soluble anhydrite (~CaSO4). This is calcium sulfate with a trace of bound water in the crystal structure and it has a very approximately similar solubility in water at room temperature to hemihydrate, which in turn has a higher solubility than either gypsum or natural anhydrite.

The relative proportions and different solubilities of these various types of calcium sulfate, and of the different forms of clinker sulfate, are of importance in controlling the rate of C3A hydration and consequently of cement set retardation. Problems associated with setting and strength characteristics of concrete can often be traced to changes in the proportions of the different forms of calcium sulfate due to differences in temperatures during grinding. Variations in cooling rate of the clinker in the kiln and subsequent changes in the proportions or size of the C3A crystals may also have an effect on setting characteristics.

If the rate at which sulfate is supplied by the different sulfate salts exceeds the rate at which the clinker minerals, principally the aluminate phase, can react, the pore solution may become supersaturated with respect to gypsum. Crystals of gypsum form which cause the mix to stiffen in what is known as a false set.

Conversely, if the rate at which sulfate is supplied is insufficient to control aluminate reaction, the mix will stiffen due to the fomation of monosulfate crystals, or possibly of calcium aluminate hydrate; this is known as flash set.

For set regulation, the most important feature of aluminate is not necessarily the absolute amount present, but the amount of surface which is available to water for reaction. This will be governed by many factors, such as the surface area of the cement, the grinding characteristics of the different phases and also the size of the aluminate crystals. Over-large crystals can lead to erratic setting characteristics.

Articles like this one can provide a lot of useful material. However, reading an article or two is perhaps not the best way to get a clear picture of a complex process like cement production. To get a more complete and integrated understanding of how cement is made, do have a look at the Understanding Cement book or ebook. This easy-to-read and concise book also contains much more detail on concrete chemistry and deleterious processes in concrete compared with the website.

Almost everyone interested in cement is also concerned to at least some degree with concrete strength. This ebook describes ten cement-related characteristics of concrete that can potentially cause strengths to be lower than expected. Get the ebook FREE when you sign up to CEMBYTES, our Understanding Cement Newsletter - just click on the ebook image above.

cement grinding stage - amrit cement

At the Cement Grinding Stage, 90 95% of the clinker is mixed with gypsum and ground in a Cement Ball Mill to produce quality cement OPC 43 and OPC 53. In case of PPC Cement, there is an addition of Fly Ash.

cement grinding unit | cement grinding plant | epc project

Cement grinding unit, or called cement grinding plant, is an individual grinding plant in the finished cement production. The new-type cement grinding units adopt pre-grinding technology. The cement grinding units not only reduce the particles of feeding materials, but also help to produce cracks and flaws inside the particles, which largely increase production capacity of cement mill, reduce the energy consumption.

The cement grinding machine includes cementdryer, cement ball mill, cement roller press, powder selecting machine, conveyor and other cement equipment. We can provide scheme design for free according to the requirements of users, and offer appropriate equipment to ensure smooth production with less investment and high profit.

The cement grinding process include the admixture crushing, cement batching station, pre-crushing, high and fine powder grinding, fine powder separation, dust treatment, automatic control and other technologies. The finished cement can meet the requirement for high yield, high quality and energy saving.

As a professional EPC project provider, we have the ability to provide the custom-design solutions to cement manufacturing for every client. Our solution covers all stages including equipment installation, operation, production, maintenance, we will send technicians to guide training, until the customer is satisfied.

For the structure, of the cement mill, we adopts the advanced obstructing equipment for internal powder, add the activated device to the fine grinding chamber, and there is the special grate plate at the end of the cement mill, which can reduce the size of the grinding medium, greatly improve the grinding efficiency, and achieve the purpose of high output and low energy consumption.

The cement grinding unit has advantages of simple structure, strong controllability, and easy to operate or maintain. The discharge concentration is less than 50mg/Nm3, lower than the discharge standard, and protect the environment of the production site effectively.

The production process overcomes the disadvantages of traditional equipment. The fineness of product is easy to be adjusted, apply for different requirement of fineness. The equipment also has advantages of large heat dissipation area and low temperature inside mill. The finished product of new-type cement grinding unit has high quality and large capacity.

As for the layout, the cement grinding unit is built near the sales market of cement. The most of cement admixture is the industrial residue, and cement grinding plant can greatly consume the industrial residues like slag, fly ash, slag, coal gangue. So the cement grinding unit brings green benefits well.

Traditional cement factories belong to high-pollution and high-energy consumption industries and do not meet the goals of energy conservation and consumption reduction in the development of modern industry. Cement grinding unit is a key link in cement plant. To achieve energy saving and consumption reduction in the cement industry, we must proceed from the cement grinding unit. Through a series of research and analysis, the following seven methods can effectively reduce the energy consumption of cement grinding plant.

Make full use of various industrial waste slags, such as blast furnace slag, steel slag, fly ash, and burned coal gangue. These waste slags are treated at high temperature and help to improve the performance of cement. At the same time, it also reduces the particle size of the grinding material, which can reduce the power consumption per unit product.

Under the fixed process conditions, when the 45m sieve residue and specific surface area of cement are controlled at a reasonable level, particles below 3m and above 45m can be limited, so as to obtain good cement performance and lower production costs. Compared with other methods, this fineness control method has the advantages of simple operation and effective control. As long as a sample is taken for a sieve analysis test and specific surface area determination, it can provide a basis for the operation of the cement mill.

In the cement grinding process, a small amount of admixture is added to eliminate the adhesion and aggregation of the fine powder, accelerate the grinding speed of the material, improve the grinding efficiency, and also increase the content of 3-30m by 10-20%, which is conducive to the high-quality, energy-saving and high-yield of cement ball mill. Such additives are collectively referred to as grinding aids. In most cases, the use of grinding aids can increase the output of the cement ball mill without changing the cement production process, the purpose of improving the quality of cement production line, reducing costs, and producing green high-performance environmentally friendly cement can be achieved.

In the cement production process, the use of new technologies and processes can greatly reduce energy consumption, save investment, and achieve economic benefits. Such as multi-point feeding, multi-point retrieving, circulating grinding process; extruder combined grinding process, etc. At the same time, we must vigorously promote to use energy-saving cement grinding equipment. For example, instead of traditional cement ball mills, select new type cement vertical mills, trapezoidal mills, and other energy-saving cement grinding equipment.

At present, these two measures are the main reform directions for the cement mill optimization. The following effects can be achieved: the output is increased by 30% -60%, the power consumption is reduced by 25% -40%, and the power consumption per ton of cement is reduced by 8-12kWh. It can effectively increase the strength of cement. Generally speaking, the strength of cement can be increased by more than 5MPa.

Frequency conversion technology is one of the energy-saving technologies. This technology can help some auxiliary equipment in cement plant to save energy, such as fans, batching metering, and variable-speed equipment. At the same time, it also meets the characteristics of low-speed operation and large starting torque of the cement ball mill and realizes the continuous adjustment of the running speed of the cement ball mill. There is no inrush current when the motor starts, the starting distance is sufficient, and the protection function is perfect. Ensure the quality of process control and save maintenance costs.

The use of low-voltage motor full-phase control technology, it has the advantage of automatically adjusting the output power of the motor when the load is lightly loaded and heavily loaded so that the motor can save electricity by 15% at the time. Adopt a green lighting system, through these technical measures, greatly improve the power factor, reduce the reactive power demand, and reduce the loss of active power.

In general, correctly coordinating the relationship between quality, output, and the energy consumption is the basic way for cement grinding to save energy and reduce consumption. Traditional cement mills have low energy utilization rate and do a lot of useless work. The energy-saving technology should be the basic task of transforming the cement grinding process, and all these need to be established on the basis of a certain quality index control. To achieve a reasonable particle ratio of cement materials, it should meet the gradation requirements of performance, and add an appropriate amount of industrial waste residue.

surya gold cement manufacturing - cement grinding process

The clinker is combined with small quantity of gypsum and then it is finely ground in a separate mill to get the final product. The mill is a large revolving cylinder containing steel balls that is driven by a motor. The finished cement is ground so fine that it can pass through a sieve that will hold water.

gypsum addition - an overview | sciencedirect topics

There is usually found to be an optimum SO3 content (2%3%) for binders, beyond which (>4%) compressive strength begins to decline, particularly at early ages. Frigione and Marra28 confirmed this statement using ISO-RILEM mortar prisms, for cements with a wide range of particle size distributions (gypsum was added to preground clinker), and found that the maximum in strength was not sensitive to particle size grading.

The compressive strength of 100-mm concrete cubes containing up to 2.2% SO3 additions (gypsum) were found to show significant strength regression at levels of addition above 4.2% SO3 for a Portland cement concrete, but incorporation of 20% or 40% natural pozzolana allowed the strength level to be maintained up to and including 13.2% SO3. It was pointed out that the maximum allowable sulfate in a concrete is dependent on many factors, including the C3A content, the curing temperature, the particle size of the gypsum and the presence of chloride in the mix.

The effects of increased sulfate content (total SO3 2.26%5.26%) on the early compressive strength of concrete cubes at up to 56 days were examined and showed a definite reduction at levels of SO3 above 4.0% for two Portland cements (3.8% and 8.9% C3A, 2.34 and 2.76% SO3) and fly ash blended cement mortars.114 Similar data have been reported elsewhere.95

The variation in SO3 level between about 1.5% and 4.5% was found to have very limited influence on compressive strengths at 28 days or 1 year,93 but had a very marked effect on the creep of concretes; the higher the sulfate level, the smaller the creep strain.115 A fall in cement strength with increasing SO3 content in low-alkali clinker by up to about 10% has been reported.104 The decrease in strength is less than could be expected from the reduction in the amount of alite forming, suggesting that activation of the belite phase is occurring.

The amount of SO3 in clinker is generally low. Here SO3 is present predominantly in the form of alkali sulfates. A larger fraction of SO3 is added to cement in the form of calcium sulfate, either as gypsum, CaSO4 2H2O, or anhydrite, CaSO4. During cement grinding, a fraction of the gypsum may be decomposed to hemihydrate CaSO4 H2O. The main role of calcium sulfate is to control cement setting, although strength may also be affected.

Data on the effect of calcium sulfate on the progress of hydration and on cement strength are not uniform, though it is generally believed that optimum strength values are obtained at an optimum gypsum value, depending on the clinker quality, cement fineness and hydration time. A strength maximum occurred with increasing gypsum in the cement.116 This maximum shifted to higher SO3 values with increasing hydration time.

The effect of anhydrite added to ordinary Portland cement was found to form a strength maximum at about 5% CaSO4 (at room temperature).117 This maximum shifted to higher values at 65C. At optimum anhydrite addition, the early hydration of C3S was retarded and the degree of hydration at later ages increased. The development of strength was accelerated.

The optimum gypsum value may depend on clinker composition and fineness and also on the hydration temperature, increasing with increasing temperature. The strength optimum shifted to higher gypsum values with increasing hydration time.118 The effect on strength for six cement brands, manufactured in full-scale plants, of changes in SO3 contents was reported.93 The strength was usually independent of, or linearly related to, the SO3 content of the cement. At longer ages the association between strength and C3A content varied considerably with the content of SO3. The effect of gypsum on strength was studied on laboratory-made cements, with variable C3A/C4AF ratios.119 In all instances, the amount of SO3 had a considerable effect on strength; however, this effect and the optimum level of SO3 depended on the C3A/C4AF ratio. In addition to strength, the progress of hydration was affected as the amount of gypsum present.

Only slight differences in strength were found by varying the content of gypsum in both Portland and Portlandblastfurnace slag cements.32 At the same time,114 other data showed a distinct optimum shifting gradually to higher SO3 values if either hemihydrate or gypsum were used as set regulators.114

Soroka and Relis120 compared the strength development of compacts made from identical ground clinkers with and without gypsum additions (2.93% SO3). The effect of gypsum on strength was most beneficial after 1 day, and exhibited a minimum after 7 days. The degree of hydration was decreased except after 1 day. It was shown that the higher strength was associated neither with a higher degree of hydration nor with lower porosity, but with an improved quality of the hydrates formed.

It has also been found that the resultant compressive strength may be affected by the rate of sulfate dissolution, which depends on the kind of calcium sulfate used and its fineness. Differences in both 1 and 28 day strengths have been found after adding identical amounts of different forms of calcium sulfate (gypsum, natural anhydrite, hemihydrate or phosphogypsum) to the same clinker.121 Greater strengths have been reported by using a gypsum with a fineness 554 m2/kg as compared with one with 10,000 m2/kg.118

It appears that the effect of calcium sulfate on strength is a rather complex one. Even though calcium sulfate may affect the rate of hydration, it appears likely that the observed variations of strength are mainly due to an alteration of the binding properties of the hydrates formed.

The incorporation of calcium chloride in the raw material mixture for Portland clinker production by utilising molten salt technology, has enabled the temperature of clinker formation to be reduced by 400C500C. This clinker contains alinite, a structural variant of alite (tricalcium silicate) incorporating chloride ions.256,291,292 The quantitative content by weight of the mineral phases present in alinite clinker varies within the following limits: alinite 60%80%, belite (-dicalcium silicate) 10%30%, calcium chloroaluminate (Ca6A107Cl) 5%10%, dicalcium ferrite 2%10%. Weak CaCl bonds are developed which result in alinite clinker being softer than alite and requiring less energy for grinding. Gypsum addition is reported293 to intensify strength development rather than principally functioning as a regulator of set.

Alinite has also been produced by clinkering steel plant wastes such as fly ash from an in-house power generating plant, limestone fines, mill scale and magnesite dust with calcium chloride as a sintering aid at 1150C.294 The optimum calcium chloride addition to the raw mix was found to be 7%8% by weight. These cements have been found to be relatively insensitive to the various impurities in the raw mix and can tolerate higher levels of MgO than Portland cements. This low-temperature clinkering route offers scope for the conversion of industrial wastes into hydraulically setting cements. Alinite cement is compatible with Portland cement and additions of 20% by weight of fly ash can be satisfactorily accommodated. Alinite is stable in impure systems with different elements, but is unstable in the pure system CaOSiO2Al2O3CaCl2. Typical alinite clinker contains alinite (65%), belite (20%), mayenite (C11A7CaCl2; 10%) and C4AF (5%).295 Alinite was ascribed the formulation Ca21Mg[Si0.75A10.25O4]8O4C12. The presence of magnesia appears to be essential for alinite formation.296 Jasmundite [Ca22(SiO4)8O4S2], which has S2 instead of Cl ions in the crystal lattice, is poorly hydraulic.297 Later work showed alinite not to have a fixed composition and to be best represented as Ca10Mg1(x/2)x/2 [(SiO4)3+x(A1O4)3+x(AlO4)1xO2Cl] where 0.35

Calcium silicate sulfate chloride [Ca4(SiO4)(SO4)Cl2], a derivative of alinite having an orthorhombic structure,300 is formed at only ca. 600C800C. It has appreciable hydraulic activity,301 greater than that of belite. Compressive strengths of 25MPa at 28days have been found.301

The bulk of the oxide components passing through a cement kiln are essentially nonvolatile. However, some have appreciable vapour pressures at clinkering temperatures: this leads to volatilisationcondensation cycles which have important implications for clinker phase compositions and kiln operation. The most important volatile components are chlorine, SO3, which is reduced to SO2 + O2 at low O2 pressure and/or high temperature, and the alkalis which are reduced to alkali metal gas at low O2 pressure and/or high temperature. Chlorine is easily the most volatile followed by SO3, K2O and Na2O58. Excessive condensation of the volatiles in the cooler regions of the kiln and preheater cyclones cause blockages restricting the flow of the process air. Whilst chloride concentrations in the clinker are normally too low to affect the mineralogy, alkali sulfates play a crucial role in the clinker mineralogy and performance of the cement in general. Most major cement producers set guidelines for the molar ratio of SO3 to alkalis (or sulfatisation degree) which should ideally be as close to unity as possible. Excess SO3 over alkalis tends to stabilise belite requiring higher temperatures to form alite as explained further in the next section. Excess alkalis over SO3 are incorporated into the other phases, mainly the belite and C3A. This leads to the conversion of cubic C3A to orthorhombic C3A59 which is usually more hydraulically reactive and more difficult to control with gypsum addition to the cement. At sulfatisation degrees higher than 1.0, which nowadays is more common with the widespread use of sulfur-rich fuels (particularly petroleum coke), sulfate phases form as shown in Fig. 3.21 for the K2SO4CaSO4 binary system.60 The role played by Na2O is relatively minor, since concentrations are usually less than half the content of K2O, and about half of the total Na2O is incorporated in the other phases. What little remains forms a solid solution with K2SO4 (aphthitalite).61 Fig. 3.21 shows the path taken by a melt with an SO3/alkali molar ratio of 3.0 from the maximum burning temperature of 1450C as the clinker cools. Based on microscopic evidence and the fact that the sulfate is immiscible with the oxide melt62 this binary system can be used in isolation to describe the reactions on cooling. However, account needs to be taken of the SO3 and alkalis incorporated in the oxide phases. When the melt cools to the liquidus curve at about 1300C anhydrite begins to crystallise, and the melt follows the path shown as it is depleted in CaSO4 (F = 1); refer to Section 3.2.1 for further details on interpretation. When the melt cools to the peritectic temperature (F = 0, i.e. invariant) previously formed anhydrite now reacts with the melt to form Ca-langbeinite. In this example both anhydrite and the melt will fully react and Ca-langbeinite is the only phase to cool below the peritectic temperature. In practice, however, equilibrium cannot always be assumed, particularly with modern efficient coolers, where the range of temperature over which rapid cooling takes place includes the peritectic temperature. Fractional crystallisation can occur where anhydrite is effectively left stranded in a fine or even glassy K2SO4-rich matrix as shown in Fig. 3.22. Since anhydrite does not dissolve rapidly in water this can lead to problems with set regulation.

Fig. 3.22. Backscattered electron micrograph showing lath shaped crystal of anhydrite, which was the first sulfate phase to crystallise, in K2SO4 rich matrix that did not have time to fully react with anhydrite at the peritectic temperature.

Calcium sulfosilicate, C5S2S, which forms from condensation and oxidation of SO2 in the cooler regions of the kiln decomposes at around 1200C before the burning zone. However, it is sometimes found in Portland cement owing to flushes of unburned material through the kiln during unstable conditions. It is poorly hydraulic and can also lead to problems with set regulation from insufficient soluble sulfate in the cement.

The CaOAl2O3(C4AF)-SiO2SO3 system holds significant interest in relation to the synthesis of calcium sulfoaluminate (CSA) cement. This represents an important and potentially low-temperature cement with rapid setting and good strength development.63 Phase equilibria in part of the system CaOAl2O3SiO2Fe2O3MgOCaSO4K2SO4 were studied by Kaprlik et al.64 in relation to calcium sulfoaluminate clinker manufacture. Three seven-phase subsolidus (950C) assemblages were identified in the relevant part of this system as follows:

In oxidising conditions, Fe2O3 is taken into C4AF and small amounts of alkali can cause significant changes in the phase composition of the sulfate phases C4A3S and C5S2S. In more recent work on the CaOAl2O3Fe2O3SO3 system the phase boundaries and the path followed by the melt on heating and cooling were identified as well as the distribution of iron between the C4A3S and ferrite phases.65 Most of the work on the CSA cements has focused on combinations of belite and C4A3S. There has been less interest in alite+C4A3S combinations since C4A3S normally decomposes above about 1350C, and temperatures higher than this are normally needed for significant C3S formation to take place. An interesting approach to overcome this has been to re-burn high sulfate clinker at 1250C,66 but this has obvious limitations in an industrial process. A more cost effecting method can be to lower the temperature of alite formation through mineralisation67 as discussed in the next section.

Defined green features of sustainable construction materials can occur at all stages of the materials life cycle, from the point of being a raw material to the point of disposal of its wastes (Kim and Rigdon, 1998). Sustainable materials are normally produced from natural, local or recycled materials. The manufacturing process can reduce waste and prevent pollution because of recycling and advanced technologies. During construction and its performance, a product should be nontoxic and durable and should provide an energy-saving building process and result. At the end of its life, in terms of waste management, a green, sustainable material needs to be either biodegradable, or recyclable or reusable. The total energy consumption accumulated by a product during all stages of its life cycle, called embodied energy, should be as low as possible.

Embodied energy is the accumulative energy spent for a products total life cycle, from raw mining to disposal, and it is considered to be incorporated in the material itself. The estimation of a construction materials from cradle-to-gate embodied energy is an estimation of the energy spent from the time of its extraction until the time it reaches the factory gate. Because of the wide range of manufacturing methods and the complexity of calculations, transportation distances and other variables for some building products, values of embodied energy vary from study to study (BEDB, 2009).

Carbon footprint value, which is estimated along with embodied energy, relates to the accumulated greenhouse gases caused by a product during its life cycle. According to research results presented by the University of Bath (United Kingdom), the embodied energy of gypsum plaster is about 1.8MJ/kg, and its carbon footprint is 0.12kg CO2 per 1kg of product. This could be compatible with terrazzo tiles with corresponding values of 1.4MJ/kg and 0.12kg CO2 per 1kg of product. In addition, gypsum plasterboard has an embodied energy of 6.75MJ/kg and 0.38kg CO2 per 1kg of product, which is comparable to clay tile values of 6.5MJ/kg and 0.45kg CO2 per 1kg of product (Hammond and Jones, 2006).

The embodied energy and carbon footprint of gypsum products for walling assemblies in the United States along with other relevant materials according to the data of the Buildings Energy Data Book (BEDB, 2009) are shown in Fig. 25.26. In general, a 60-year building life time was estimated. As can be seen, gypsum board partitions with wood studs have the lowest values of these parameters when compared to the other products.

Direct reuse of gypsum products after demolition is unusual. Thus careful demolition of plasterboards and panels is needed in order to reuse them. Normally, recycling procedures are used for gypsum products, mostly plasterboards and wastes of gypsum manufacturing.

Disposal of gypsum products has become a serious issue because they still contain various substances such as organic matter (paper and wood), drywall (made mainly of lining paper and gypsum CaSO42H2O) and heavy metals. The wastes are regarded as the major contributor to hydrogen sulphide (H2S) generation in landfills (Musson et al., 2007). Wastes formed by the production of gypsum products can be recycled instantly at the point of processing. Lump wastes of gypsum concrete or gypsum plasters at large enterprises can be ground and returned to the manufacturing process. Gypsum dust accumulated in dust-collecting systems can be used as a setting regulator in the mixtures or can be added to gypsum binder during homogenisation (Ferronskaya, 2004).

Construction and demolition wastes require preliminary treatment (removal of impurities, etc.). There are developed technologies for distribution of gypsum and organic matters in demolition and construction wastes. Heavy liquid separation can be applied to determine the density range in the places of the highest concentration of gypsum and organic wastes. However, as heavy liquid separation requires water and hazardous chemicals to be applied, is more advantageous (Montero et al., 2010).

Different kinds of equipment are used for waste processing, such as wasteboard crushers, which are developed especially for recycling rejected gypsum boards and their wet and dry wastes (Gypsum Technik, 2014). Gypsum Recycling International, a mother company for the operating units in the Gypsum Recycling Group, has developed mobile gypsum recycling units. Trailer-based and transported by ordinary trucks, these units can recycle 100,000tonnes per year. The units recycle both new gypsum waste and waste from reconstruction and demolition (Lund-Nielsen, 2007). According to calculations of Gypsum Recycling International, 200kg of CO2 is saved for each recycled tonne of waste. One hundred percent of paper recycling is achieved for gypsum paper recycling. Developed in Denmark in 2001, such recycling activity has spread to other European countries, North America and Asia (Lund-Nielsen, 2007).

Other recycling technologies exist, and they deal mostly with demolition wastes, recycled at plasterboard manufacturing lines. Yoshino Gypsum Co., Ltd. (Japan) uses by-product gypsum (50%), imported raw gypsum (45%) and collected waste plasterboards (5%) for domestic plasterboard production; 100% of recycled paper is applied for production (Environmentally-Friendly Gypsum Products, n/a). The Australian company ReGyp Pty Ltd suggests solutions not only for manufacturing construction products but also for agriculture. Ninety-five percent of recycled gypsum dehydrate are applied soil addings (http://www.regyp.com.au). The level of application of recycled and synthetic gypsum varies from country to country. In the United States in 2014, more than four million tonnes of gypsum scrap generated by wallboard manufacturing, wallboard installation and building demolition were recycled. The recycled gypsum was used primarily for agricultural purposes and as feedstock for the manufacture of new wallboard. Other potential markets for recycled gypsum include athletic field marking, cement production as a stucco additive, grease absorption, sludge drying and water treatment. In 2014 in the United States, synthetic gypsum accounted for approximately 50% of the total domestic gypsum supply (MCS, 2015). In the United Kingdom, 38% of imported gypsum stone, 51.6% of FGD gypsum (one-third of it imported), 5.6% of titanogypsum and 4.8% of recycled gypsum are used for plasterboard production (ELCAP, 2010).

Use of gypsum wastes and synthetic gypsum is vital for the countries where natural gypsum deposits are rather rare (Japan, South Korea, New Zealand and Australia, etc.). Share of FGD gypsum in European Union plasterboard production has reached 60% in 2010, and it was close to zero in the middle of 1970s (Lund-Nielsen, 2010). The increasing demand to meet the standards of sustainable construction (LEED in the United States, BREAM in the United Kingdom, etc.) is promoting green technologies related to the manufacturing, use and disposal of gypsum products.

According to Lund-Nielsen (2008) the reasons plasterboard enterprises want to become involved in recycling schemes vary depending on the region (Table 25.6). As can be concluded, the limited resources of natural gypsum as well as the higher cost of disposing and transporting it are dominant factors.

Life-cycle assessments (LCAs) of gypsum products are generally considered to be from cradle-to-gate. The use and disposal stages are excluded from the assessments. Numerous data have been collected for different gypsum products, but mostly for wallboard manufacture. The LCA of the most commonly recycled gypsum product, gypsum board, is based on case-study research conducted by the order of the International Gypsum Association (EPD NAGB, 2014; LCAS, 2013). LCAs have been developed according to ISO 14025 and 21930 principles and requirements.

Calculations were conducted for 1000sq ft (1007kg) of 5/8 type X gypsum board at the facilitys gate. The calculation included 2117.66lbs (960.52kg) of gypsum material consisting of natural gypsum ore (40.5%), FGD synthetic gypsum (57.5%) and post-consumer gypsum (2%). Gypsum paper (both facing and backing) consumption was 84.6lbs (38.37kg). Additives were used: 1348.6lbs (611.71kg) of starch, vermiculite, fibreglass, dispersant, retarder, potassium sulphate, dextrose, clay (kaolin), boric acid, land plaster, foaming agent (soap), BM accelerator, ammonium sulphate, edge paste, shredded paper and water. The Cradle-to-Gate Gypsum Board Manufacturing System is shown in Fig. 25.27. Basic LCA results are shown in Table 25.7.

Data from EPD NAGB, 2014. Environmental Product Declaration. Typical (5/8 Type X) North American Gypsum Boards. Declaration number: FPI/GA/01/2014 Issued May 2014 Valid until May 2019. Available at: http://www.gypsum.org/wp/wp-content/uploads/2013/12/Gypsum-2014-FINAL-May-13-.pdf.

Fig. 25.28 shows the relative contributions from the raw material input supply, input transport to a manufacturing facility and gypsum board manufacturing, showing their dominance in the impact categories. Gypsum board manufacturing has a much greater influence on the environment than do raw supply and transportation, particularly on global warming, acidification, ozone depletion and abiotic depletion potentials.

The Environmental Product declaration prepared by GIPS Bundesverband der Gipsindustrie e.V. (Germany) suggests a comparative LCA analysis of different gypsum- based products (GBP EPD, 2009). The LCA was conducted in compliance with DIN ISO 14040. Primary energy consumption is shown in Fig. 25.29. For each product, the nonrenewable energy is more than 90% of the total energy consumption.

According to this document, the highest values of global warming potential and acidification potential among all ready-to-use products result from gypsum fibreboards (0.308kgCO2eq./kg and 0.39E03SO2eq./kg correspondingly) and fibreboard flooring elements (0.303kgCO2eq./kg and 0.38E03SO2eq./kg correspondingly). The highest values of eutrophication potential are peculiar to all of the plasterboards: for perforated plasterboard, 8.21E05kgPO4eq./kg; for fire-retardant plasterboard, 8.05E05kgPO4eq./kg; for precast screed plasterboard, 7.66E05kgPO4eq./kg.

The products with the greatest potential for creating ozone are gypsum perforated (4.01E05kgC2H4eq./kg) and fire-retardant plasterboard (3.87E05kgC2H4eq./kg), fibreboards for walling (3.7E05kgC2H4eq./kg) and flooring (3.76E05kgC2H4eq./kg). Of all the considered parameters, the products with the lowest potential are those with supplementary jointing gypsum materials and adhesive binders. Therefore, the potential for global warming is equal to 0.108kgCO2eq./kg; for photochemical ozone depletion, the potential is 1.33E05kgeth4eq./kg, and a acidification potential of 0.14SO2eq./kg; and for the eutrophication, the potential is 1.55E05kgPO4eq./kg (GBP EPD, 2009).

The areas for optimisation of quality in the cement mill relate to the processes and chemical reactions which occur within the mill. The primary function of a cement mill is to reduce the size of the clinker from being measured in millimetres to being measured in a few microns (a micron being 1/1000th of a millimetre). Cement milling comprises two stages, crushing the large clinker grains down to a powder, then grinding the powder to the size required. In some instances these processes are separated to the extent that the crushing stage is accomplished in a roller press and the grinding in a small finish mill. In most cases, however, the two are combined within a single mill, either as two chambers separated by a diaphragm in a ball mill or as the feed travels from the centre to the edge of the grinding table of a vertical spindle mill.

The mill operator judges when the product is ready to collect from the mill by the extent to which the size has been reduced. Historically this has been measured by the surface area of the cement. As a clinker nodule is cracked in half then the total surface of the two halves is greater than that of the original nodule because a new surface has been created on the fracture surface of each half. Size reduction is therefore also a process of increasing the surface of the clinker presented to the water when the cement is used in practice. Because the water can only act initially on the surface of each grain to form the hydration products which give strength, the more surface there is, the stronger the cement will be at a given early age until the hydration front has moved through each grain.

In an open circuit cement mill the whole of the feed needs to remain in the mill until the product has reached the desired fineness. Because some parts of the cement will grind more easily than others there will be some of the material which is ground to an exceedingly fine powder before the coarser grains are sufficiently fine to give an average surface area as required. The throughput of the mill cannot be increased until all the material is on average within specification.

For this reason closed circuit mills have been developed in which the finer material is removed by a separator while the coarser grains are returned to the mill for another circuit. This can produce a cement with the same surface area as the open circuit mill but without the very fine material and without the coarser material. While this makes for a very much more efficient system of milling cement, it means that the very fine material which had the higher surface area and therefore reacted faster is absent. The even size of the grains also has implications for the use of the cement in concrete. Fig. 4.36 shows in diagrammatic form a theoretical cement with the ultimate tight particle size distribution as all the grains are the same size and shape. The result of this is that for the grains at the top left of the field there is a lot of free space between the grains which will need to be filled with water to give workability to a concrete mix. The two most important features of a cement for concrete manufacturers are water demand and workability. Closed circuit mills therefore provided a challenge to the users of cement in concrete.

To counter this loss of workability another very fine material is used to fill in the spaces, as in the cluster of grains at the bottom right of the field in Fig. 4.36 and the most common material to be added is limestone. Most standards now allow up to 5% limestone in the basic Portland cement. Limestone grinds more easily than clinker and therefore, together with gypsum addition which also grinds more easily, provides a very fine material to fill in the spaces. The high surface area material then is no longer reactive clinker and many plants control the fineness of the product by the residue on a 45 m sieve, which is predominantly clinker, instead of the surface area.

Sulfate (SO3) is used to control the way in which cement sets. Several forms of sulfate are found in cement. Within the cement clinker there may be alkali sulfate (various forms of sodium and potassium sulfate) or calcium sulfate, or a mixed sulfate with calcium and potassium known as calcium langbeinite. To this is added more SO3 at the cement mill usually in the form of gypsum or anhydrite or a combination of both. Historically natural gypsum rock has been used as this source but it is becoming more common to utilise sulfate sources formed as a result of other industrial processes and frequently called synthetic gypsums. While natural gypsum as used in a cement mill is usually a combination of CaSO42H2O and CaSO4 (gypsum and anhydrite respectively), synthetic alternatives may consist of a slurry of CaSO42H2O, a dry plasterboard product (also CaSO42H2O) or a powder or cake of anhydrite. An appropriate mix of gypsum and anhydrite is necessary because the CaSO42H2O will dehydrate to some extent in the heat of the cement mill to form hemi-hydrate, also known as plaster of Paris. This will cause a false (plaster) set if present in excessive amounts, but a small amount enhances the solution of SO3 which is necessary to prevent the flash set of C3A from the cement clinker. The reactions of sulfate with the clinker phases are discussed in Section 4.12.

Because of the complexity of the various types of sulfate present and the effects of the milling temperature on the changes which may occur, there are three aspects relating to SO3 which can be varied at the finish milling stage to affect the quality of the cement. The first is the type of sulfate added. Frequently it has been found that a blend of half gypsum and half anhydrite is suitable. The second is the overall quantity of SO3 present in the cement. The optimum SO3 can most easily be found by milling for a short time with a low sulfate addition, taking samples then increasing the addition gradually over the period of a day, taking regular samples. The SO3 can then be measured at each stage and compared with the standard test results on the cement, particularly setting times and strengths (concrete as well as mortar). Plotting the SO3 against each parameter will provide evidence of the optimum level for each parameter. Very often the optimum will not be the same for all parameters, so the most important in a given market needs to be optimised.

The third control at the mill is the material exit temperature. When gypsum is above 105C it will be starting to dehydrate to hemi-hydrate. Above 110C this process will be accelerated and at higher temperatures still there will be little CaSO42H2O left in the cement. Efficient cooling of the cement mill is essential to prevent excessive hemi-hydrate formation and variable setting times and workability.

Increasingly cements are being made with components other than clinker, gypsum and a minor mineral addition. The European cement standard EN 1971 lists 27 types of cement with varying quantities of fly ash, ground granulated blastfurnace slag, limestone, silica fume, burnt shale or pozzolana.

While the general properties of these additions are well documented, the details are frequently less clear and the optimum for a particular blend of materials in a particular cement mill is sometimes a surprise. Fig. 4.37 shows the results of a laboratory trial with limestone additions in excess of the 5% allowed in most standards as a minor addition but not as high as is usually used in Limestone Cement. In the standard mortar strength test to fixed water content, even without the advantages of water reduction referred to above, the limestone gives an advantage in strength at early age with 12% limestone giving a 3.5 N/mm2 higher than the control at 7 days. The improvement is probably due to the extra surface area generated by the fine limestone which provides extra sites for precipitation of the cement hydrates. The 28 day strength was maximised at 4% limestone but even at 12% had just returned to the control level.

When using pozzolanic additions or latent hydraulic binders the situation becomes more complex because there is also an element of strength provided by the additions. In the United States, ACI 21170 recommends the use of optimum ash curves to determine the most appropriate addition rate. Whichever method is used and whatever the material, the only sure way to find out the optimum quality combination is to carry out trials.

The term, Grinding aids today is taken to include a variety of small volume additions to the cement mill to improve quality of the cement as well as to improve the throughput of the mill. The properties of the additives include: admixtures such as water reducers, air entrainers, strength enhancers (see Chapter 15) and of course grinding aids.

In principle the object of the quality manager is to produce the quality at the front end of the process and having been trained to make clinker to the best quality it is sometimes difficult to view grinding aids with an open mind. However, in practice they have a place in modern cement making. This became apparent with the EN 1971 restrictions not only on low strength cement but on high strength within certain strength bands. In a competitive market the cement as tested in mortar prisms to a fixed water content needed to be within the maximum allowed in the band, however in concrete it needed to be as strong as possible. At some plants the addition of an air entrainer was able to reduce mortar strengths due to decrease in density, but to improve concrete performance due to the improvement in workability and reduced water demand.

From there it was easier to accept the addition of water reducers, common practice already at the concrete plant, as well as strength enhancers, to improve the quality of cements. Today the use of additives is very widespread and has led to the offering of a more consistent product to the market.

Gypsum is a common building material which is widely used in building construction work. In addition, gypsum is usually found in partition walls and it is always use in the interior side of a wall as a cladding element [54]. This guarantees the used of most of the thermal inertia when PCMs are integrated. Therefore, gypsum is preferred to be used as a medium for PCM applications in building. Furthermore, the PCMs-gypsum composite products are easily produced and could be easily marketed by existing manufacturers. The products also could be easily installed in existing and renovated buildings or buildings under construction as well as light weight buildings. This encourages comprehensive on the fabrication and thermal characterization of PCM-gypsum composite boards.

Shossig et al. [58] investigated the incorporation of microencapsulated paraffin into gypsum wallboard. The microcapsule system prevents the interaction between the building matrix material and the paraffin. The microcapsules could also prevent leakage of paraffin during the lifetime of the building element. In addition, microencapsulation of PCMs could solve the problems of immersion technique or macro-capsules mentioned above. Fig. 4 shows the SEM micrograph of microencapsulated paraffin with an average diameter of 8m in gypsum wallboard. The microencapsulated PCM are homogeneously dispersed between the gypsum crystals. The reduction of the indoor temperature fluctuations of approximately 2C was found compared with the walls without microencapsulated PCM.

Su et al. [68] developed microencapsulated-paraffin/gypsum composite board for TES in building application. The paraffin was microencapsulated using in-situ polymerization method. The paraffin was used as PCM and methanol-modified melamine-formaldehyde was used as a shell. The microencapsulated-paraffin/gypsum composite was prepared by mixing the gypsum powder with the microencapsulated paraffin. The thickness of the microencapsulate-paraffin/gypsum composite board was 1.0cm. The results indicate that the microencapsulated paraffin/gypsum composite board has thermo-regulated ability and can store the time-dependent and intermittent solar energy. The results also shows that the microencapsulated paraffin/gypsum composite board has the ability to shift heating or cooling load from peak to off peak electricity periods, which is helpful for reducing the electricity tariff.

Oliver, [54] studied the gypsum board with 45% PCMs. They used commercial microencapsulated PCM (Micronal DS 5001X, melting temperature of 26C, latent heat of fusion of 110kJ/kg). They concluded that the gypsum board with 45% PCMs can stores 5 times more energy per unit mass than a thermal brick, 9.5 times more than a brick wall, and almost 3 times more energy per unit mass than a common gypsum board.

Toppi and Mazzarella [69] fabricated smart gypsum wallboards containing commercial microencapsulated PCM product (MicronalPCM). They found that small amount of microcapsules in the gypsum wallboards presents a big effect on the thermoregulating properties of the gypsum.

Based on the above findings shows that microencapsulated PCMs can be used to increase the TES capacity of building materials while keeping the insulating properties but tend to be costly [70,71] and can adversely affect structure integrity [72].

The effect of FGDG on the uptake/enrichment of toxic trace elements (As, B, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, Zn) in various crops have been investigated with the safe uses of crop produced for human consumption [19,109]. Concentrations of toxic trace elements added to land, regardless of the source, should be monitored and be within established standard limits. The changes in concentrations of trace elements found in plants are listed in Table 6.

Note: represents increasing the accumulation of the toxic trace elment in plant; represents decreasing the accumulation of the toxic trace elment in plant; N represents FGD gypsum having no effect on the accumulation of the toxic trace elment in plant; represents making no analysis on the accumulation of the toxic trace elment in plant.

Note: represents increasing the accumulation of the toxic trace elment in plant; represents decreasing the accumulation of the toxic trace elment in plant; N represents FGD gypsum having no effect on the accumulation of the toxic trace elment in plant; represents making no analysis on the accumulation of the toxic trace elment in plant.

FGDG with a large amount of available Si may increase Si content in roots and reduce the root absorption of heavy metals. It was reported that the deposition of Si in the vicinity of the root endodermis might partially physically block the apoplast bypass flow across the roots [110]. The heavy metal sequestration and detoxification are attributed to the chemical and physical effects of Si on forming co-precipitation with heavy metals and blocking the metal transfer in plants [111]. Concentrations of B, Se and As did not accumulate significantly in all of the plant species tested (cotton, corn, soybean and radish crops) in a greenhouse-based pot study and a large scale mesocosm experiment [112,113]. No significant difference in the concentrations of Mo, Zn, Pb, Ni, Cd, Mn, Cr, and Cu in corn leaves was observed between the FGDG addition and the control [40]. The contents of almost all of the metals (B, Cr, Mn, Ni, Cu, As, Cd, Pb) in the corn grains in the FGDG plots were almost the same or lower than those in the control plot [39]. The concentrations of Cr, Pb, Cd, As, and Hg in the seeds of corn and alfalfa grown in China were far below the tolerance limits regulated by National Food Standards of China [24]. After applying FGDG, the concentrations of Cd, Cr, Cu, Pb, and As in sweet potato were reduced by 31.34%, 70.57%, 22.17%, 79.49%, and 100%, respectively [105]. Tissue Mn, Cu, Ni, and Zn concentrations decreased from near phytotoxic levels to normal levels after applying FGDG in mined soils [114].

However, some studies also showed that the application of FGDG could increase the accumulation of toxic metals, and using FGDG as a soil amendment for the purpose of growing agronomic and horticultural crops was not recommended. The elemental concentration of the maize tissues indicated a characteristic elevation of B, As, Se and Mo [105]. Alfalfa yields were unaffected by these rates of residue applications, but shoot concentrations of B in the second cutting and B, Mo in the third cutting were increased by FGDG applications [115]. Cheng et al. [116] evaluated the environmental impact associated with the land application of FGDG by considering the overall mass balance of an element in a life cycle starting from the coal combustion process to the stage of land application. The author believed that considering only the amounts of trace element uptake in surface vegetation may underestimate the overall release of the trace elements from FGDG and that the mobility of trace elements varies when FGDG produced from different processes were used.

Although FGDG has become an effective soil amendment for soil reclamation, it might deposit extra heavy metals into the soil environment. Some studies showed that FGDG does not contain significant quantities of heavy metals and that the application of FGDG does not cause soil contamination [117]. Briggs et al. [118] reported that total Hg concentrations measured were similar for reclaimed and unclaimed soils after FGDG application. Chen et al. [26] reported that As and Hg concentrations in the soils were found to be positively correlated with FGDG added. Yang et al. [119] conducted a soil column leaching experiment to analyze the changes in heavy metals from reclaimed tidal flat soil that used FGDG and found that the highest removal rates of Cd and Pb in the upper soil layers (030cm) were 52.7% and 30.5%, respectively. The application of FGDG (two times) and the extension of the leaching interval time to 20 days increased the heavy metal removal rate in the upper soil layers. The heavy metals desorbed from the upper soil layers were re-adsorbed and fixed in the 3070cm soil layers [119]. Although there might be a slight increase in heavy metals, the concentrations of Cr, Pb, Cd, As, and Hg in the treated soils are far below the background values stipulated by the Environmental Quality Standard for Soils (GB 15618-1995) [27,120]. Chen et al. [45] reported that No soil contamination problems were observed even at high application FGDG rates and that it can be safely applied to agricultural soils. Chun et al. [40] also reported that no significant differences in the concentrations of Mo, Zn, Pb, Ni, Cd, Mn, Cr, Cu, and Al in sodic soils were observed between the FGDG addition and the control. Chen et al. [48] found that compared to the conventional reclamation, Ca, S, B, and Zn concentrations at 020cm depth were generally increased by the treatments with FGDG at a 280Mgha1 application rate in an abandoned surface coal mine in Ohio, while other trace metals measured were generally not increased in short or long term measurements.

As FGDG brings significant amounts of Ca2+ and SO42 to the soil, it could affect the Pb sorption by Ferrihydrite and humic acid in soil. Therefore, FGDG has the potential use as an in situ Pb stabilizer in contaminated soils. The previous study showed that FGDG effectively reduced the Pb leaching in two contaminated soils under leaching conditions [121].

There are growing concerns regarding the effects of toxic trace elements on water quality when FGDG is applied to agricultural fields. Applying FGDG into acid forest soils has the potential to provide growth benefits to a commercially important tree species (red oak), but care would need to be taken to avoid the release of toxic levels of B. Crews and Dick [9] reported that S concentration increased from less than 10mgl1 to 234mgl1 in the leachate in four months after the application of FGDG and that B also approached toxicity concentrations ( l mg l1) in the leachate from the soil treated during the initial leaching but that the concentrations tended to decline with time. Punshon et al. [113] conducted a greenhouse-based pot study and a large scale mesocosm experiment and found that leachate pH was unaffected by FGDG but that salinity rose sharply with increasing application rates of FGDG. Moreover, leachates contained higher concentrations of B with small increases in Se and As. While concerns about heavy metal leaching are not without basis, concentrations of B, Se and As do not exceed drinking water standards [112,122].

For the effects on surface water, Chen et al. [76] reported that the concentrations of Ca, S and B in surface runoff generally increased by the treatments with FGDG in both short- and long-term measurements in a field plot experiment and that the concentrations of the trace elements were generally not statistically increased in surface runoff over the 20-year period. Stehouwer et al. [49] reported that leachate pH, EC, dissolved organic C, Ca, Mg, and S tended to increase with the increasing application rate of FGDG in a leaching experiment of mine soils, and with FGDG of 120gkg1 or less, leachate concentrations of most elements of environmental concern were less than the levels mandated by drinking water standards. The amount of FGDG that can be applied to mined soils is probably limited by soluble salts and initially high pH levels rather than by the trace element loading of soil or water [49]. Jenkins et al. [123] reported that FGDG applications may be a management practice that reduces microbial contamination of surface waters from manure applied to agricultural fields in the southeastern United States. Salmonella was not detected in the runoff, and after 3 years of FGDG applications, the highest rate of FGDG resulted in decreased flow-weighted concentrations and total loads of Escherichia coli. Torbert and Watts [124] carried out a rainfall simulation to examine the impact of FGDG application on runoff nutrient losses and found that a maximum of 61% reduction in soluble reactive P concentration in runoff with the application of 8.9Mgha1 FGDG and the concentrations of heavy metals in runoff were all found to be below detection limits.