Experimental Investigation and Modeling of Sulfoaluminate Cement

Article pubs.acs.org/IECR

Experimental Investigation and Modeling of Sulfoaluminate Cement Preparation Using Desulfurization Gypsum and Red Mud Wenlong Wang,*,1 Xujiang Wang,1 Jianping Zhu,2 Peng Wang,1 and Chunyuan Ma1 1 2

National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, China 250061 School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China 454000 ABSTRACT: Desulfurization gypsum, the byproduct from wet flue gas desulfurization, and red mud, from the production of aluminum oxide, are two bulk industrial solid wastes that trigger many local environmental problems in China. This study aims to jointly utilize them. Through experimentation and modeling using FactSage, it has been found to be feasible to prepare sulfoaluminate cement using these wastes. The calcination temperature in the preparation was as low as 1250−1300 °C, and the main mineral phases of the cement clinker were 3CaO·3Al2O3, CaSO4, β-2CaO·SiO2, and 2CaO·Fe2O3. The cement clinkers tested showed excellent mechanical strength performances. This process was found to be an efficient way to consume industrial solid wastes, with the total proportion of desulfurization gypsum and red mud over 70−90% by mass in the raw materials. The sulfoaluminate cement products have outstanding cost superiority over Portland cement because of their low material costs, low material pretreatment costs, and low calcination temperature. Moreover, this technology could bring about immense environmental and social benefits in terms of waste consumption, energy conservation, and CO2 reductions. This technology has considerable prospects, and it is worth undertaking further research into its potential applications.

1. INTRODUCTION Wet limestone-gypsum flue gas desulfurization (FGD) technology is the predominant SO2 removal technology in power plants because of its high efficiency and reliability.1 However, a large quantity of desulfurization gypsum is generated, which is becoming a new environmental problem. For example, according to the installed capacity of desulfurization systems, over 100 million tons of FGD gypsum have been generated since 2010 in China. There have been many attempts to utilize this waste product, for example, as a soil amendment, as a building material, or as an additive in cement production.2−7 However, so much of this byproduct is produced that much of it is being stored, which uses up valuable land and causes environmental pollution. Hence, there is an urgent need to research and develop new technologies for the large-scale utilization of this material. Red mud, from alumina plants, is another industrial waste that is generated on a massive scale. Generally, 0.8−1.5 tons of red mud is generated per ton of alumina produced.8 In 2011, over 50 million tons of this waste was created in China. The disposal of red mud tailings costs the industry about US$3 per ton of alumina production.9 Furthermore, the enormous quantity of red mud takes up large amounts of valuable land and poses a very serious pollution risk. In recent years, many researchers have investigated the utilization of red mud, and a considerable amount of research has been carried out all around the world. So far, red mud has mostly been used in building materials, as a catalyst carrier, and in cement materials, among others.9−14 Use in the recovery of metals, such as Fe, Sc, and Ti, is also a potentially attractive use for red mud.15,16 In recent years, red mud has been investigated as an adsorbent for gas cleansing and wastewater treatment.17,18 Nevertheless, the consumption capacity of these methods is very limited because of the high alkalinity, fineness, and high water content of red © 2013 American Chemical Society

mud. Thus far, the total utilization rate has not been more than 15%, which means that innovative utilization methods are urgently needed. Dealkalization is the key step in the large-scale utilization of red mud. Because of the promotion of stricter environmental policies and the requirement for sustainable development, some large alumina enterprises began to boost the dealkalization of red mud to try to obtain a balance between environmental costs and economic benefits. For instance, an alumina plant in Shandong Province, China, has already built one process line to dealkalize red mud on a large scale. The hydrothermal method, using lime to remove Na2O, is the conventional dealkalization method for red mud.19,20 The plant uses carbide slag instead of lime. After dealkalization, the red mud is rich in Ca, Si, Al, and Fe and is therefore suitable for cement production. If the dealkalized red mud is used as a raw material in Portland cement, then the overall proportion of red mud has to be very limited because the high Fe content means that the mixture has to be modified with Si and Ca. However, sulfoaluminate cement has no strict requirement for iron content. The contents of dealkalized red mud, combined with FGD gypsum, almost meet the chemical requirements of this special cement. Their joint use could trigger the large-scale utilization of these two industrial wastes. It could also bring great social benefits in terms of energy conservation and CO2 emissions. Previous researchers have prepared different types of cements with original red mud and demonstrated that doing so is possible, but the red mud proportion was limited, and the effects of Na2O were neglected.21,22 So far, no research has Received: Revised: Accepted: Published: 1261

May 25, 2012 October 4, 2012 January 2, 2013 January 2, 2013 dx.doi.org/10.1021/ie301364c | Ind. Eng. Chem. Res. 2013, 52, 1261−1266

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Composition of the Raw Materials (Mass %) sample

CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

TiO2

loss

total

FGD gypsum red mud bauxite limestone

41.55 32.94 2.27 53.52

2.86 12.92 16.58 1.49

1.8 12.17 57.08 0.56

0.47 11.66 4.8 1.01

− 0.42 0.22 0.04

51.02 0.12 − −

− 2.74 3.05 −

2.09 26.28 14.23 43.21

99.72 99.25 98.23 99.83

ambient temperature to 900 °C, hold for 30 min at 900 °C, heat to 1300 °C, hold again for 30 min, and finally cool the samples using a fan. Following this treatment, three cement clinker samples were obtained. An ARL Quant’x X-ray fluorescence (XRF) spectrometer was used first to determine the chemical compositions of the clinker samples. The crystalline phases of the cement clinkers were analyzed by powder X-ray diffraction using a D/MAX2500 V diffractometer at a reflection angle range of 2θ = 10−80°. After being ground, the clinkers were formed into 20 mm × 20 mm × 20 mm cubic blocks, with a water/cement ratio of 0.35 and a gypsum/cement ratio of 0.05. The cubic blocks were cured in a moist cabinet at 96% humidity and 21.2 °C for 24 h and then demolded and placed in an isothermal curing water bath at 21.2 °C until the desired testing ages of 3, 7, and 28 days were reached. Mechanical tests were carried out to determine the compressive strength and to help judge the performance of the cement products.

been reported for the preparation of cement with dealkalized red mud as the major material. In this work, bench-scale experiments to prepare sulfoaluminate cement clinker were carried out using dealkalized red mud and FGD gypsum as the main materials. Mineral formation was modeled through the thermodynamical software package FactSage.23 The main objective was to confirm the feasibility of the above concept and thus enable the disposal of the two wastes on a large scale.

2. EXPERIMENTS Sulfoaluminate cement clinker is usually prepared by calcinating mixtures of limestone, gypsum, and bauxite at 1300−1350 °C with dicalcium silicate (Ca2SiO4 or 2CaO·SiO2) and calcium sulfoaluminate (Ca4Al6O12SO4 or 3CaO·3Al2O3·CaSO4) as the main mineral phases.24 In this study, the FGD gypsum acted as the source of sulfur instead of natural gypsum, and the dealkalized red mud provided the necessary silica, aluminum, ferrum, and most of the calcium. Any calcium deficit was made up using limestone, and the aluminum content was modified by the addition of bauxite. 2.1. Materials. Dealkalized red mud samples were obtained from an alumina plant in Shandong Province, China. FGD gypsum was obtained from a local power plant equipped with a limestone-gypsum desulfurization process. Bauxite and limestone were supplied by a sulfoaluminate cement plant in Henan Province, China. The chemical compositions of the raw materials are listed in Table 1. The raw material proportions present in the three batches tested are shown in Table 2. The results for each batch were

3. RESULTS AND DISCUSSION 3.1. XRF and XRD Results. Table 3 reports the XRF analysis results for the clinker samples. As expected, increasing Table 3. Chemical Compositions of Clinker Samples by XRF (Mass %)

Table 2. Proportioning of Raw Materials for Typical Samples (Mass %) batch

FGD gypsum

red mud

bauxite

limestone

1 2 3

12.12 15.14 15.18

78.61 63.84 55.67

0 6.38 11.14

9.27 14.64 18.01

cement clinker sample

SiO2

Al2O3

Fe2O3

CaO

TiO2

SO3

1 2 3

13.95 13.23 13.01

12.73 14.59 16.79

13.26 11.19 9.87

48.31 47.32 46.63

3.28 3.12 2.92

7.17 8.37 8.24

the amount of bauxite in the raw material batches caused the Al2O3 content in the clinker samples to increase gradually from sample 1 to sample 3. Although the Fe2O3 content decreased gradually, it remained fairly high in all of the clinkers compared to that of common Portland cement or sulfoaluminate cement. It is important not to overlook the TiO2 content, as it will consume a certain amount of CaO. The XRD patterns of the sulfoaluminate cement clinker samples are given in Figures 1−3. As can be seen, calcium sulfoaluminate (A), β-dicalcium silicate (B), and calcium ferrite (C) are clearly the dominant crystalline minerals in all of the diffractograms. Some minor phases, including calcium titanate (D), calcium sulfate (E), and gehlenite (F), can also be distinguished. The main mineral structures show that all three samples are similar to sulfoaluminate cement, in which calcium sulfoaluminate and dicalcium silicate are usually dominant. In view of the high calcium ferrite content, the three clinkers should fall into the high-iron sulfoaluminate cement category. The iron took the form of 2CaO·Fe2O3, not calcium iron aluminum oxides, which indicates that Fe2O3 does not entirely compete with CaSO4 in grabbing Al2O3 during the solid-phase reactions. In reality, 2CaO·Fe2O3 forms easily in the sintering of iron ore.25,26

what would generally be expected for each raw material and batch treatment. Batch 1 was mainly made up of red mud and FGD gypsum, whereas batches 2 and 3 included different amounts of bauxite to increase the Al2O3 content. In each batch, limestone was employed to adjust the proportions of CaO by mass. It can be seen that the proportions of red mud were considerable in all of the batches. 2.2. Experimental Methods. The raw material mixtures were blended in a ball mill, with the final particle size being less than 180 mesh. Because both red mud and FGD gypsum were in the form of fine powder, the grinding process was very short and easy. Each raw material mixture was compressed in a steel mold at a pressure of 30 MPa to obtain samples with dimensions of 50 cm ×50 cm × 1.0 cm. The samples were then dried in an air oven at 90 °C for 4 h and subsequently calcined in an electrically heated furnace. The heating rate was 5 °C/ min, and the firing cycle was programmed as follows: heat from 1262

dx.doi.org/10.1021/ie301364c | Ind. Eng. Chem. Res. 2013, 52, 1261−1266


Article

slightly too low. This was beneficial to the clinker because, in these experiments, even a slight excess of CaO might have caused the formation of 12CaO·7Al2O3, which can make the cement set too rapidly. In the proportions used in this study, CaSO4 was added in excess to guarantee sufficient formation of calcium sulfoaluminate. Its surplus and the sufficient formation of 3CaO·3Al2O3·CaSO4 in the diffractograms meant that adding CaSO4 in excess might have been unnecessary. Because the short-term strength of sulfoaluminate cement is mainly attributed to the contribution of calcium sulfoaluminate, the content of this material in the clinker is very important to the quality of the cement. By comparing the three diffractograms, it can be seen that the intensity sequence for the peaks of calcium sulfoaluminate (A) was clinker 3 > clinker 2 > clinker 1, which mirrored the corresponding decrease in the mineral content of the three samples. In particular, the peaks for 3CaO·3Al2O3·CaSO4 were strongest in sample 3. This change can be attributed to the different Al2O3 contents in the raw material batches, as the most bauxite was added to batch 3 whereas no bauxite was added to batch 1. Therefore, the addition of bauxite had an important effect on the mineral components of the cement clinkers and could be an effective means of changing the fraction of the hydraulic mineral, 3CaO·3Al2O3·CaSO4.27 3.2. Mechanical Strength Performance. Table 4 presents the compressive strength results for clinker samples

Figure 1. XRD pattern of cement clinker sample 1.

Table 4. Results of Mechanical Strength Tests of Clinker Samples 1−3 compressive strength (MPa) sample

added agent

water/cement ratio

3 days

7 days

28 days

1 2 3

5% gypsum 5% gypsum 5% gypsum

0.35 0.35 0.35

32.7 45.6 53.9

36.8 55.1 56.2

41.9 54.8 61.3


1−3 after different lengths of curing. All of the clinkers showed very good strength values, with noticeable increases from 3 days to 7 days and then to 28 days. The good early strength performance is a feature of sulfoaluminate cement, whereas the increase in the later strength is similar to that of Portland cement. Hence, the clinkers obtained in these experiments have the combined features of these two types of cement. The strength characteristics can be attributed to the type of mineral components in the samples. Calcium sulfoaluminate and dicalcium silicate are both important cement minerals with good hydraulicity properties. Calcium sulfoaluminate has a high hydration rate and is characterized by its excellent early strength. Dicalcium silicate has a much lower hydration rate, but it can exert strength continuously for over 2 years.28 According to the literature, the iron phase (2CaO·Fe2O3) makes only a small contribution to strength increase in cement.24 Therefore, the combination of calcium sulfoaluminate and dicalcium silicate provides good mechanical strength to these experimental clinker samples at both the early and late hydration stages. In addition, because the nonhydraulic gehlenite (2CaO·Al2O3·SiO2) shown in the XRD results contributes little to the mechanical strength, the strength performance still has a margin for improvement if the CaO dose is adjusted. In Table 4, significant increases in strength values from clinker 1 to clinker 2 to clinker 3 can also be observed. Clinker


The titanium combined with the calcium and appeared in the clinkers as CaO·TiO2. Singh et al. also found the formation of calcium titanate in their preparation of iron-rich cement.21 Therefore, the consumption of CaO by titanium is unavoidable. The weak peaks of gehlenite (2CaO·Al2O3·SiO2) indicate that the CaO content used in the proportions of raw materials was 1263



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Table 5. Theoretical Mineral Compositions of the Clinker Samples by FactSage cement clinker sample

β-2CaO·SiO2

3CaO·3Al2O3·CaSO4

2CaO·Fe2O3

CaO·TiO2

2CaO·Al2O3·SiO2

CaSO4

1 2 3

35.785 33.896 32.249

20.432 24.349 27.538

22.573 19.049 16.802

5.583 5.311 4.970

6.693 6.416 8.033

7.634 8.801 7.868

shown to be the definite titanic phase, so it was selected rather than CaO·4TiO2. The compound 2CaO·Al2O3·SiO2 usually forms in solid-phase reactions when there is insufficient CaO. Its not-inconsiderable percentage value indicates that the amounts of β-2CaO·SiO2 and 3CaO·3Al2O3·CaSO4 could be increased if more CaO were added. CaSO4 was left after the reaction because it was added in excess to ensure that the reaction was extensive enough to form 3CaO·3Al2O3·CaSO4. By comparing the mineral composition differences between the three clinkers, it can be seen that there is a clear increase in the content of 3CaO·3Al2O3·CaSO4 from sample 1 to sample 3. The increasing formation of this mineral is consistent with the characteristics of the XRD patterns and the mechanical strength results, thus confirming the correctness of the raw material proportion design. This confirms that increasing the proportion of Al2O3 in the raw materials was an effective way to increase the formation of calcium sulfoaluminate and improve the mineral component structure of the cement clinker.

3 has about 20 MPa higher strength values at each curing time than clinker 1. This can be attributed to the addition of bauxite to the raw material mix, which directly increases the content of Al2O3 in the clinker, as shown in Table 3. It has been demonstrated that an appreciable increase in Al2O3 content can trigger an amplified increase in the percentage of calcium sulfoaluminate in the clinkers because the mass ratio between 3CaO·3Al2O3·CaSO4 and Al2O3 at the same number of moles of aluminum is 1.993.27 The increase in 3CaO·3Al2O3·CaSO4, which is the main contributor to the strength improvement, was also shown by the XRD results. Other clinker characteristics were also tested. For example, the initial setting time could be adjusted from tens of minutes to several hours with the addition of some retarding agents, such as boric acid. A pilot-scale experiment in a 20 t/h rotary cement kiln is being prepared, and detailed engineering performance features using large-scale clinkers, which should give a more typical set of data, will be reported in the future. In conclusion, both the XRD results and the mechanical strength results confirmed the feasibility of producing sulfoaluminate cement using FGD gypsum and red mud. Furthermore, the performance of the cement could be improved by increasing the Al2O3 content. Although the generation of calcium ferrite was significant because of the high Fe2O3 content in red mud, no large negative effects on the mechanical strength were encountered. This means that iron might not limit the utilization of red mud in the production of sulfoaluminate cement.

5. APPLICATION PROSPECTS The experiments and modeling reported herein confirm the feasibility of jointly utilizing red mud and desulfurization gypsum to prepare sulfoaluminate cement. The cement clinker produced contains calcium sulfoaluminate, β-dicalcium silicate, and calcium ferrite as the main mineral phases. It shows the combined features of routine sulfoaluminate cement and Portland cement. The sulfoaluminate cement available commercially has calcium sulfoaluminate as the dominant mineral phase, which can set and harden rapidly and has a high early mechanical strength. The mass proportion of dicalcium silicate is usually much less than that of calcium sulfoaluminate. Therefore, sulfoaluminate cement should be regarded as a special cement, mechanically distinguished by its high early strength, but with little increase in later strength. Furthermore, because of its strict material requirements (e.g., high-grade bauxite is needed) and high material costs, this cement has so far mainly been applied in some special fields, such as salvage engineering and glass fiber reinforced concrete(GFRC) products.30 Portland cement, which exhibits its mechanical performance mainly through tricalcium silicate (Ca3SiO5), is widely applied in all kinds of engineering. The sulfoaluminate cement prepared in these experiments had a lower proportion of calcium sulfoaluminate than commercial products and therefore sacrificed a certain early strength performance. However, it had a strength similar to that of Portland cement because the proportion of dicalcium silicate had been increased, which improved the later strength performance. With regard to commercial production, cement preparation using red mud could follow a number of routes. Without any change to the composition of dealkalized red mud, common high-silica sulfoaluminate cement can be produced. With the addition of some bauxite or the removal of Fe2O3, high-grade sulfoaluminate cement can be produced because the Al2O3 content in the clinker should increase, which would improve the performance of the cement product.

4. MODELING To further understand the mineral formation mechanism when sulfoaluminate cement is prepared with desulfurization gypsum and red mud, FactSage 6.3 was employed to model the solidphase reactions in the calcination process. The theoretical mineral components of the cement clinker samples were calculated using the core module of the software package, Equilib, which was built following the principle of minimal Gibbs free energy. Through the expansion of the thermodynamic database with some key minerals, such as 3CaO·3Al2O3·CaSO4, it became possible to model the sulfoaluminate cement system.23,29 In the calculation, the chemical compositions of the clinker samples (Table 3) were set as the initial reactants. All of the solid phases in the CaO−SiO2−Al2O3−Fe2O3−TiO2−SO3 system were taken into consideration as possible resultants in the calculation. The calcination temperature was set to 1300 °C. The theoretical mineral compositions of the clinker samples were then obtained and are listed in Table 5. Table 5 shows that β-2CaO·SiO2, 3CaO·3Al2O3·CaSO4, and 2CaO·Fe2O3, whose total proportion by mass is about 80%, are the main mineral phases in all of the clinker samples and that the minor minerals are CaO·TiO2, 2CaO·Al2O3·SiO2, and CaSO4. These theoretical mineral components were in accordance with the XRD results. It should be pointed out that, in the calculation, CaO·4TiO2 was found to form more easily than CaO·TiO2. In the XRD patterns, CaO·TiO2 was 1264



Article

Notes

There are a number of advantages to this cement technology. (1) Most of the materials are industrial wastes, with the total proportion of dealkalized red mud and desulfurization gypsum amounting to 70−90%, so the material costs are low and the waste consumption capacity is high. (2) The calcination temperature is only 1250−1300 °C, which is almost 200 °C lower than the requirement for Portland cement calcination. (3) Because both the red mud and the desulfurization gypsum have excellent particle fineness, the milling system for the raw material can be considerably simplified. (4) The low costs of materials, pretreatment, and calcination reduce the production costs for this cement. In addition to the attractive economic benefits, the environmental and social benefits are also considerable in terms of waste consumption, energy conservation, and reductions in CO2 emissions. Therefore, sulfoaluminate cement preparation technology using red mud and desulfurization gypsum has considerable potential and could have very wide application prospects in the future.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of National Natural Science Foundation of China (Grant 50906046) and Program for New Century Excellent Talents in University (NCET-100529).

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(1) Kikkawa, H.; Nakamoto, T.; Morishita, M.; Yamada, K. New Wet FGD Process Using Granular Limestone. Ind. Eng. Chem. Res. 2002, 41, 3028−3036. (2) Clark, R. B.; Zeto, S. K.; Ritchey, K. D.; Baligar, V. C. Growth of forages on acid soil amended with flue gas desulfurization by-products. Fuel 1997, 76, 771−775. (3) Chen, L.; Dick, W. A.; Nelson, S. Flue gas desulfurization byproducts additions to acid soil: Alfalfa productivity and environmental quality. Environ. Pollut. 2001, 114, 161−168. (4) Leiva, C.; Arenas, C. G.; Vilches, L. F.; Vale, J.; Gimenez, A.; Ballesteros, J. C.; Fernández-Pereira, C. Use of FGD gypsum in fire resistant panels. Waste Manage. 2010, 30 (6), 1123−1129. (5) Tzouvalas, G.; Rantis, G.; Tsimas, S. Alternative calcium-sulfatebearing materials as cement retarders: Part II. FGD gypsum. Cem. Concr. Res. 2004, 34, 2119−2125. (6) Chandara, C.; Azizli, K. A. M.; Ahmad, Z. A.; Sakai, E. Use of waste gypsum to replace natural gypsum as set retarders in portland cement. Waste Manage. 2009, 29, 1675−1679. (7) Guo, X. L.; Shi, H. S. Thermal treatment and utilization of flue gas desulphurization gypsum as an admixture in cement and concrete. Constr. Build. Mater. 2008, 22 (7), 1471−1476. (8) Zhang, N.; Liu, X. M.; Sun, H. H.; Li, L. T. Evaluation of blends bauxite-calcination-method red mud with other industrial wastes as a cementitious material: Properties and hydration characteristics. J. Hazard. Mater. 2011, 185, 329−335. (9) Amritphale, S. S.; Anshul, A.; Chandra, N.; Ramakrishnan, N. A novel process for making radiopaque materials using bauxite-Red mud. J. Eur. Ceram. Soc. 2007, 27, 1945−1951. (10) Liu, W. C.; Yang, J. K.; Xiao, B. Application of Bayer red mud for iron recovery and building material production from alumosilicate residues. J. Hazard. Mater. 2009, 161, 474−478. (11) Sushil, S.; Batra, V. S. Catalytic applications of red mud, an aluminum industry waste: A review. Appl. Catal., B 2008, 81, 64−77. (12) Paredes, J. R.; Ordóñez, S.; Vega, A.; Díez, F. V. Catalytic combustion of methane over red mud-based catalysts. Appl. Catal. B 2004, 47, 37−45. (13) Tsakiridis, P. E.; Agatzini-Leonardou, S.; Oustadakis, P. Red mud addition in the raw meal for the production of Portland cement clinker. J. Hazard. Mater. 2004, 116, 103−110. (14) Zhang, Y. N.; Pan, Z. H.; Li, D. X.; Xu, Z. Z. Study on red mud used as a property modifying mineral admixture for high performance cement. J. Nanjing Univ. Technol. 2004, 26, 20−26. (15) Zhou, H. L.; Li, D. Y.; Tian, Y. J.; Chen, Y. F. Extraction of scandium from red mud by modified activated carbon and kinetics study. Rare Met. 2008, 27 (3), 223−227. (16) Ç engeloğlu, Y.; Kir, E.; Ersöz, M. Recovery and Concentration of Al(III), Fe(III), Ti(IV), and Na(I) from Red Mud. J. Colloid Interface Sci. 2001, 244, 342−346. (17) Wang, S. B.; Ang, H. M.; Tadé, M. O. Novel applications of red mud as coagulant, adsorbent and catalyst for environmentally benign processes. Chemosphere 2008, 72, 1621−1635. (18) López, E.; Soto, B.; Arias, M.; Núñez, A.; Rubinos, D.; Barral, M. T. Adsorbent properties of red mud and its use for wastewater treatment. Water Res. 1998, 32, 1314−1322. (19) Zheng, X. F.; Hu, J.; Jiang, M.; Xue, Z. X. Study on optimization of dealkalization process on adding lime to red mud produced by low temperature Bayer process. Light Met. 2010, 4, 21−23.

6. CONCLUSIONS Red mud and desulfurization gypsum are two bulk industrial solid wastes that trigger many local environmental problems in China. This research, through experimentation and modeling, has shown that the idea of jointly utilizing red mud and desulfurization gypsum to prepare sulfoaluminate cement is feasible. It was found that, through calcination at about 1300 °C, desulfurization gypsum and red mud can be converted to cement clinker with dicalcium silicate (2CaO·SiO2) and calcium sulfoaluminate (3CaO·3Al2O3·CaSO4) as the main minerals. In this process, the desulfurization gypsum acts as the source of sulfur and some of the calcium, whereas the red mud provides the necessary silica, aluminum and most of the calcium. The desulfurization gypsum and red mud can make up 70−90% by mass of the total raw materials. The cement clinkers tested showed excellent mechanical strength performance. This technology provides an efficient method for disposing of some industrial solid wastes. The waste consumption capacity is considerable. Because the material costs, material pretreatment costs, and calcination temperature are all much lower, the sulfoaluminate cement products from this technology should show outstanding cost superiority to Portland cement, and its mechanical strength performance could be even better than that of Portland cement. Moreover, this technology could bring about considerable environmental and social benefits in terms of waste consumption, energy conservation, and reductions in CO2 emissions. Sulfoaluminate cement preparation technology using red mud and desulfurization gypsum has excellent potential application prospects. However, there should be further research into this technology that investigates areas such as the optimal proportions for these waste raw materials, the best calcination system and the mechanical and chemical performances of the clinker products. This research continues, and the working group that carried out the research covered in this article will widen the potential applications of this technology to the benefit of the environment.

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REFERENCES

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(20) Wang, Q.; Li, J.; Zhao, Y.; Luan, Z. K. Study on the dealkalization of red mud by suspension and carbonation. Chin. J. Environ. Eng. 2009, 3, 2275−2280. (21) Singh, M.; Upadhayay, S. N.; Prasad, P. M. Preparation of iron rich cements using red mud. Cem. Concr. Res. 1997, 27, 1037−1046. (22) Singh, M.; Upadhayay, S. N.; Prasad, P. M. Preparation of special cements from red mud. Waste Manage. 1996, 16, 665−670. (23) Wang, W. L.; Luo, Z. Y.; Shi, Z. L.; Cen, K. F. Experiments and modeling of ash mineral evolution in burning high-sulfur coal with lime. Energy Fuels 2011, 25, 130−135. (24) Wang, Y. M.; Su, M. Z.; Zhang, L. Sulfoaluminate Cement; Beijing University of Technology Press: Beijing, China, 1999. (25) Hida, Y.; Okazaki, J.; Itoh, K.; Sasaki, M. Formation mechanism of acicular calcium rerrite of iron ore sinter. J. Iron Steel Inst. Jpn. 1987, 73, 1893−1900. (26) Zhang, S. J.; Wang, S. T. Formation mechanism of acicular calcium ferrite. Iron Steel 1992, 27, 7−12. (27) Wang, W. L.; Wang, P.; Ma, C. Y.; Luo, Z.Y.. Calculation for mineral phases in the calcination of desulfurization residue to produce sulfoaluminate cement. Ind. Eng. Chem. Res. 2010, 49, 9504−9510. (28) Taylor, H. F. W. Cement Chemistry; Thomas Telford Publishing: London, U.K., 1997. (29) Wang, W. L.; Chen, X. D.; Chen, Y.; Ma, C. Y. Calculation and verification for the thermodynamic data of 3CaO·3Al2O3·CaSO4. Chin. J. Chem. Eng. 2011, 19, 489−495. (30) Quillin, K. Performance of belite-sulfoaluminate cements. Cem. Concr. Res. 2001, 31, 1341−1349.

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