Belite Cement Clinker from Coal Fly Ash of High Ca Content

The optimization of parameters of synthesis of belite cement clinker from coal fly ash of high Ca content is presented in this paper. ... Nevertheless...
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Environ. Sci. Technol. 2004, 38, 3209-3213

Belite Cement Clinker from Coal Fly Ash of High Ca Content. Optimization of Synthesis Parameters A . G U E R R E R O , * ,† S . G O N ˜ I,† I. CAMPILLO,‡ AND A. MORAGUES§ Instituto de Ciencias de la Construccio´n “Eduardo Torroja”(CSIC), Serrano Galvache s/n, 28033 Madrid, Espan ˜ a, Centro Tecnolo´gico LABEIN, Cuesta de Olabeaga, 16. 48013 Bilbao, Espan ˜ a, and Universidad Polite´cnica de Madrid, Escuela Te´cnica Superior de Ingenieros de Carninos Canales y Puertos, Ciudad Universitaria, S/N, 28040 Madrid, Espan ˜a

The optimization of parameters of synthesis of belite cement clinker from coal fly ash of high Ca content is presented in this paper. The synthesis process is based on the hydrothermal-calcination-route of the fly ash without extra additions. The hydrothermal treatment was carried out in demineralized water and a 1 M NaOH solution for 4 h at the temperatures of 100 °C, 150 °C, and 200 °C. The precursors obtained during the hydrothermal treatment were heated at temperatures of 700 °C, 800 °C, 900 °C, and 1000 °C. The changes of fly ash composition after the different treatments were characterized by X-ray diffraction (XRD), FT infrared (FTIR) spectroscopy, surface area (BETN2), and thermal analyses. From the results obtained we concluded that the optimum temperature of the hydrothermal treatment was 200 °C, and the optimum temperature for obtaining the belite cement clinker was 800 °C.

Introduction This work is a part of an extensive investigation in which different like-belite cement clinkers of low-energy are being synthesized as an alternative to the conventional Portland cement clinker. Our investigations were promoted by the serious environmental climate change which has arisen from the greenhouse CO2 emissions produced by the cement industry and particularly by the Portland cement clinker fabrication. It is estimated that the global cement industry produces around 1.4 Bt of CO2 per year, which represents about 6% of the total global man-made CO2 production (26.9 Bt now) (1, 2). According to R. McCaffrey (2) global cement production increases at about 3%/year, along with the corresponding CO2 emissions increase. In the particular case of Spain, the CO2 equivalent emissions and its reduction for 2010 according to the Kyoto emission targets can be seen in Figure 1 (the emissions included six greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)) (1). * Corresponding author phone: +34 91 3020440; fax: +34 91 3020700; e-mail: [email protected] [email protected]. † Instituto de Ciencias de la Construccio ´ n “Eduardo Torroja”(CSIC). ‡ Centro Tecnolo ´ gico LABEIN. § Universidad Polite ´ cnica de Madrid. 10.1021/es0351589 CCC: $27.50 Published on Web 05/01/2004

 2004 American Chemical Society

FIGURE 1. Total greenhouse CO2 equivalent emissions of Spain and reduction from projection according to Kyoto target (source ref 1).

Calcination of CaCO3 to form CaO is responsible for 54% of CO2 emissions, the rest being due to 34% by the combustion of fuel in the kiln and 12% by the use of electricity. An average of 0.83 tonnes of CO2 is emitted per ton of finished product (cement with 80% clinker) (3). In this sense, the formation of alite (Ca3SiO5), which is the main component of the Portland cement clinker, produces a greater amount of CO2 emissions than the formation of belite (Ca2SiO4). But CO2 emissions can be strongly reduced or even totally avoided if the raw material has CaO, as is the case of coal fly ash of high Ca content. The industrial byproducts such as fly ash of low lime content or slag have the potential to reduce CO2 emissions and are being mainly used by the cement industry by replacing a part of the clinker in cement. Since 1997 we have been investigating the use of industrial wastes and byproducts as alternative secondary raw materials for synthesizing reactive low-energy belite cements. The belite phase β-Ca2SiO4 of dicalcium silicate is a component of the Portland cement, which does not contribute to the gain of strength during the first 28 days of hydration owing to its very low hydraulic activity. Nevertheless, the growing interest in this phase is related to several interesting properties such as energy-saving, CO2 emission-saving, and, mainly, better durability against sulfate and carbonation attacks (4, 5). Our investigations focused on the hydrothermal-calcination-route for making reactive belites by using wastes as raw materials such as fly ash from coal combustion of low lime content and fly ash and bottom ash from incineration of municipal solid wastes. The hydrothermal-calcination-route was used by Jiang and Roy in the year 1992 for the first time (6). The authors used fly ash of low lime content from coal combustion, as raw material for synthesizing low-energy cement, what they called Reactive Fly Ash Belite Cement (FABC). The fabrication process included a prehydrothermal treatment of the fly ash and CaO, in which the hydrated precursors of the cement were obtained. The subsequent dehydration of those phases, by controlled heating, gave rise to the highly reactive belite phases (β-Ca2SiO4), mayenite Ca12Al14O33 and CaCO3. The Jiang and Roy methodology was applied in our laboratory with some modifications for the case of Spanish fly ash of low CaO content (7, 8). During the hydrothermal treatment (200 °C for 4 h and 1.24 Mpa of pressure) the fly ash pozzolanic reaction is strongly activated leading to hydraulic products: katoite (C3ASH4) (Ca3Al2 (SiO4)(OH)8) together with CSH gel (Ca1.5SiO3.5‚xH2O) and R-Ca2SiO4‚H2O, VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Chemical Composition of the Starting Fly Ash FA-1 (% by Weight) FA-1 a

LOIa

CaO

SiO2 (total)

Fe2O3

Al2O3

MgO

SO3

Na2O

K2O

SiO2b (reactive)

BET-N2 (m2/g)

22.1

37.6

19.4

3.0

11.2

0.94

3.9

0.35

0.83

16.8

35

LOI ) ignition loss.

b

Reactive silica determined according to Spanish standard UNE-0-224.

which are the precursors of the cement. In the second step those precursors are dehydrated by heating at 800 °C, where a mixture of β- and R′L-Ca2SiO4, mayenite, and calcite were obtained. The BET-N2 surface area of cement was 4.4 m2/g (8). This cement at 28 days had a compressive strength value similar to that of Spanish Portland cement type CEM I-32.5 (9). Besides, as the durability study in sulfate aggressive solution of 48 000 ppm of sulfate concentration demonstrated (10), the cement was very resistant to severe sulfate attack, taking into account the great amount of aluminate of starting fly ash. On the basis of those studies, we are currently investigating the possibilities of coal fly ash of high Ca content as 100% of raw material for synthesizing belite cements (11). In this paper, the optimization of parameters of synthesis is presented. The synthesis process is based on the hydrothermal-calcination-route of the fly ash without extra additions. The hydrothermal treatment was carried out in water and a 1 M NaOH solution for 4 h at the temperatures of 100 °C, 150 °C, and 200 °C. The precursors obtained during the hydrothermal treatment were heated at temperatures of 700 °C, 800 °C, 900 °C, and 1000 °C for obtaining the belite phase. The changes of fly ash composition after the different treatments were characterized by X-ray diffraction (XRD), FT infrared (FTIR) spectroscopy, surface area (BET-N2), and thermal analyses.

Materials and Methods Spanish coal fly ash of high Ca content (ASTM Class C), from the thermal station of CERCS (Catalonia), called FA-1, was used as raw material. The chemical composition of the fly ash (Table 1) was determined according to the Spanish standard UNE-EN 196-2. The fly ash and demineralized water at a water-to-solid ratio of 3 was first hydrothermally activated at 100 °C, 150 °C, and 200 °C for 4 h with continuous stirring. An equivalent hydrothermal treatment was carried out in a 1 M NaOH solution. After this process, the reactor was cooled, and the solid was filtered and dried at 80 °C. The dried product was then heated at a rate of 10 °C/min (up to 600 °C) and 5 °C/min from 600 °C to 700 °C, 800 °C, 900 °C, and 1000 °C. The hydrothermal treatment was carried out with a Parr model 4522 (100 mL pump with split-ring closure and a PID model 4842 temperature controller). XRD patterns were recorded on a Philips PW 1730 diffractometer with a graphite monochromator and Cu KR1 radiation. FTIR spectroscopy study was carried out on a Atimattson Genesis FTIR TM instrument and KBr pellets containing 0.5% of sample. The surface area measurements were made by the BET multipoint method with a Micromeritics ASAP 2010 device, a previous sample degasification at 50 °C during 24 h up to 0.05 µm Hg pressure, and N2 gas as absorbate. The surface areas were calculated from the isotherm data using the BrunauerEmmett and Teller (BET) equation in the relative pressure range of 0.003-0.3. Thermal analyses were recorded with Netzsch equipment with STA 409 simultaneous analysis system using 50 mg samples and a dynamic nitrogen stream (flow rate ) 100 cm3/min) at a heating rate of 10 °C/min.

Results and Discussion A. Hydrothermal Treatment of Fly Ash. The changes of starting fly ash as a consequence of the hydrothermal 3210

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FIGURE 2. Evolution of X-ray diffraction patterns of starting fly ash after different hydrothermal treatments in water and 1 N NaOH solution.

treatment in water and NaOH can be seen in Figure 2. In Table 2, the JCPDS cards of the crystalline compounds are given. The main crystalline compounds of the starting fly ash (Figure 2(a)) are the following: calcite (CaCO3), ettringite (Ca6Al2(SO4)3(OH)12‚26H2O), hydrated monosulfo calcium aluminate (Ca4Al2(SO4)O6‚12H2O), gypsum (CaSO4‚2H2O), and quartz (SiO2). Although the CaO/SiO2 molar ratio of the fly ash is 2.1, as shown in Table 1, 18% of CaO was carbonated probably due to meteorization during ambient exposure outside of the thermal station. The CaCO3 content was 32.1% (according to the thermogravimetric analysis carried out in the sample previously dried at room temperature (Figure 4 (a))). That means that 18% of CaO from CaCO3 will probably not react during the hydrothermal treatment, owing to the fact that calcite is a very stable compound. One possible way to avoid this fact could be to heat previously the fly ash to decompose calcite and to obtain reactive free lime, but we preferred to use the fly ash as raw material without any pretreatment. When the fly ash is hydrothermally treated in demineralized water at 100 °C (Figure 2(b)), gypsum transformed in bassanite (CaSO4‚0.5H2O) and the amount of monosulfo calcium aluminate (Ca4Al2(SO4)O6‚12H2O) increased; the reflection at 29.5, 2θ angular zone, increased strongly, probably due to the formation of CSH gel (Ca1.5SiO3.5‚xH2O). At 150 °C (Figure 2(c)), the amount of bassanite increased, with the reflections disappearing corresponding to ettringite and monosulfo calcium aluminate; insoluble orthorhombic anhydrite (CaSO4) together with katoite (Ca3Al2(SiO4)(OH)8) begins to appear. At 200 °C (Figure 2(d)), bassanite dehydrated leading to the massive formation of the anhydrite aforementioned. In the case of the hydrothermal treatment in a 1 M NaOH solution, the main differences are the absence of crystalline sulfate compounds and the formation of aluminum-tobermorite (Ca5Si5Al(OH)O17‚5H2O), which begins to be detected

TABLE 2. JCPDS Cards of the Crystalline Compounds compound

JCPDS no.

compound

JCPDS no.

ettringite (Ca6Al2(SO4)3(OH)12‚26H2O) Ca4Al2(SO4)O6‚12H2O gypsum (CaSO4‚2H2O) calcite (CaCO3) bassanite (CaSO4‚0.5H2O) katoite (Ca3Al2(SiO4)(OH)8) R-quartz (SiO2) insoluble anhydrite (CaSO4) C-S-H gel (Ca1.5SiO3.5‚xH2O)

41-1451 45-158 76-1746 72-1651 41-224 38-368 46-1045 37-1496 33-306

lime (CaO) aluminum tobermorite (Ca5Si5Al(OH)O17‚5H2O) zeolite A Na6[AlSiO4]6‚4H2O R’L-Ca2SiO4 β-Ca2SiO4 mayenite (C12A7) gehlenite (Ca2Al2SiO7)

37-1497 19-52 42-216 36-642 33-302 48-1882 35-755

TABLE 3. Combined Water and Calcite Content Calculated by Thermogravimetric Analysis (Weight %)

combined water calcite

FA-1

water 100 °C

water 150 °C

water 200 °C

NaOH 100 °C

NaOH 150 °C

NaOH 200 °C

12.3 32.1

8.4 34.1

6.3 33.5

5.6 34.1

8.8 33.4

7.3 33.9

5.1 32.5

FIGURE 3. Evolution of FTIR spectra of starting fly ash after its hydrothermal treatment in water and 1 N NaOH solution at 200 °C. FIGURE 5. X-ray diffraction patterns of cement fly ash precursors heated at 700 °C.

FIGURE 4. Evolution of TG curves of starting fly ash after its hydrothermal treatment in water and NaOH at 200 °C. at 150 °C (Figure 2(f)), and zeolite (Na6[AlSiO4]6‚4H2O) type sodalite, which is formed at 200 °C (Figure 2(g)). These differences were confirmed by FTIR (Figure 3). In the case of untreated fly ash (Figure 3(a)) the vibrations of the OH group produce the bands centered at 3678 and 3633 cm-1; the vibration of lattice water at 3450 and 1660 cm-1; [CO3]2- group produces the bands centered at 1450 and 877

FIGURE 6. X-ray diffraction patterns of cement fly ash precursors heated at 800 °C. cm-1; the band appearing at 1130 cm-1 is due to the vibration of [SO4]2 group, and the band at 990 cm-1 is due to the vibration of [SiO4]4- and [AlO4]5- groups. After the hydrothermal treatment in water at 200 °C (Figure 3(b)), the main change is produced in the band at 1130 cm-1, which is VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Comparison of crystalline belite and calcite contents after heating the different precursors at 800 °C.

transformed in a shoulder, whereas the intensity of the band at 990 cm-1 increased (probably by the contribution of CSH gel). When the hydrothermal treatment is in NaOH at 200 °C (Figure 3(c)), the intensity of the [SO4]2 band at 1130 cm-1 strongly decreased, whereas the intensity of the band at 990 cm-1, attributed to the vibration of [SiO4]4- and [AlO4]5groups, from tobermorite, zeolite and katoite, increased. Calcite remained irrespective of the hydrothermal treatment conditions (see Table 3). The calcite content was determined by thermogravimetric analysis from the weight loss produced between 500 °C and 1000 °C and the combined water content from that produced between 25 °C and 500 °C (see Figure 4). B. Cement Precursors Dehydration: Influence of Heating Temperature. The precursors phases formed during the hydrothermal fly ash activation were heated at temperatures ranging from 700 °C to 1000 °C. 700 °C (Figure 5): In all the conditions the hydrated phases disappeared. At this temperature, an amorphous halo appeared between 31 and 34 of 2θ angular zone, which could correspond to some of the Ca2SiO4 varieties, probably the R′L-Ca2SiO4; the intensity of anhydrite reflections increased with the temperature of hydrothermal activation. In samples treated in NaOH no anhydrite appeared (see Figure 5(d-f)); the amorphous halo aforementioned diminished, and dehydrated zeolite is detected at 150 °C and 200 °C (Figure 5(e),(f)). 800 °C (Figure 6): At this temperature R′L-Ca2SiO4 of belite variety is formed and began to appear as lime (CaO) from calcite decomposition; the anhydrite remained uncharged. In the case of the hydrothermal treatment in NaOH (Figure 6(d-f)), the intensity of R′L-Ca2SiO4 reflections is higher compared with equivalent samples treated in water, whereas calcite reflections are lower. In both cases, the optimum temperature of the hydrothermal treatment seems to be 200 °C where the maximum of R′L-Ca2SiO4 and the minimum of calcite and lime are obtained. This can be better seen in Figure 7 where the intensity of the R′L-Ca2SiO4 reflection at 32.5 of 2θ angular zone together with calcite contents (from TG curves) for all the hydrothermal conditions studied is presented. The maximum intensity of R′L-Ca2SiO4 is reached for the sample hydrothermally treated in NaOH at 200 °C, which is about two times higher than that obtained when the hydrothermal treatment is in water at 200 °C. This fact is reflected in the corresponding compressive mechanical strengths (12). 900 °C (Figure 8): The main change observed at this temperature is the formation of gehlenite (Ca2Al2SiO7 (C2AS)) together with β-Ca2SiO4; traces of mayenite (Ca12Al14O33 (C12A7)) were also detected. Calcite disappeared, and the intensity of lime reflections increased. 1000 °C (Figure 9): The intensity of gehlenite and β-Ca2SiO4 increased, with the appearance of CaAl2O4 (CA). 3212

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FIGURE 8. X-ray diffraction patterns of cement fly ash precursors heated at 900 °C.

FIGURE 9. X-ray diffraction patterns of cement fly ash precursors heated at 1000 °C.

The semiquantitative evolution with the heating temperature of the crystalline phases detected from XRD, in the case of fly ash hydrothermally activated at 200 °C, is presented in Figure 10. Although the higher amount of belite variety is reached at 900 °C, nevertheless, the lime content is the maximum for the sample treated in water, furthermore a considerable amount of gehlenite is formed. Both free lime and gehlenite are not desirable since gehlenite has small hydraulic activity and lime can produce expansion during hydration. Consequently, we concluded that the heating

FIGURE 10. Semiquantitative evolution of the main crystalline phases obtained from heating of precursors obtained through hydrothermal treatment of fly ash in water and NaOH solution.

FIGURE 11. Evolution of the surface area of starting fly ash after the different treatments. temperature of 800 °C is the optimum for obtaining the optimum belite cement clinker. The BET-surface area decreased as the temperature of heating increased due to the sinterization of particles, as can be seen in Figure 11. The optimum belite cement clinkers synthesized at 800 °C have a surface area of 10 m2/g and 12 m2/g for previous hydrothermal treatments in water and NaOH, respectively. These high values have an important contribution to the respective reactivity with water (12).

Conclusions 1. The optimum belite cement clinker was allowed when the temperature of the hydrothermal treatment of the fly ash was 200 °C and when the hydrated precursors were heated at 800 °C. At this temperature the main crystalline belite variety was the R′L-Ca2SiO4. 2. In the case of the hydrothermal treatment of fly ash in a 1 N NaOH solution and subsequent heating at 800 °C, the amount of R′L-Ca2SiO4 duplicated practically that formed when the hydrothermal treatment was in demineralized water. 3. At temperatures higher than 800 °C, gehlenite and free lime (CaO) were formed. Both free lime and gehlenite are not desirable since gehlenite has a small hydraulic activity and lime can produce expansion during hydration. Furthermore, the R′L-Ca2SiO4 partially converted in β-Ca2SiO4, and traces of mayenite (Ca12A14O33) were formed.

Acknowledgments The authors gratefully acknowledge the financial support by the Minister of Science and Technology (Project no. MAT 2002-04023-CO1-CO2-CO3) and the Thermal Station of Cercs (Catalonia) for the fly ash supplied.

Literature Cited (1) Intergovernmental Panel on Climate Change: Special report on Emissions 2001. http://www.grida.no/climate. (2) McCaffrey, R. Climate change and the cement industry; Global Cement and Lime Magazine; Environmental Special Issue 2002. (3) European Cement Industry: Cembureau (Organisation of the Cement Industry in Europe). http://www.cembureau.be (4) Chatterjee, A. K. High belite cements-Present status and future technological options: Part I. Cem. Concr. Res. 1996, 26(8), 12131225. (5) Chatterjee, A. K. Future technological options: Part II. Cem. Concr. Res. 1996, 26(8), 1227-1237. (6) Jiang, W.; Roy, D. M. Hydrothermal Processing of New Fly Ash Cement. Ceram. Bull. 1992, 71(4), 642-647. (7) Guerrero, A.; Gon ˜ i, S.; Macı´as, A.; Luxa´n, M. P. Hydraulic Activity and Microstructural Characterization of New Fly Ash-Belite Cements Synthesized at Different Temperatures. J. Mater. Res. 1999, 14(6), 2680-2687. (8) Gon ˜ i, S.; Guerrero, A.; Luxa´n, M. P.; Macı´as, A. Dehydration of pozzolanic products hydrothermally synthesized from fly ashes. microstructure evolution. Mater. Res. Bull. 2000, 35(8), 13331344. (9) Guerrero, A.; Gon ˜ i, S.; Macı´as, A.; Luxa´n, M. P. Mechanical Properties, Pore-Size Distribution and Pore Solution of Fly AshBelite Cement Mortars. Cem. Concr. Res. 1999, 29, 1753-1758. (10) Guerrero, A.; Gon ˜ i, S.; Macı´as, A. Durability of new Fly ashBelite Cement Mortars in Sulfated and Chloride Medium. Cem. Concr. Res. 2000, 30(8), 1231-1238. (11) Gon ˜ i, E. S.; Guerrero, B. A.; Moragues, T. A.; Tallafigo, V. M. F.; Campillo, S. I.; Sa´nchez, D. J.; Porro, F. A. New Belite Cement Clinkers from Fly ash of Coal Combustion of High Ca Content. Spanish Patent No. 200301504, solicited 27, June, 2003. (12) Guerrero, A.; Gon ˜ i, S.; Moragues, A.; Dolado, J. S. Microstructure and Mechanical Performance of Belite Cements from High Calcium Coal Fly Ash. J. Am. Ceram. Soc. Manuscript in preparation.

Received for review October 17, 2003. Revised manuscript received March 15, 2004. Accepted March 25, 2004. ES0351589

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