CaO-Based Pellets Supported by Calcium Aluminate Cements for

Environ. Sci. Technol. , 2009, 43 (18), pp 7117–7122. DOI: 10.1021/es901258w. Publication Date (Web): August 12, 2009. Copyright © 2009 American Ch...
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Environ. Sci. Technol. 2009, 43, 7117–7122

CaO-Based Pellets Supported by Calcium Aluminate Cements for High-Temperature CO2 Capture VASILIJE MANOVIC AND EDWARD J. ANTHONY* CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1

Received April 27, 2009. Revised manuscript received July 24, 2009. Accepted July 27, 2009.

The development of highly efficient CaO-based pellet sorbents, using inexpensive raw materials (limestones) or the spent sorbent from CO2 capture cycles, and commercially available calcium aluminate cements (CA-14, CA-25, Secar 51, and Secar 80), is described here. The pellets were prepared using untreated powdered limestones or their corresponding hydrated limes and were tested for their CO2 capture carrying capacities for 30 carbonation/calcination cycles in a thermogravimetric analyzer (TGA). Their morphology was also investigated by scanning electron microscopy (SEM) and their compositions before and after carbonation/calcination cycles were determined by X-ray diffraction (XRD). Pellets prepared in this manner showed superior behavior during CO2 capture cycles compared to natural sorbents, with the highest conversions being >50% after 30 cycles. This improved performance was attributed to the resulting substructure of the sorbent particles, i.e., a porous structure with nanoparticles incorporated. During carbonation/calcination cycles mayenite (Ca12Al14O33) was formed, which is believed to be responsible for the favorable performance of synthetic CaO-based sorbents doped with alumina compounds. An added advantage of the pellets produced here is their superior strength, offering the possibility of using them in fluidized bed combustion (FBC) systems with minimal sorbent loss due to attrition.

1. Introduction The need to mitigate CO2 emissions from thermal power generation by developing technologies for CO2 capture and sequestration is now widely accepted (1-3). Unfortunately, technologies like amine scrubbing that would allow removal of CO2 from flue gas are relatively expensive, energy intensive, and largely unproven. In this context, CO2 looping cycle technology, employing CaO looping, has attracted increasing attention, owing to a number of its potential advantages, namely the relatively small efficiency penalty which it imposes upon a power station (estimated at 6-8%, including compression of the CO2) (4-6); its potential use in large-scale circulating fluidized beds (CFBC) (7), (a mature technology, as opposed to the need to significantly scale up solvent scrubbing towers, which would be required for amine scrubbing); the excellent opportunity for its integration with cement manufacture (potentially decarbonizing both industries): and its extremely cheap sorbent (crushed limestone) (8), coupled with the fact that any final solid waste * Corresponding author phone: (613) 996-2868; fax: (613) 9929335; E-mail: [email protected]. 10.1021/es901258w CCC: $40.75

Published on Web 08/12/2009

 2009 American Chemical Society

products are environmentally benign. This technology is also interesting for gasification applications and enhanced production of hydrogen from shift reactors (9). Like chemical looping combustion (10), this technology gains its potential efficiency advantages due to the fact that the energy consumption in one reaction (in this case calcination), is regained in the capture reaction. Moreover, the carrying capacity of CaO is very high, since 56 g CaO can capture 44 g CO2, i.e., the carrying capacity is 786 mg/g, which is an order of magnitude better than hydrotalcites, and double or triple some of the alternative and more expensive materials such as lithium silicates and zirconates (9). This high carrying capacity means in practice that if the decay in activity can be reduced, then the addition of a moderate amount of binder, and/or some reaction of the binder with the active CaO, is relatively unimportant in terms of the sorbent performance if in turn the binder improves the overall sorbent performance and its mechanical stability. These issues will be discussed in detail below. The most widely investigated CaO-based CO2 carriers are natural materials: calcite (CaCO3) and dolomite (CaCO3 · MgCO3) (4, 11, 12), which also have the best thermodynamic properties among metal oxides for high-temperature CO2 capture (13). Here, the process of CO2 capture by CaO-based sorbents is possible because carbonation is a reversible chemical reaction: CaO(s) + CO2(g) ) CaCO3(s) ∆H < 0

(1)

The thermodynamics of the reaction system can be described by the following equation (14): log10 PCO2[atm] ) 7.079 - (8308/T[K])

(2)

Employing this reaction in practice, to transport CO2 from a gas with lower concentration (e.g., flue gas, ∼15% CO2) to a concentrated stream (∼90% CO2) ready for liquefaction and sequestration, implies CO2 capture (carbonation) at lower temperatures (650-700 °C) and sorbent regeneration (calcination) at higher temperature (>900 °C). As noted above, this process requires transfer of solid sorbent from one chemical environment to another and fluidized bed combustion (FBC) systems represent an excellent way of achieving this (7, 15, 16). Unfortunately, FBCs also cause enhanced mechanical degradation of the solid materials (17), i.e., attrition, and consequent elutriation (18). Attrition is more pronounced than in normal FBC operation because, in addition to mechanical stresses due to solid circulation and fluidization, particles are also subjected to thermal stresses due to temperature cycling between the carbonator and calciner. Moreover, enhancement of the CO2 carrying capacity of the sorbent by steam reactivation causes further softening and powdering of the sorbent particles (19, 20). Significant sorbent loss has important negative economic consequences to the technology because of the need for makeup of fresh sorbent and disposal of elutriated sorbent. The other major challenge in developing CO2 looping cycle technology involves the loss of sorbent activity, due to sintering, in successive cycles. The decay of activity or CO2 carrying capacity can be described qualitatively (21, 22) and quantitatively (23, 24) by simple models based on the idea of sorbent sintering (21, 25, 26) and there is now extensive research directed to improving sorbent carrying capacity over “long” series of capture cycles. Probably the most promising approach is sorbent hydration by steam before use in CO2 cycles (27) or of spent sorbents (28). It has been shown that VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the reactivation of spent sorbent is even possible with moist air (29). Another method of enhancing activity is by doping natural sorbents (30). Experiments on doping with sodium carbonate (Na2CO3) indicated that small amounts added by washing with very dilute solutions might be favorable for long-term reactivity (26). However, it should be pointed out here that most of the additives investigated to date negatively affected sorbent carrying capacity (30). An alternative approach is to produce synthetic sorbents doped with alumina compounds (31-34). In some of these studies, reagent-grade powdered CaO and aqueous Al(NO3)3 solution were used in the synthesis. The conversions in longer series of CO2 cycles are high in comparison with those for natural sorbents. After optimization of the synthesis procedure, conversions >40% were recently reported for a series of 45 cycles (33). These results support further investigation of CaO-based sorbents containing alumina compounds, but using less expensive materials and procedures. The use of suitable binders for pelletization has shown that calcium aluminate cements have the most promising performance (35). These cements display good setting performance and are refractory materials suitable for use in corrosive environments. Moreover, their chemical composition is also desirable because they contain mainly calcium- and alumina-containing compounds and their compositions and properties are described in detail elsewhere (36). The properties of CaO-based pellets formed using calcium aluminate cements, together with encouraging preliminary results (35) led us to study them in more detail. This study presented here, describes their characteristics and gives their performance in multicycle CO2 capture experiments.

Materials and Methods Three limestones: Cadomin (CD) and Havelock (HV) from Canada, and Katowice (KT) from Poland (Upper Silesia) were used for pelletization. Particle sizes used were CD and HV limestones 0.25-1.4 mm; and KT 0.4-0.8 mm. The elemental analysis of the limestones used is given in Supporting Information (SI) Table S1. The distinguishing feature in the elemental compositions is the high SiO2 (5.47%) and Al2O3 content (1.54%) in CD limestone, indicating the presence of silicate and aluminosilicate impurities. Four commercial calcium aluminate cements were used here. CA-14 and CA-25 were produced by Almatis Inc., having Al2O3 contents of 70 and 80%, respectively. Secar 51 and Secar 80 are produced by Kerneos Inc., and were chosen because of their wider difference in Al2O3 content, 50 and 80%, respectively. They are produced in large quantities, are relatively inexpensive, and are used when refractory propertiessresistance to corrosion and chemicalssand rapid setting are required (36). This also recommends them as potential binders for CaO-based pellets for CO2 capture. Pellets were prepared using powdered limestone or hydrated lime, and 10% of the binder, based on the results of our most recent study (35). Here, sorbent/binder ratios are calculated on the initial limestone properties and represent optimal ratios, taking into account the need for pellet strength, CO2 carrying capacity (expressed on the pellet mass), and costs. When hydrated lime was used, the limestone was calcined at 850 °C for 2 h before hydration. Weighed amounts of limestone (or lime after calcination) and cement were mixed in a glass beaker, and water was added with stirring to obtain a gel, similar to mortar. When lime was used, water was slowly added to minimize the effects of heat release due to the exothermic hydration process. The gel obtained was then extruded through a 1.0 mm sieve to produce uniform pellet diameters and the resulting pellets were then air-dried for 24 h. Typically, final pellet diameters were ∼0.8 mm. It was noted that pellets behave differently 7118

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depending on the particle size and shape; thus, two other pellet sizes were also prepared, with diameters of 2 and 4 mm and these three sizes were designated as P1, P2, and P4, respectively. The length of the pellet was similar to its diameter, i.e., 1, 2, and 4 mm, respectively. Some fresh pellet particles were also shaped by hand, using protective gloves, to obtain spheres. The pellets were tested to determine their crushing strength by a custom-made apparatus. The crushing strength of a single pellet sphere was determined by gradually increasing the force required to break the sphere when it was placed between the two plates. Additionally, pellet strength is tested by examining the effect of the fall of pellet particles from 1 m to a concrete surface. We believe that these simple laboratory tests are a good starting point to determine an optimal cement/limestone ratio appropriate for use in FBC systems. CO2 carrying capacities for pellets were determined by a Perkin-Elmer TGA-7 apparatus using ∼30 mg samples suspended in a quartz tube (i.d. 20 mm) on a platinum pan (i.d. 5 mm). The gas flow rate was 0.04 dm3/min and the temperature and gas used were controlled by Pyris software. Data on sample mass during the experiments were monitored and conversions were calculated on the basis of mass change, assuming this occurs only due to formation/decomposition of CaCO3. The experiments were done isothermally at 850 °C, with carbonation in 100% CO2 and calcination in 100% N2, both for 10 min. The carbonation temperature and CO2 concentration were more severe than that expected in real capture systems (650-700 °C, 15% CO2), but conditions for calcination were milder than those anticipated in a real process (900-950 °C, >90% CO2) (4, 7, 11, 18). The larger amounts of cycled pellets necessary for X-ray diffraction (XRD) analysis were obtained from a tube furnace (TF). Here, about 10 g of pellets were placed in the TF and 30 CO2 cycles were performed isothermally at 850 °C, with carbonation in 100% CO2 and calcination in 100% N2, both for 20 min. The gas flow rate was controlled by a flowmeter at 0.5 dm3/min. The increased carbonation/calcination time compared to that in the TGA was chosen because of the larger sample mass in the TF, which causes slower interparticle mass transfer and consequently longer reaction times. The sample morphologies were observed with a Hitachi S3400 scanning electron microscope (SEM) with 20 kV of accelerating voltage under high vacuum. The samples were coated with gold/palladium before SEM examination and images obtained by secondary electrons. Identification/quantification of compounds in the pellet samples before and after carbonation/calcination cycles were done by XRD, with data collected on a Bruker D500TT diffractometer over the angular range 10-70° (2θ) in 0.02° steps and 20 s per step. The phases were identified and quantitative analyses of the samples were obtained using alpha-alumina (Al2O3) as an internal spiking standard.

Results and Discussion In our recent study (35) on screening of binders for CaObased pellets for CO2 capture, different limestone-to-cement ratios were used (5-40% cement) and it was shown that 10% cement was sufficient to ensure good pellet hardness. An increase of the cement/limestone ratio improves pellet strength but reduces the amount of active CaO per total mass of the sorbent and, also increases cost. The 10% cement used here results in sorbent that, in the calcined form, contains ∼10% Al2O3 in alumina compounds, which is comparable with that seen in other studies on alumina-based synthetic sorbents (31-34). Results of carbonation conversion for CA-14 cement/CD pellets are presented in Figure 1. Carbonation conversions

FIGURE 1. Carbonation conversions during CO2 cycles in TGA of pellets: powdered CD limestone (PP) or hydrated CD lime (HP) and 10% CA-14 cement, particle size ∼1 mm. Conditions: carbonation in 100% CO2, calcination in 100% N2, both for 10 min, isothermally at 850 °C. Filled symbols are for samples pretreated in 100% N2 at 1000 °C for 6 h.

FIGURE 2. Carbonation conversions during CO2 cycles in TGA of pellets: hydrated CD lime and 10% of different calcium aluminate cements, particle size ∼1 mm. are calculated taking into account only CaO from limestone because the “CaO” content in the cement does not contribute to CO2 capture (35). However, some of the CaO from limestone also reacted with the cement to form mayenite (Ca12Al14O33). It was not feasible to determine the amount of active CaO in each cycle/each TGA carbonation/calcination run and conversion was, therefore, calculated assuming that all CaO from limestone is available. This means that conversions presented in this paper are somewhat conservative and the “real” conversion may be higher by up to 10% (relatively). It can be seen from Figure 1 that the highest conversions were obtained with pellets prepared with hydrated lime and cement (HP-CA-14), indicating that this pellet production procedure preserved the advantages of hydration as an activation or reactivation step. The superior performance of hydrated lime pellets is believed to be due to a superior pore structure of CaO obtained from Ca(OH)2 (19, 27, 28). Moreover, using hydrated lime and alumina cement appears to enhance the favorable influence of alumina compounds from the cement (31-34). Effects of the use of different cements with hydrated CD lime are presented in Figure 2. It can be seen that conversions are similar with the exception of those for pellets using CA14 cement, which are significantly higher than those for pellets obtained with other cements. However, with increasing cycle number the difference decreases; after 30 cycles the conversion is only better by 5-7%. The effect of increased sorbent activity in the beginning cycles after pretreatment is also pronounced, but unfortunately this beneficial effect is not maintained for a series of 30 cycles or more. Pellet spheres of diameter ∼4 mm (P4) were prepared with hydrated HV lime and their conversions during CO2 cycles are shown in Figure 3. Note that the influence of cement type used is very small. The conversions obtained for pellets

FIGURE 3. Carbonation conversions during CO2 cycles in TGA of pellets: hydrated HV lime and 10% of different calcium aluminate cements, particle size ∼4 mm.

FIGURE 4. Carbonation conversions of pellet (P4, ∼4 mm) prepared from hydrated CD lime and 10% CA-14 cement. After 15th cycle the particle was crushed and the run quickly recommenced. with cement addition were always higher than those prepared solely from hydrated lime (solid line). This additionally highlights the favorable effect of cement not only with regard to pellet strength, but also for CO2 uptake during multiple reaction cycles. It is also interesting that conversions shown in Figure 3 are significantly lower (∼10% for the original and ∼30% for pretreated pellets) than those presented in Figure 2 for hydrated CD lime. The differences between these two sets of experiments were lime type (CD and HV) and pellet size (∼1 and ∼4 mm); these effects were resolved in additional experiments described below. Spherical pellets (P4) were crushed before CO2 cycles in TGA. The results obtained were compared with those from the original-size pellets. It was found that higher conversions were obtained with crushed pellets and this effect was more pronounced for pretreated samples. The change of activity with pellet size is illustrated in Figure 4. A spherical pellet (∼4 mm, CD lime and 10% CA-14) was subjected to TGA carbonation/calcination cycles. After 15 cycles, the TGA run was stopped, the particle was crushed, and the run was immediately restarted to minimize any undesirable influence of moist air. Comparing results, it can be seen that conversions are significantly lower than those for smaller particles (∼1 mm) obtained from the same materials (Figure 2). During the initial 15 cycles conversions were very similar to those for the same particle size prepared with the HV hydrated lime. However, during the subsequent 15 cycles with the crushed residue, the conversions did not continue to decrease (as seen in Figure 2); in fact they increased slightly. This is confirmation of the influence of particle size on pellet CO2 capture performance in looping cycles. It can also be seen that conversions in Figure 4 after the crushing are lower than those seen for smaller particles after the 15th cycle (Figure 2). This means that differing sorbent changes, most likely morphological, occurred during cycling of different particle sizes, and conversions after crushing are not determined solely by particle size. VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Carbonation conversions during CO2 cycles in TGA of pellets: hydrated spent sorbent from a pilot plant FBC reactor (sample designated as Carbonator no. 4 (19)) and 10% of CA-14 or Secar 80 cements, particle size ∼2 mm (S2), and ∼4 mm (S4). Similar CO2 carrying capacity was found for pellets prepared from KT limestone (results are presented in SI Figure S1). The qualitative influence of particle size, cement used and pretreatment is similar to that seen for CD- and HVbased pellets. Previous research (19, 28) on hydration of spent sorbent from CO2 looping cycles showed that sorbent powder and fragile sorbent particles were obtained, which are unlikely to be suitable for FBC systems. Hence, we also prepared pellets from a spent and hydrated sorbent sample and tested them for CO2 carrying capacity. The spent sample from the carbonator of a pilot-scale dual FBC reactor (19) was obtained after 7 h and 35 min operation and pelletized here with 10% cement. Figure 5 shows the results, in the case of CA-14 cement; after 30 cycles conversions of ∼40% were achieved for larger S4 spheres and >45% for the S2 sample. The influence of particle size and pretreatment on carbonation conversion is similar to that seen for pellets prepared from fresh hydrated lime. However, noticeably lower conversions were obtained for pellets prepared with Secar 80. A possible explanation for this result is the higher Al2O3/CaO ratio in this cement, suggesting that more CaO from limestone is bound as mayenite to satisfy its stoichiometry. However, the main results seen from Figure 5 are high conversions, significantly higher than those for natural limestones. CO2 concentration during carbonation in this study was 100% to ensure carbonation at 850 °C. However, Dennis and Pacciani (34) reported that significantly higher conversions were obtained with synthetic CaO/Ca12Al14O33 sorbent when higher CO2 concentrations were used. Thus, the CO2 carrying capacity of some samples produced here was determined using 15% CO2 in the carbonation stream. The temperature was decreased to 750 °C to ensure carbonation occurred. The typical sample mass and conversion changes during 35 cycles are presented in Figure 6. The comparison of conver-

sions when 15 and 100% CO2 were used for carbonation was presented in SI Figure S2. It can be seen that sorbent performance after 30 cycles is even better than that when 100% CO2 was present. These results are consistent with those that show that natural CaO-based sorbents give better performances at lower CO2 concentration/temperature (37). This can be explained based on the hypothesis that sintering causes loss of sorbent activity during cycles (21). It is expected that reduced temperature and CO2 concentration during carbonation result in less pronounced sintering, and consequently, in better sorbent performance with longer series of cycles. Moreover, a very interesting result from Figure 6 is the high conversion, 67%, which is almost constant from the 15th to the 30th cycle. These conversions are comparable to, or even higher than, those obtained with synthetic CaObased sorbents produced in other studies (31-34). To explain the superior CO2 carrying capacity of pellets prepared with calcium aluminate cements, further XRD and SEM studies were done. The results of XRD quantitative analysis of the pellet sample before and after CO2 capture cycles in the TF are presented in SI Table S2 and corresponding XRD spectra can be seen in SI Figure S3. Here, the main alumina compound after cycles is mayenite, 23.1%. Some alumina is also present in calcium aluminum silicate (Ca12Al10Si4O35, 4.2%), but this most likely comes from impurities in limestone. Mayenite formation during cycles is an interesting result and provides a simple explanation for the superior performance of pellets prepared using calcium aluminate cements (31-34). The ratio of limestone to cement used in this study was 9:1. The content of Al2O3 in the cements used was 70-80% (with exception of Secar 51, ∼50% Al2O3), meaning that the Al2O3 content in the calcined pellets is ∼10%. Thus, the maximum content of mayenite in the calcined pellets is ∼20%, implying total conversion of alumina compounds to mayenite (23.1%, SI Table S2). In other words, using the pellet preparation procedure presented in this study, the resulting sorbent produced during the CO2 looping cycles becomes a mixture of CaO and Ca12Al14O33. The final CaO/Ca12Al14O33 ratio is ∼80:20, or somewhat lower, depending on the impurity content in the limestone and cement. This composition is in the range of those for synthetic CaO/Ca12Al14O33 sorbents recently discussed in the literature (31-34). However, the important difference for practical application of the pellets prepared in this study is that the approach used involves a much easier preparation procedure and uses naturally abundant precursors or spent residue from CO2 cycles, and inexpensive, commercially available binders. It can be calculated that, apart from the CaO present in the cement compounds, additional CaO from limestone is needed for total conversion of the Al2O3 in the cement to mayenite. Taking into account that CaO bound in mayenite is inactive, actual conversion of CaO available for carbonation

FIGURE 6. Sample mass change during CO2 cycles in TGA of pellet: hydrated CD lime and 10% CA-14 cement, particle size ∼1 mm. Conditions: carbonation in 15% CO2, calcination in 100% N2, both for 10 min, isothermally at 750 °C. 7120

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must be up to 10% higher. For example, conversions from the 15-30th cycles in Figure 6 may be up to 74% if the maximum amount of mayenite was formed. However, practically speaking, the most important property is the amount of CO2 captured per sorbent mass, which is 8.5 mol CO2/kg pellet in the case of the experiment presented in Figure 6. For comparison, Li et al. (31) reported ∼5 mol CO2/ kg sorbent (carbonation: 30 min, 690 °C, 14% CO2; and calcination: 10 min, 850 °C, 100% N2), and Martavaltzi and Lemonidou (33), after optimization of the synthesis procedure, obtained ∼7 mol CO2/kg sorbent (carbonation: 30 min, 690 °C, 15% CO2; and calcination: 10 min, 850 °C, 100% N2). The morphology of pellets was investigated by SEM and characteristic images obtained at 2000× and 20 000× magnification are presented in SI Figure S4. The images were taken from the interior of broken pellet spheres before (a and b) and after 30 CO2 cycles (c and d). Similar sorbent morphology can be seen before and after CO2 cycles, and two types of pores are present: large macropores on the 1 µm scale and mesopores on the 10-100 nm scale (SI Figure S4b and d). These nanosized pores, which did not disappear during repeated reaction cycles, are responsible for carbonation conversion because they are the main contributor to the sorbent surface area and micro- and meso-porosity necessary for storage of more voluminous product, CaCO3. Thus, we conclude that the superior performance of CaO-based pellets is due to the uniform dispersion of mayenite. This binder provides a stable nanosized framework among pellet particles, which retards the sintering of active CaO sites represented by nanosized subgrains (31-33). This also explains the better performance of pellets obtained from hydrated lime than those prepared from powdered limestone (Figure 1). Namely, uniform mixing on the nanoscale level is more difficult when powdered limestone particles of size ∼10 µm are used instead of Ca(OH)2-gel. The enhanced performance of sorbents with nanosized grains has also been reported for flame-made CaO-based nanosorbents (38). The encouraging property of the pellets prepared here is their high particle strength, noted during handling/ cycling, which is suitable for FBC systems. The use of inexpensive natural materials such as limestone ($10/t) or spent lime-based sorbent (that is practically valueless, as it incurs a disposal cost) can provides us with very low costs for these pellets. Calcium aluminate cements are also relatively inexpensive binders when used at the industrial scale ($1200-1300/t). It has been assumed here that an optimal limestone/cement ratio is 9:1. Thus, including costs for limestone calcination, hydration, shaping, and drying of pellets, sorbent costs should not be higher than $150/t, i.e., ∼$200/t of calcined pellet ready for CO2 capture. To meet a price of ∼$10 of processed sorbent/t of captured CO2 (8), enabling a very competitive cost for avoided CO2 (∼$20/t) (5, 8), pellets should last ∼60 cycles with an uptake of ∼350 kg CO2/t. The results presented here suggest that this is in fact achievable. However, loss of activity due to sulphation and attrition should be minimized.

Acknowledgments This work was funded by the Panel on Energy Research and Development (PERD), Natural Resources Canada. We also gratefully acknowledge Dr. Dale Zacherl, Almatis, Inc., and Dr. Charles Alt, Kerneos, Inc. for supplying samples of calcium aluminate cements, and for a number of useful suggestions about the uses of these cements.

Supporting Information Available Elemental compositions of the limestone used, XRD information on the pellet compositions, additional results on carbonate conversions and SEM of the pellets. This material

is available free of charge via the Internet at http:// pubs.acs.org.

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