CO2 Capture by Using Blended Hydraulic Slag Cement via a Slurry Reactor

Mitigation and adaptation are viable strategies for resolving climate change issues which may pose significant challenges to both ecosystems and human populations around the world. Aqueous carbonation is a promising process for mitigating CO2, due to the permanent storage of gaseous CO2 into carbonate precipitations (CaCO3 and/or MgCO3). In this study, aqueous carbonation of blended hydraulic slag cement (BHC) for CO2 sequestration was investigated and evaluated under various operating conditions, i.e., different reaction temperatures and CO2 concentrations, in a slurry reactor. The suspension BHC slurry was strongly alkaline (pH ~11.4) before carbonation, whereas the pH of the slurry dropped rapidly to nearly a weakly acidic solution (i.e., pH ~6.3) after introducing CO2 gas into the reactor. The results show that the maximum CO2 capture capacity was 181 g CO2 per kg BHC at a reaction time of 120 min, a CO2 concentration of 10%, and a gas flow rate of 2.5 L/min at 65°C. The reaction temperature slightly influenced the carbonation conversion of BHC, with an increasing temperature resulting in relatively higher conversion. In addition, the SEM and XRD results suggest that the BHC should be carbonated with CO2 to form CaCO3 in a slurry reactor. It was thus concluded that the CO2 could be successfully captured by the carbonation of BHC in this manner. Furthermore, the experimental data were utilized to determine the rate-limiting mechanism based on the shrinking-core model (SCM), which was validated by the observations of SEM images. The SCM results indicate that the overall carbonation reaction of BHC in a slurry reactor was controlled by the ash-layer diffusion mechanism.


INTRODUCTION
Since the first Industrial Revolution in 18 th century, the increasing atmospheric concentration of greenhouse gases (GHGs), e.g., CO 2 , CH 4 , N 2 O, has threatened the global environment and led to global warming.The noxious gases produced from industrial and fire power plant boilers and the increasing usage of fossil fuel resulted in more GHG emissions.However, reducing the usage of fossil fuel, implementing carbon capture, utilization, and storage (CCUS) technologies in the industrial process are regarded as practical approaches in reducing CO 2 emission.Post-combustion capture technologies include absorption process using aqueous absorbents (e.g., MEA, PZ), adsorption process by solid adsorbent (e.g., natural ore, alkaline wastes), ionic liquid (IL), metal organic frameworks (MOFs), membrane separation, enzyme-base system, and bio-algae capture (Pan et al., 2012;Yu et al., 2012).For instance, accelerated carbonation is one of the promising processes of the CCUS technologies by using the alkaline wastes such as BHC produced from the steel manufacturing industry.The reacted products from the carbonation process could be further utilized as the construction materials.However, utilization of alkaline solid wastes must comply with environmental regulations and standards, for protecting the public health and environmental quality.
Carbonation of alkaline solid waste has been proved to be an effective way to capture CO 2 as well as to eliminate the contents of Ca(OH) 2 in solid residues, thus improving the durability of concrete blended with the carbonated residues (Pan et al., 2012).There are both chemical and physical changes in the structure and surface characteristics of steelmaking slag properties after carbonation.Fernández-Bertos et al. (2004a) found that the carbonated product is a solid of lower porosity, lower tortuosity, and lower pore area with calcite infilling the pore space.Furthermore, the carbonated particles became coarser due to agglomeration, which might be beneficial for use in aggregate manufacturing (Fernández-Bertos et al., 2004b).
Carbonation reaction could be accelerated in the presence of moisture and/or operated under a high CO 2 concentration.The first reaction step for carbonation is the hydration of gaseous CO 2 to form carbonic acid and the second step is the dissolution of calcium ions from alkaline solid as shown in Eqs. ( 1) and (2), respectively: CO 2(g) + H 2 O (l)  H 2 CO 3 * (aq) (1) (CaO) x (SiO 2 ) y(s) + H 2 O (l)  Ca 2+ (aq) + (CaO) x-1 (SiO 2 ) y(s) + 2OH - (aq) (2) Calcium carbonate can be nucleated and crystallized under a high saturation ratio, i.e., > 1, as shown in Eq. ( 3): The use of industrial wastes, e.g., steelmaking slag, in the carbonation process can accelerate the carbonation reaction due to their high reactivity with CO 2 and typically rich in calcium contents, which are the suitable materials for the carbonation reaction.Carbonation is an effective way to improve the durability of concrete because relatively insoluble CaCO 3 is formed from the soluble Ca(OH) 2 .The reaction could permanently bind CO 2 with carbonates while at the same time strengthen the waste material.In addition, the formation of CaCO 3 could immobilize the heavy metals, such as Cd, Pb, and Cr, and minimize the metal leaching from the alkaline solid waste by metal sorption onto the surface of the new formed products.The leachings of Pb, Zn, Cr, Cu, and Mo are markedly reduced upon carbonation for both APC and BA (Fernández-Bertos et al., 2004a;Cappai et al., 2012).
Steelmaking slag, e.g., blast furnace (BF) slag, exhibits cementitious behavior (latent hydraulic activity) and/or some pozzolanic characteristics (reaction with portlandite) due to their chemical and mineral compositions, which can be potentially used as mineral addition in replacement of clinker in blended cements (Kourounis et al., 2007;Kolani et al., 2012).Nowadays, blended hydraulic-slag (BF slag) cements CEM II/A,B-S and CEM III are commonly used in several countries in Central and Northern Europe, although their applications in concrete engineering structures are rare (Giergiczny et al., 2009).Only the grounded granulated BF slag can be used as a partial Portland cement replacement.On the other hand, steelmaking slag can be regarded as a low strength hydraulic material due to the presence of C 3 S, C 2 S, C 4 AF and C 2 F in steelmaking slag which endorses its hydraulic properties (Kourounis et al., 2007).
This investigation intends to evaluate the performance of carbonation conversion of blended hydraulic slag cement (BHC) in a slurry reactor.The effects of the operational conditions, including reaction time, reaction temperature, and CO 2 concentration, on the performance of the carbonation process were evaluated.In addition, the carbonation kinetics was described by the shrinking core model (SCM) and the effective diffusivity was determined for various CO 2 concentrations and reaction temperatures.

Experiment
The aqueous carbonation of BHC was conducted in a slurry reactor that contained cold-rolling wastewater (CRW).The BHC with a diameter of approximately 1 cm was provided by the China Hi-ment Corporation (Kaohsiung, Taiwan).All BHC were grounded and sieved to less than 37 μm for all experiments.The BHC contains an uniform blend of Portland cement and fine granulated blast furnace (BF) slag.The BHC used in this investigation is classified as CEM III/C (~90% BF slag content) according to EN standards (European Standards).Before experiment, BHC was dried at 105°C in an oven and stored in an electronic damp proofer case.The CRW was stored at 4°C in a refrigerator.
The slurry reactor, where fine solid particles were suspended in a liquid solution, is frequently used in chemical and biochemical industries.A schematic diagram demonstrating the carbonation of BHC in a slurry reactor is shown in Fig. 1.The operational parameters investigated in this study include the reaction time (i.e., 0, 5, 10, 20, 40, 80, and 120 min), CO 2 concentration (i.e., 10 vol%, 30 vol%, 100 vol%), and reaction temperature (i.e., 25°C and 65°C).All experiments were performed in a slurry reactor with an L/S ratio of 20:1 (i.e., 17.5 g BHC in 350 mL CRW) at a gas flow rate of 2.5 L/min, a gas pressure of 14.7 psi.When the desired temperature has reached, CO 2 was injected into the reactor continuously at a constant pressure and flow rate.During the whole experiment, temperature fluctuating is under controlled within 5°C difference from the settings.The samples of reacted slurry were taken from the reactor periodically at various reaction times of 5, 10, 20, 40, 80, and 120 min.
After the reaction, the samples of reacted slurry were immediately filtered through a PTFE membrane filter (Millipore, 45-μm pore size and 47 mm diameter) by a vacuum pump, and then the reacted BHC was dried at 105°C in an oven for 3 hr.The conversion of the carbonation products was determined quantitatively by thermogravimetric-differential scanning calorimetry (TG-DSC) and qualitatively by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD).

Physico-Chemical Properties Analysis
The density of the sample was analyzed with a Micromeritrics Accupyc1340 pycnometer.The BET surface area of the BHC was determined using a low-temperature N 2 -adsorption BET apparatus (Micromeritrics ASAP2010).The particle size distribution (PSD) of the BHC in tap water was obtained by laser diffraction (Malvern, Hydro 2000MU), which was adapted from the ISO 13320-1 method, with a range of 0.02 to 2000 μm.In addition, prior to examination of the capacity for CO 2 capture, the chemical properties of BHC were measured in accordance with ASTM method C114 for concrete using XRF (PW2430, Phillips).

TG-DSC, SEM-EDX, and XRD
The carbonation conversions of the BHC before and after carbonation reaction were examined with a TG-DSC (STA6000, PerkinElmer), which determines the sample weight loss at different temperatures as well as the phase transformation of sample.TGA is commonly used in determining the physico-chemical properties of solid wastes (Lekakh et al., 2008;Chang et al., 2012;Wang et al., 2012).
Approximately 15-25 mg aliquots of the BHC samples were placed inside a platinum crucible, and then the samples were heated linearly in the temperature range from 50 to 900°C at a heating rate of 10 °C/min in an inert atmosphere with a nitrogen flow rate of 10 mL/min.In a DSC, the sample and reference cells were heated equally into a temperature regime.When the transformation of sample occurs, the temperature difference between the sample and reference cell appear.The device will increase or reduce the heat input to the sample cell to maintain a zero temperature difference between the sample and reference cell, establishing a "null balance" (Speyer, 1994).The quantity of the electrical energy supplied to the sample cell is usually expressed in terms of energy per unit time (Watts).Morphological investigations of fresh and carbonated BHC were performed with an SEM-EDX (JSM-4500, JEOL), which is a type of electron microscope capable of producing high-resolution two-dimensional images of sample surfaces.The accelerating voltage is setting at 10 kV.The distributions of elements in the fresh and carbonated BHC were also detected using EDX, which is useful in identifying CaCO 3 formed on the surface of the BHC during the carbonation reaction.
The mineral compositions of BHC before and after carbonation were qualitatively identified and characterized by XRD (Bruker D8 Advance).The X-ray source was a Cu anode operating at 40 kV and 40 mA using Cu Kα radiation with a diffracted beam graphite-monochromator.Data were usually collected between 20° and 80° in 2θ with a step of 0.008° and count time of 0.5 s per step.The principle of XRD was that a reflection could be produced based on the Bragg law for constructive interference, and the relative peak height generally is proportional to the number of grains in a preferred orientation.

Degree of Carbonation Conversion of BHC
The degree of carbonation conversion of the BHC before and after carbonation reaction was examined with a TG-DSC.It was noted that the calcium carbonate (reaction product) would typically decompose into calcium oxide and CO 2 within the 500-850°C temperature range (Chang et al., 2011a).Therefore, the weight loss ( .%100% The carbonation conversion (δ CaO ) was defined as the amount of CO 2 actually captured in the dry mass of each sample compared with the calculated theoretical extent of carbonation based on the reactive-oxide content of BHC.It was assumed that calcium-bearing compound should be the major chemical components in the accelerated carbonation reaction.Therefore, the carbonation conversion of BHC was calculated using Eq. ( 5

Physico-Chemical Properties of BHC
Table 1 presents the physico-chemical properties of the fresh BHC which indicates that the major components of the fresh BHC were CaO (52.8%),Al 2 O 3 (8.4%),and MgO (4.72%).Minor amounts of SiO 2 , Fe 2 O 3 , and SO 3 were also observed.The CO 2 -capturing capacity of the BHC is assumed to be attributed mainly to the CaO components.Assuming that all CaO was converted to CaCO 3 , the theoretical capacity of the BHC in removing CO 2 was 415 g CO 2 per kg dry solid.
Figs. 2 (a) and (b) show the weight variations of the fresh and carbonated specimen obtained by using TG-DSC.Between 50°C and 400°C, the TG/DSC curves of fresh BHC show a slight weight decrease and an endothermic phenomenon, which correspond to the sample dehydration.Then DSC curves present an exothermic phenomenon from 400°C to 900°C.According to the TG results, the weight loss of the fresh BHC between 500 to 850°C was insignificant, corresponding to a carbonation conversion of 2.4% based on Eq. ( 5).It was observed that the initial carbonate contents of the fresh BHC could be negligible.Compared to the TGA curves of carbonated specimen, the content of calcium hydroxide was eliminated due to lack of a DTG peak at temperatures between 300°C and 500°C, whereas the CaCO 3 content significantly increased.The DSC curve of the reacted BHC exhibits a small endothermic peak from 740°C to 780°C, which is attributed to the decomposition of the 15.4% of calcium carbonates.In addition, the decomposition of CaCO 3 at higher temperatures (above 800°C) in the carbonated BHC can be neglected due to the lack of a peak above 800°C in the DTG curves.It could be explained by the fact that the dissociation of calcium hydroxide in the fresh BHC occurred after dispersion of the BHC into the solution; thereby forming calcium ions and enhancing the alkalinity of the solution, which is beneficial to carbonation.Figs. 3 (a) and (b) present the SEM-EDX images of the fresh and carbonated BHC, respectively, which indicate that each BHC was observed to possess a smooth surface in the absence of carbon prior to carbonation.Comparisons of the SEM images of the feedstock before and after carbonation showed that cubic particles were adhered to the feedstock after carbonation.The EDX analyses as shown in Fig. 3(b) show that the carbonated BHC is composed primarily of Ca, O, and C. Therefore, the cubic-like particles formed during the carbonation reaction were found to be calcium carbonate (CaCO 3 ) according to the SEM-EDX results.
On the other hand, the mineralogical characterizations of fresh and carbonated BHC were determined based on the XRD patterns shown in Figs. 4 (a) and (b), which reveal that the major crystal of the fresh BHC was identified to be Ca 2 SiO 4 (Larnite), CaSiO 3 (Wollastonite), minor phase of Ca 27 O 45 Si 9 (Hatrurite) and Cu 2.5 Fe 0.5 S 2 (Bornite).In contrast to the XRD results from fresh BHC, CaCO 3 was identified as the primary phase in the reaction products.The peaks in the XRD analysis of the carbonated material appeared at 2θ values of 23.02°, 29.41°, 35.97°, 43.15°, 47.49°, 48.50°, 57.40°, 60.68°, and 64.68°, which are indicative of calcium carbonate.
Based on the SEM-EDX results and XRD patterns, it was determined that the cubic particles were composed of calcite (CaCO 3 ) with a diameter ranging from 1 to 2 μm, which is consistent with the observations reported by Chang et al. (2011a, b).It suggests that the BHC should be carbonated with CO 2 to form CaCO 3 in a slurry reactor for a long-term storage.

Effects of Reaction Time, CO 2 concentration, and Reaction Temperature on Carbonation
Fig. 5 shows the pH profile and the effect of reaction time and CO 2 concentrations on the carbonation conversion of the BHC under various temperatures (i.e., 25°C and 65°C).The suspension solution became strongly alkaline (pH = 11.4) before carbonation, whereas the pH of the slurry dropped rapidly to nearly a weakly acidic solution after introducing CO 2 gas into the system.The solution ultimately stabilized at a pH value of approximately 6.3, which suggests that the carbonation process should consume alkalinity.In addition, the carbonation rate decreased as the reaction time increased.The reaction leveled off after 40 min, indicating that the carbonation reaction had a stationary phase due to the formation of a CaCO 3 and/or SiO 2 barrier, which can effectively block the reactive surface sites and inhibits the release of calcium ions from the BHC.These effects exhibit a limited conversion of CO 2 during the carbonation reaction.Therefore, the maximum carbonation conversion (i.e., 43.6%) of BHC was found to be operated under a CO 2 concentration of 10% and a reaction time of 120 min at 65°C, corresponding to a capture capacity of 181 g CO 2 per kg BHC.
To assess the effective CO 2 concentration for the carbonation of BHC in a slurry reactor, the CO 2 concentration of the inlet gas stream was varied at 100%, 30%, and 10% with a fixed reaction temperature, a pressure of 14.7 psig, and an L/S of 20 mL/g.As shown in Fig. 5, the carbonation conversion of BHC is higher under a lower concentration of CO 2 in the gas stream.A previous study made by Chang et al. (2011b) suggests that a higher pH condition (e.g., > 10) should be beneficial to increase the conversion degree of aqueous carbonation in promoting environmental pollution prevention and mitigation technology.With a constant gas inflow rate, a gas stream in higher CO 2 concentration would rapidly acidify the suspension slurry from alkaline to weakly acid than that in a lower CO 2 concentration, thereby leading to a lower carbonation conversion of BHC.However, it needs more reaction time to reach the maximum conversion of BHC for the gas stream with a lower CO 2 concentration.
The effect of temperature on the carbonation was assessed by varying the reaction temperature to 25°C and 65°C, respectively, with a fixed reaction time of 120 min and a CO 2 pressure of 14.7 psig.As shown in Fig. 6, the carbonation conversion of BHC slightly increased with increasing temperature due to the higher leaching rate of calcium at higher temperatures.The dissolution kinetics of the calcium species could be enhanced by increasing the temperature.However, at higher temperatures, the nucleation and growth of CaCO 3 were retarded due to the decreased solubility of CO 2 which is detrimental to the carbonation reaction.A previous study reported by Chang et al. (2011a) suggests that the CO 2 solubility should be the key factor affecting the carbonation conversion at higher temperatures case.It was evident that the temperature slightly influenced the carbonation conversion of BHC in a slurry reactor, with increasing temperature resulting in a relatively higher conversion.
Many researchers have attempted to capture CO 2 with lower power and chemical usage to optimize the CCUS process.Table 2 presents the comparison of operating conditions and the corresponding results in the literature and this study.The CO 2 capture capacity depends on the CaO contents in the solid wastes, where the BHC exhibits the highest CaO content, i.e., 52-54% among the cement kiln dust (CKD), municipal solid waste incinerator air pollution control (MSWI-APC) ash, and stainless steel slag (SSS).Huntzinger et al. (2009) found that a higher carbonation conversion of CKD could be achieved in the batch absorber operated a long time, i.e, 11,520 min.It was also observed that accelerated carbonation of BHC in an autoclave reactor possessed a relatively higher conversion (69%) at the expense of higher temperature (160°C) and pressure (700 psig) for a long reaction time of 12 hr (Chang et al., 2011b), which, however, would lead to additional CO 2 emissions.On the other hand, the results of BHC carbonation in DI water reported by Chang et al. (2011a) exhibit a carbonation conversion of 44% at 70°C for 2 hr in a slurry reactor, which were similar to those observed in this study.However, the gas flow rate used (i.e., 0.1 L/min) was much smaller than that in this study (i.e., 2.5 L/min).In this study, the highest conversion for the BHC was 43.6%, corresponding to a capacity of 180 g CO 2 per kg of BHC, when the aqueous carbonation was perfor med with 10% CO 2 in the CRW at 65°C in a slurry reactor.CO 2 capture by accelerated carbonation process would enable on-site recycle and reuse of wasted materials.However, the operating conditions must be further understood, since they determine the economic viability of the technology as well as the most favor reaction.In addition, CCS is an energy-intensive process which would consume additional energy and lead to further emissions of CO 2 .Therefore, additional energy consumption from the developed processes should be critically assessed using life cycle assessment (LCA).

Kinetic Modeling of Carbonation Reaction
Shrinking core model (SCM) can be performed to describe the carbonation kinetics during trials of carbonation conversion of calcium oxide (Park et al., 2006;Castellote and Andrade, 2008).The reaction mechanism is assumed to be: (a) film diffusion controls: diffusion of a fluid reactant through the fluid film surrounding the solid particles; (b) ash-layer diffusion controls: penetration and diffusion of the reactant through the layer of solid product until it reaches the surface of the un-reacted core; and (c) chemical reaction controls: reaction over the surface of the core.A research made by Castellote and Andrade (2008), which investigated the carbonation kinetics of cementitious matrixes by SCM, suggested that the chemical reactions are infinitely faster than the CO 2 diffusion through the reacted layer.In addition, the measurements of the carbonated material by the SEM in conjunction with XRD provide evidence indicating the suitability of using the shrinking core model (Chang et al., 2011a).The surface composition and molecular structure were found to be changed in the course of carbonation.The fluid-solid reaction of the following general expression was suggested: The stoichiometric coefficient of unit for the carbonization of CaO (b) was assigned a value of one here and the overall radius of the solid (R) is assumed to remain constant.When the mass transfer of the reactants through the boundary layer at the liquid-solid interface is the rate-limiting step, Eq. ( 7) shows the relationship between the conversion (X B , -) and reaction time (t, s): where ρ B is the molar density of the BHC of 2.77 × 10 -2 mol/cm 3 , R is the particle size of 2 × 10 -3 cm, C Ag is the molar concentration of CO 2(aq) (mol/cm 3 ) which can be calculated by the Henry's law and the Van't Hoff equation (Morel and Hering, 1993), and τ (s) is the time for the complete conversion of a reactant particle to product.When the diffusion of reactants through the ash layer is the rate-limiting step, Eq. ( 8) illustrates the dependence of the reaction time on a conversion and the effective diffusivity of a reactant in the ash layer (D e , cm 2 /s): When the chemical reaction between reactants is the ratelimiting step, Eq. ( 9) shows the relationship among the conversion, reaction time, and the first-order rate constant for the surface reaction (k " , s -1 ): According to the SCM results, the overall carbonation reaction of BHC in a slurry reactor was controlled by the ash-layer diffusion mechanism.Fig. 7 shows the estimated values of D e based on the experimental data by SCM, which indicates that the diffusivity (D e ) increase with increasing temperature.The D e values are in the range of 3.70 × 10 -8 to 1.66 × 10 -6 cm 2 /s with R 2 values ranged from 0.72 to 0.99.The SCM results are consistent with those findings reported in the literature (Lekakh et al., 2008;Chang et al., 2011aChang et al., , 2012)), which suggests that the carbonation of steelmaking slag should be ash-diffusion controlled.Therefore, the challenge of accelerated carbonation is to increase the rate of the ash-layer diffusion and also the mass transfer between CO 2 and liquid solution.From the point-view of process intensification, the rotating packed bed (RPB) process was proven to possess a higher interfacial mass-transfer rate, thereby achieving a relatively higher conversion in a short reaction time (Cheng and Tan, 2011;Chang et al., 2012).However, the rotation of packed bed would consume energy and lead to additional CO 2 emission, which would offset the capacity of the CO 2 -captured by the process.As a result, it suggests that future research work should be focused on the development of CO 2 capture process by combining the slurry reactor with RPB and establishment of the optimal modulus and operating conditions by RSM (Response Surface Methodology) and LCA.

CONCLUSIONS AND RECOMMENDATIONS
CO 2 capture by aqueous carbonation of blended hydraulic slag cement (BHC) was conducted in a slurry reactor that contained cold-rolling wastewater (CRW).The operational parameters including the reaction time (i.e., 0, 5, 10, 20, 40, 80, and 120 min), CO 2 concentration (i.e., 10 vol%, 30 vol%, 100 vol%), and reaction temperature (i.e., 25°C and 65°C) were evaluated.The conversion of the carbonation products was determined quantitatively by TG-DSC and qualitatively by SEM-EDX and XRD.Since the major components in fresh BHC were CaO (52.8%), the theoretical CO 2 -capturing capacity of the BHC was determined to be 415 g CO 2 per kg dry solid.
The rate of carbonation reaction decreased with the reaction time increased and the carbonation process was found to consume alkalinity.Meanwhile, the reaction temperature was found to be slightly influenced the carbonation conversion of BHC in a slurry reactor, with increasing temperature resulting in relatively higher conversion.The unique findings of this research work were summarized as follows: (1) A maximum BHC carbonation conversion of 43.6% was achieved under a CO 2 concentration of 10% and a reaction time of 120 min at 65°C, corresponding to a capture capacity of 181 g CO 2 per kg BHC.
(2) The reaction leveled off after 40 min, indicating that the carbonation reaction had a stationary phase due to the formation of a CaCO 3 and/or SiO 2 barrier, which can effectively block the reactive surface sites and inhibits the release of calcium ions from the BHC.(3) The major crystal of the fresh BHC was identified to be Ca 2 SiO 4 (Larnite) and CaSiO 3 (Wollastonite).Based on the SEM images and XRD patterns, the reacted product in the form of cubic particles were identified to be calcite (CaCO 3 ) with a diameter ranging from 1 to 2 μm.(4) The overall carbonation reaction of BHC in a slurry reactor was found to be controlled by the ash-layer diffusion mechanism determined by SCM.The diffusivity (D e ) values are in the range of 3.70 × 10 -8 to 1.66 × 10 -6 cm 2 /s, which increase with increasing temperature.It was thus concluded that the CO 2 could be successfully captured by the carbonation of BHC in a slurry reactor.It suggests that the overall energy utilization in conjunction with net CO 2 emission from the developed processes should be critically assessed by using LCA.
this range is mainly caused by the release of CO 2 .The amounts of CO 2 captured in dry mass of BHC by carbonation were expressed as shown in Eq. (4):

Table 1 .
Physico-chemical properties of blended hydraulic slag cement (BHC) used in this study.