Comparative Study on the Carbonation-Activated Calcium Silicates as

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Comparative Study on the Carbonation-Activated Calcium Silicates as Sustainable Binders: Reactivity, Mechanical Performance, and Microstructure Yuandong Mu,†,‡ Zhichao Liu,†,‡ and Fazhou Wang*,†,‡ †

School of Materials Science and Engineering and ‡State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China

Downloaded via ALBRIGHT COLG on March 21, 2019 at 21:13:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Calcium silicate minerals can react with CO2 to form calcium carbonate and have been proposed to be a sustainable binder as a potential CO2 sinker. In this study, the carbonation characteristics are comparatively assessed among calcium silicates having different calcium/silica (Ca/Si) ratios and polymorphs (CS, C3S2, γ-C2S, β-C2S, C3S). Calcium silicate compacts exposed to a 100% CO2 environment at a 0.4 MPa pressure were tested for carbonation temperature evolution, degree of carbonation (DOC), mechanical properties, and microstructural characterization. Results indicate γC2S is the most reactive, reaching a DOC of 50% in 24 h, followed by C3S2, CS, β-C2S, and C3S, which generally agrees with the pattern of the cumulative normalized temperature increase. Meanwhile, carbonated β-C2S compact attains the highest compressive strength of 80 MPa in 24 h, followed by γ-C2S, C3S2, and C3S, while CS only reaches 20 MPa. Calcite and aragonite are the preferable polymorphs of calcium carbonate in the carbonated C3S, γ-C2S, β-C2S, and C3S2, while only the carbonation of CS generates vaterite in addition to calcite and aragonite. The unreacted grains coated by a thin rim of calcium-modified silica gels are encapsulated by the continuous calcium carbonates, which composes the skeleton of the carbonated calcium silicates. KEYWORDS: Calcium silicate, Accelerated carbonation, Mechanical property, Carbonation products, Microstructure

■

ambient conditions, which are γ-C2S (calcium-olivine), C3S2 (rankinite, there is no specialized Latin symbols to distinguish polymorphs of C3S2), and β-CS (pseudowollastonite), while C3S has no stable form under ambient conditions. Thus, the distinct polymorph of each calcium silicate will not be denoted except for β-C2S and γ-C2S hereinafter. Usually the most stable polymorph of each kind of calcium silicates also draws more attention as it is easier to be synthesized and might exist in nature. Specifically, C3S and β-C2S acquired by quenching of the corresponding high-temperature phase are hydraulic at ambient conditions, composing more than 80 wt % of ordinary Portland cement clinkers, while γ-C2S, C3S2, and β-CS are nonhydraulic. The carbonation of calcium silicate can be generalized by the following equation, as shown in eq 1. The consumption of CO2 in this process creates a potential way of substituting the traditional cement clinker and benefits the sustainable development of the building materials. Thus, research on the

INTRODUCTION The cement industry is a significant contributor to global CO2 emission mainly from fossil fuel combustion and limestone calcination, which accounts for around 7% of total anthropogenic emission.1,2 According to the International Energy Agency (IEA) Reference Technology Scenario (RTS), the direct carbon emissions from the cement industry will witness a 4% increase by 2050 due to the projected cement growth by 12−23% globally. Thus, there is a pressing need for the sustainable development of the cement industry. This leads to the proposal of a technology roadmap known as the low carbon transition by the IEA and the Cement Sustainability Initiative (CSI) to achieve the goal of 24% reduction in direct CO2 emission below the current level by 2050. One emerging strategy for carbon mitigation is the utilization of carbonation instead of hydration as the hardening mechanism for producing binders, among which calcium silicates are the frequently investigated carbonation-based clinkers. There are essentially four calcium silicates of different Ca/Si molar ratios, namely, tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium disilicate (C3S2), and monocalcium silicate (CS), based on the CaO−SiO2 binary phase diagram.3,4 C2S, C3S2, and CS have polymorphs that are stable under © XXXX American Chemical Society

Received: December 29, 2018 Revised: February 11, 2019 Published: March 4, 2019 A

DOI: 10.1021/acssuschemeng.8b06841 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

type of calcium silicate were measured utilizing the Le Chatelier Flask method. The paste specimens were prepared by mixing the same volume of silicate minerals and 1.5 g deionized water for 2 min and compacted into cylindrical tablets of 20 mm in diameter and 20 mm in height such that the cylinders had the same solid phase volume fraction. The densities of calcium silicates and mix characteristics of the compacts are listed in Table 1.

carbonation-activated binders have gained growing interest recently. H 2O

CaxSi yOx + 2y + xCO2 ⎯⎯⎯→ xCaCO3 + ySiO2 (gel)

(1)

5,6

As the pioneer researchers, Bukowsk et al. made a comparison of the carbonation behavior of compacted nonhydraulic γ-C2S and CS as well as the hydraulic C3S and β-C2S mortar under high CO2 pressure, separately. The results indicated the carbonation rate and strength development are faster in γ-C2S than CS, and β-C2S carbonates slower than C3S initially, but both achieve a compressive strength of more than 50 MPa. The recent research7 revealed that compacted γ-C2S carbonates two times faster than β-C2S but gains a strength much lower when subjected to a 0.2 MPa CO2 pressure. Bodor et al.8 assessed the susceptibility of various calcium silicate minerals subjected to different carbonation scenarios, demonstrating the significance of carbonates composition and morphology on the reaction process. Several studies were conducted9−11 regarding the calcium silicates pastes under an atmospherical CO2 pressure, and findings were reported on the carbonation dynamics, reaction products, and micromechanical properties. At the same time, the commercially available products in the current market using this newly developed clinker technology remain restricted to only one or two calcium silicates and suffer from limitations in different aspects. This creates a need for the enhanced understanding of the carbonation characteristics and mechanical properties among different calcium silicates and their relationships to the microstructure of the carbonated matrix. As a result, this study presents a comprehensive experimental program to compare the carbonation behavior of the above-mentioned typical calcium silicates and their microstructural characteristics of the carbonated matrix in relation to the mechanical properties. The results will hopefully shed more light on the development of calcium silicate-based sustainable binders with the added benefit of low carbon emission.

■

Table 1. Densities of the Calcium Silicates and the Compositions of the Calcium Silicate Compacts type

density (g·cm−3)

mass (g)

water (g)

solid phase volume (cm3)

C3S β-C2S γ-C2S C3S2 CS

3.158 3.268 2.907 2.986 2.963

10.86 11.24 10.00 10.27 10.19

1.50 1.50 1.50 1.50 1.50

3.44 3.44 3.44 3.44 3.44

The cylindrical compacts were placed in a steel-made pressure tank positioned in a constant-temperature chamber of 20 °C to minimize the temperature fluctuation. Additionally, a glass of saturated potassium sulfate solution was put into the tank, aiming to maintain the relative humidity of 98%. CO2 gas of 99.99% purity was introduced to purge the tank with the gas outlet open for 30 s. Then the outlet was shut off, and CO2 was kept pumping in until the pressure in the tank reached 0.4 MPa; this procedure should be finished in 15 s. After that the carbonation curing began, and the carbonated compacts with different curing ages of 1, 5, and 24 h were prepared. Testing Procedures. The particle size distributions of the grinded calcium silicate minerals were obtained utilizing a Malvern Mastersizer 2000 particle size analyzer with ethanol as the dispersion medium. The free lime contents of the minerals were tested by the alcohol−glycerol method.14 The X-ray diffraction (XRD) patterns of powdered samples mixed with 10 wt % corundum as the internal standard were conducted using an Empyrean X-ray diffractometer (Malvern Panalytical) with Cu Kα radiation at 40 kV and 40 mA and the 2θ value ranging from 10° to 70°. The scanning rate was 4°/min. Temperature evolution during carbonation was monitored by a thermocouple inserted in a predrilled perforation (∼5 mm in depth and 2 mm in diameter) on the center of the top surface of the compacted calcium silicate samples. The thermocouple was connected to a data logger for continuous temperature measurement at a rate of 1 points/s. A compacted silica quartz powder was also tested for the temperature evolution as the control. The compressive strength of the carbonated compacts was evaluated using a CMT5105 testing system with a loading rate of 0.5 mm/min, and the average value from 6 individual specimens was reported along with the standard deviation. The bulk elastic modulus was calculated according to the strain−stress curves attained from the strength test. The fragments near the compact surface were collected for morphological observation. The scanning electron microscope (SEM) images of the platinum-coated calcined powders and fractures of carbonated pastes were obtained using a FEI QUANTA FEG 450 ESEM with an accelerating voltage of 5 and 15 kV and a working distance of around 10 mm. The collected fragments for cross-sectional analysis were first embedded in transparent epoxy resin and then polished with diamond auxiliary agents down to 1 μm. The backscattered scanning electron (BSE) images and energy-disperse X-ray spectroscopy (EDS) analysis of the polished samples without platinum sputtering were obtained at the low-vacuum mode with a 20 kV voltage. The remaining fragments were grinded for the following tests. Thermogravimetry coupled with a differential scanning calorimeter and mass spectrum analyzer (TG-DSC-MS) was used to analyze the carbonated pastes with a heating rate of 10 °C/min from 50 to 1000 °C under a nitrogen atmosphere. The degree of carbonation (DOC)

MATERIALS AND METHODS

Materials and Preparation. The stoichiometric composition of C3S, β-C2S, γ-C2S, C3S2, and CS was produced from analytically pure reagents Ca(OH)2 and SiO2 (Sinopharm Chemical Reagent Co., Ltd.), except for β-C2S with an addition of 0.5 wt % of H3BO3 as the crystal stabilizer. The chemical compositions of Ca(OH)2 and SiO2 can be seen in the previously published papers.12,13 The weighed powders were sufficiently homogenized in a urethanes jar mill for 3 h, followed by blending with 10 wt % ethyl alcohol and compaction into plates. The plates were fired in a furnace with a heating rate of 10 °C/min to different temperatures and different holding durations corresponding to the type of calcium silicate. The calcining temperatures were 1500, 1400, 1400, 1350, and 1420 °C, and the holding spans were 3, 4, 3, 3, and 2 h, respectively, for C3S, β-C2S, γ-C2S, C3S2, and CS. Additionally, the sintered plates of γ-C2S, C3S2, and CS were furnace cooled, while the plates of C3S and β-C2S were removed and cooled rapidly by cold air. In particular, the sintered plates of C3S were grinded, compacted, and calcined repeatedly an extra two times. All of the sintered plates of calcium silicates were powdered by a vibratory cup mill, except the self-pulverized γ-C2S, which was grinded by a zirconia ball mill, until most of the powders passed the 74 μm sieve. Specimen Fabrication for Carbonation. Given that the initial carbonation rate is controlled by the sample porosity which decides the diffusion of CO2, it is essential to maintain the same shape and solid volume content in sample preparation. The densities of each B


Research Article


Figure 1. Particle size distribution of the grinded calcium silicates powders: (a) differential volume; (b) cumulative volume. was determined according to the loss on ignition (LOI) of the powdered carbonated samples from 300 to 1000 °C. Calculation of the DOC can be found in the previously published article.12 The powdered samples were mixed with 10 wt % corundum as the internal standard. The following XRD patterns were attained based on the same parameters as that of the synthesized calcium silicate minerals. The phase assemblage was analyzed using MAUD software. The average saturation porosity from 3 individual carbonated compacts was tested using a vacuum saturation apparatus. The weight and volume of the whole cylindrical compacts were registered prior to oven drying at 105 °C for 24 h until near-constant weight was achieved. Then the samples were placed in a glass vessel in a vacuum desiccator filled with kerosene for 3 h. After 12 h the samples were taken out and kerosene was wiped off the surface. The porosity was calculated based on the following formula

P = ((msat − mdry )/ρker )/Vcyl

(2)

where P is the saturation porosity (%), msat and mdry are the sample weight after kerosene saturation and drying, ρker is the density of kerosene, and Vcyl is the volume of the cylindrical sample.

■

RESULTS AND DISCUSSION Characterization of Calcium Silicates. The particle size distributions of the grinded calcium silicates are illustrated in Figure 1. A similar distribution pattern is noted between different calcium silicates and the D50 (the intercept for 50% of the cumulative volume) of C3S, β-C2S, γ-C2S, C3S2, and CS are 10.4, 14.7, 14.4, 10.9, and 13.83 μm, respectively. The comparable particle size distribution justifies the comparative nature of this study by eliminating the size effect on the carbonation reactivity and mechanical properties presented hereafter. The specific surface area (SSA) of the powders measured by the Blaine air-permeability apparatus is 427.9, 382.6, 309.3, 344.7, and 396.6 m2/kg for C3S, β-C2S, γ-C2S, C3S2, and CS, respectively. The free lime contents of the calcium silicates are 3.0, 2.1, 0.2, 1.7, and 0.2 wt %, respectively. The XRD patterns of the synthesized calcium silicates blended with 10 wt % corundum as the internal standard are shown in Figure 2. The patterns match well with the corresponding standard diffraction patterns, except for the difference in the relative height of the main peaks of γ-C2S. This difference may be attributed to the dissimilarity in the existence of foreign ions or the sintering regime. The refined results of the XRD data indicate that the amorphous contents of C3S, β-C2S, γ-C2S, C3S2, and CS are 23.9, 25.2, 21.7, 15.9, and 12.9 wt %, respectively.8 Figure 3 shows the SEM images of the grinded silicate minerals. Owing to the self-pulverization property in the phase

Figure 2. XRD patterns of synthesized calcium silicates blended with 10 wt % crystalline corundum.

transformation from β-C2S to γ-C2S, the slowly cooled γ-C2S breaks down into powders with D50 of 17.3 μm. Thus, mild grinding by a ball mill is sufficient to crush the incompletely pulverized particles while preserving the cleavage surface of γC2S (Figure 3c). At the same time, other calcium silicates remain as the compacted tablets after calcination, and a vibratory cup mill is needed to grind them to powders as seen in Figure 3a, 3b, 3d, and 3e. This harsh grinding procedure creates a smooth surface morphology and leads to generation of very fine components, which is demonstrated by the shift of the differential size distribution to the fine fraction region (Figure 1a). Carbonation Reactivity of Calcium Silicates. Carbonation of calcium silicates is highly exothermic;2,15 thus, the heat signal is an effective metric in evaluating the carbonation reactivity. However, accurate measurement of heat release in an adiabatic condition remains experimentally challenging at present due to the continuous replenishment of fresh CO2 of a lower temperature and the latent heat associated with moisture evaporation.12 In this work, the carbonation reactivity of the five calcium silicates compacts as well as a quartz compact as C


Research Article


Figure 3. Morphology of grinded calcium silicate minerals.

temperature rise because each type of calcium silicate has its unique calcium content whose dissociation and precipitation as carbonates is responsible for the heat evolution. Thus, the temperature increase curves are normalized according to the Ca/Si ratio of each mineral (Figure 4c). The normalized temperature of C3S2 grows as fast as γ-C2S and reaches the highest temperature of about 40 °C owing to its low Ca/Si ratio, while the temperature peak of C3S is suppressed to around 26 °C, which is much lower than γ-C2S and C3S2. The temperature of β-C2S grows slower than the above minerals, while CS is the slowest. Furthermore, a cumulative normalized temperature increase (short for CNTI) has been calculated by integrating the temperature curve, as shown in Figure 4d. It can be obviously seen that the 60 min CNTI value of γ-C2S is higher than C3S2, and the other three minerals are the lowest. This CNTI is reminiscent of the hydration heat during cement hydration and can be used to represent the total heat release of the calcium silicate minerals. It should be noted that carbonation is a CO2 penetration-controlled process. Hence, unlike hydration of cement, carbonation in different depths does not proceed simultaneously. The carbonation in the compact surface, where the thermocouple was positioned, precedes the interior. This explains why the temperature drops after reaching the peak (Figure 4c). The low decreasing speed

an inert benchmark was investigated by the temperature evolution, as shown in Figure 4a. There is initially a short-lived temperature increase in samples when CO2 is pumped into the purged container, which is mainly attributed to the extra work from CO2 pumping. This is demonstrated by the temperature rise in the inert quartz sample. Once CO2 pressure reaches and maintains 0.4 MPa, two processes occur simultaneously that affect the sample temperature: the heat exchange between the sample and the environment that causes a temperature decrease which is indicated in the quartz sample and the exothermic carbonation reaction in the calcium silicate samples. The measured temperature is indicative of the two superimposed effects as shown in Figure 4a. For better illustrating the carbonation reactivity of the five silicates, the temperature increase associated with the heat liberation of the carbonation reaction is calculated by subtracting the temperature of the quartz benchmark. As shown in Figure 4b, the five calcium silicates initiate the carbonation with dissimilar intensities of reaction in the sequence of γ-C2S ≈ C3S > C3S2 > β-C2S > CS based on the peak value and change rate in the temperature rise curves. Additionally, it is inaccurate to estimate the carbonation heat liberation rate of calcium silicates by the measured absolute D


Research Article


Figure 4. Carbonation temperature evolution of the five types of calcium silicate compacts: (a) actual temperature of the silicates and quartz; (b) temperature increase of the silicates; (c) normalized temperature increase; (d) cumulative normalized temperature increase.

of β-C2S and CS temperature in Figure 4a−c and their continuous CNTI increase in the later stage demonstrate that the carbonation of β-C2S and CS lasts longer than the others, although they have lower reaction intensities. To further investigate the reaction degree of the whole sample, the DOC of the five silicate compacts subjected to various carbonation durations were tested. DOC indicates the percentage of the measured CO2 uptake to the theoretically maximum uptake of a certain calcium silicate.12,16 The TGDSC-MS curves of five carbonated calcium silicates illustrated in Figure 9 in the Characterization of Carbonation Products section indicate the absence of hydration product − Ca(OH)2, whether for hydraulic or nonhydraulic minerals. In consequence, as described in the Testing Procedures section, the LOI from 300 to 1000 °C was treated as the total escaped CO2, by which the DOC could be calculated. As illustrated in Figure 5, the DOC increases with the elapse of carbonation time, characterized by the rapidity in the very early stage. In the initial 1 h, γ-C2S reaches more than 40% DOC while the DOC of C3S reaches more than 20%. This demonstrates that carbonation is a very intense process in the early stage. After 1 h carbonation, the DOC development of all of the calcium silicates tapers off gradually. The highest DOC at 24 h is achieved by γ-C2S at roughly 50%, while C3S reaches the lowest DOC at about 25%. The reason why C3S carbonates much less is more likely to be associated with its carbonated structure. During the carbonation process, C3S particles are likely to release more Ca ions than the other minerals, which will form a carbonate shell that blocks the interior of the compacts from further carbonation, while CS probably has the least blocking effect. This can also be reflected by the BSE

Figure 5. DOC of carbonated calcium silicate compacts.

images in Figure 11 that the loose structure of carbonates in CS is more favorable for CO2 penetration. Generally, the order of the five calcium silicates with respect to DOC is similar to that of CNTI in Figure 4d. It has to be mentioned that the calculation of the DOC has been normalized in the sense that it has already taken into consideration the CO2 absorption capacity of a calcium silicate mineral. Additionally, given the comparable compactness of the samples during fabrication, the penetrating rate of CO2 is alike. Thus, the DOC of the compact can be regarded as the integration of a numerous concentric cylinders of CNTI, which explains the analogy in the sequences of calcium silicates in DOC and CNTI. Mechanical Properties of Calcium Silicate Compacts. The compressive strength evolution of the five calcium silicate E


Research Article


compressive strength with carbonation time generally enhances the deformation resistance for each of the minerals with a few deviating results on account of the relatively high scattering of the stress−strain curves among individual samples. However, there is no significant difference in this mechanical indicator among carbonated C3S, γ-C2S, β-C2S, and C3S2, while carbonated CS sample exhibits the lowest resistance to deformation. It can be deduced that once the construction of the basic carbonate skeleton (Figure 12) is completed, the resistance to deformation has been established. The elastic modulus varies according to the different scales of the measurement or different sample sizes.18,19 To evaluate the deformation resistance of carbonated calcium silicate matrix, the ordinary Portland cement paste was made as a comparison. The pastes were cast to a size of 20 × 20 × 20 mm with a water/cement ratio of 0.4 and hydrated for 28 days at 98% relative humidity and 20 °C. The average elastic modulus of cement paste is tested to be 3.7 GPa with an average compressive strength of 73.4 MPa. Therefore, it is concluded that the carbonated calcium silicate compacted paste could achieve a high early strength compared to the ordinary Portland cement paste with a similar deformation resistance. Characterization of Carbonation Products. Phase assemblage (weight percentage) of the carbonated calcium silicate is shown in Figure 8. It can be seen that there are three kinds of main phases in the carbonated samples, namely, unreacted calcium silicates, crystalline CaCO3, and the amorphous phase. Calcite and aragonite are the preferable polymorphs of CaCO3 in the carbonated C3S, γ-C2S, β-C2S, and C3S2, while vaterite only exists in the carbonated CS in addition to calcite and aragonite. Their relative amount varies among the five minerals. For β-C2S, C3S2, and CS, aragonite accounts for most of the crystalline proportion of CaCO3 and calcite is the major crystalline CaCO3 in the case of C3S, while for γ-C2S, aragonite and calcite take approximately 46%. Additionally, the species and relative proportion of crystalline CaCO3 remain almost constant with the proceeding of the carbonation reaction. The negligible presence of vaterite in the carbonation products might be attributed to the fact that vaterite is the least stable among the three varieties and is a precursor to the other two polymorphs.20 The sharp temperature rise associated with the fierce carbonation reaction in C3S, β-C2S, γ-C2S, and C3S2 (Figure 4) may facilitate the conversion of vaterite to calcite or aragonite under exposure to water.21 It is speculated that the high temperature associated with carbonation of C3S and γC2S is more beneficial to formation of calcite or the transformation from aragonite to calcite given the fact that calcite is the most thermodynamically stable polymorph, while aragonite could be obtained at a subhigh temperature, corresponding to C3S2 and β-C2S carbonation.22,23 The amorphous phase is undetectable in the XRD patterns.7,11 The amorphous content before carbonation probably indicates the phase exists among the crystalline grains, which has a similar Ca/Si ratio and is distinguishable from the BSE images.24 After carbonation, the amorphous content increases with the elapse of carbonation time. The amorphous calcium silicates are speculated to be carbonated along with the crystalline calcium silicates. However, the generation of amorphous carbonation products covers up the evolution of amorphous silicates. The amorphous increment is reported to mainly consist of calcium-modified silica gel and

compacts with the elapse of carbonation time is illustrated in Figure 6. There is initially a rapid increase in compressive

Figure 6. Compressive strength of carbonated calcium silicate compacts.

strength, consistent with the DOC results in Figure 5. All of the carbonated minerals except CS reach a compressive strength of around 45 MPa in 1 h, almost 4 times as high as that of CS. The 1 h compressive strength accounts for 83%, 61%, 60%, 61%, and 62%, respectively, for γ-C2S, β-C2S, C3S, C3S2, and CS with respect to the strength after 24 h. At 24 h, carbonated β-C2S achieves the highest strength of 80 MPa, followed by γ-C2S, C3S2, and C3S, while the strength of carbonated CS only averages around 20 MPa. The strength-gaining sequence of the five calcium silicates is different from the DOC. This might be attributed to the following reasons: First, the high Ca/Si-ratio minerals will generate more carbonation products than the low Ca/Si-ratio counterparts at the same DOC, resulting in a denser matrix and a higher mechanical strength. Moreover, the carbonation products of each mineral might be different in terms of composition and distribution, leading to the various matrix structures. This will be discussed in the following sections regarding characterization of the carbonation products and microstructure. The bulk elastic modulus evaluates the resistance of an object to the elastic deformation when a stress is loaded.17 It can be computed as the slope of the stress−strain curve in the elastic deformation segment. The bulk elastic modulus of carbonated calcium silicate compacts subjected to different carbonation spans is shown in Figure 7. The growing

Figure 7. Elasticity modulus of carbonated calcium silicate compacts. F


Research Article


Figure 8. Phase assemblage of different calcium silicates carbonated for various durations (a−e), and example of XRD curve fitting by Rietveld refinement (f).

silica gel, reflected by the peak at about 100 °C in the H2O MS curve. The mass loss at the other two high-temperature ranges corresponds to the two decomposition stages (marked as Stage I and Stage II) of calcium carbonates, revealed by the peaks in the CO2 MS curves. All of the carbonated calcium silicates exhibit two peaks of CO2 release, located in Stage I and Stage II, respectively, with the exception that carbonated CS only shows one peak in Stage II. The emission of CO2 in Stage II is believed to be attributed to decomposition of calcite which is the well-crystalline form. However, there is a debate about whether decomposition of calcium carbonate in Stage I belongs to ACC or metastable calcium carbonates−aragonite and vaterite. One opinion holds that aragonite or vaterite will not be decomposed until it is irreversibly transformed to calcite upon heating and decomposes at a similar temperature as calcite;28 thus, Stage I corresponds to the ACC decomposition. The other opinion denotes that Stage I is more likely to be related to aragonite or vaterite, either of which was found to be

amorphous calcium carbonate (ACC). 9,25 The weight percentage of amorphous carbonation phases is high in carbonated C3S, β-C2S, and γ-C2S but decreases in carbonated C3S2 and CS. This is possibly related to the fact that calciummodified silica gel is produced by the reaggregation of −SiOH groups together with the absorbed calcium ions.26 According to the EDS results in Figure 11 and the related report,27 the Ca/Si ratio of the silica gel region is higher in carbonated C3S, β-C2S, and γ-C2S than that in C3S2 and CS, indicating that the calcium-modified silica gel is probably denser in high Ca/Si ratio minerals, resulting in the high mass percentage of amorphous gel. Figure 9 shows the TG-DSC-MS curves of the minerals carbonated for 24 h with a heating rate of 10 °C/min from 50 to 1000 °C under a nitrogen atmosphere. The mass losses are mainly located in three temperature ranges: 50−200, 400−600, and 600−800 °C. The mass loss before 200 °C is related to the evaporation of free water and dehydration of calcium-modified G


Research Article


Figure 9. TG-DSC-MS curves of carbonated calcium silicates.

decomposed before being transformed into calcite.29,30 In this study, Stage I is more likely to be a result of ACC decomposition, which can be partially substantiated by the QXRD results in Figure 8. Aragonite is predominant in β-C2S, C3S2, and CS; thus, this polymorph would have produced a significant mass loss in Stage I if it were decomposed before 600 °C, which is not consistent with the measured mass loss results in Figure 9. It is also interesting to note that there is no appreciable presence of chemically bound water (i.e., the water in calcium hydroxide) at temperatures above 200 °C in C3S and β-C2S, which are also very hydration active. This can be readily explained by the immediate carbonation of the hydration products upon their formation.15 Moreover, the CO2 releaseinduced mass loss between 300 and 1000 °C is used to calculate the DOC as shown in Figure 5. The total amounts of CaCO3 formed could also be calculated. The CaCO3 contents after 24 h carbonation for C3S, β-C2S, γ-C2S, C3S2, and CS are 30.9, 33.9, 46.2, 35.8, and 28.7 wt %, respectively. The contents are a bit higher than the crystalline CaCO3 contents calculated by XRD refinement, indicating the existence of amorphous CaCO3. The abundant CaCO3 contents guarantee the resistance to compression, as CaCO3 forms the skeleton of the matrix, which will be testified in the next section. However,

the content of CaCO3 is not in proportion to the compressive strength for the carbonated β-C2S and γ-C2S compacts. This might be owing to the following three reasons. First, as can be seen in Figure 10, the matrix of γ-C2S compact is not as dense as β-C2S, resulting in the low contact force of the skeleton. Second, the carbonation of β-C2S compacts is conceived to be more intense at the surface than the interior, deduced from the saturation porosity results in Figure 13. Third, the strength of the unreacted particles which act as aggregates may be lower in γ-C2S than β-C2S, since γ-C2S is much easier to be crushed than β-C2S. The three polymorphs of CaCO3 have been demonstrated to possess distinct morphological structures.31,32 Usually calcite appears in the form of scalenohedral or cubic crystals, aragonite is characterized by a needle-like structure, whereas vaterite is usually spherically shaped. The morphology from the secondary electron SEM of carbonated calcium silicates is depicted in Figure 10. The needle-like aragonite can be obviously seen in carbonated β-C2S, γ-C2S, C3S2, and CS samples. Calcite is found in C3S, γ-C2S, C3S2, and CS samples with a deformed structure because the accelerated carbonation allows for little time for growth of a well-defined crystal structure. The low-content vaterite can hardly be observed in carbonated CS. As indicated in the article,33 the morphology of H


Research Article


Figure 10. Morphology of the fracture surface of different calcium silicates carbonated for 24 h.

sluggish increment of DOC (Figure 5). It is confirmed that the CaCO3 region of γ-C2S and CS is more porous than the other minerals, compared to the more compacted carbonate zone in C3S, β-C2S, and C3S2. In particular, in carbonated CS, the clavate aragonites overlap each other and loosely fill the spaces, which is in accordance with Figure 10e. The compositional values for the average atomic Ca/Si ratio of the carbonation products were characterized by the EDS analysis technique to provide more insight into the structure of the carbonated matrix. The Ca/Si ratios of the unreacted calcium silicates match satisfactorily with the stoichiometric ratio of the corresponding mineral. The Ca/Si ratio of calcium carbonate reaches more than 5 in carbonated C3S and β-C2S and then decreases gradually to 1.45 in carbonated CS. Various values of Ca/Si ratios have been reported for the calciummodified silica gel,6,27,36,37 which can be attributed to the inherently large variability in the output of the EDS analysis and more relevantly the difference in the specimen preparation and carbonation conditions (i.e., the initial water content, CO2 partial pressure, and temperature) . The water to solid ratio in this work is much lower than the others, generating a thinner moisture film on the particle surface. This restricts the mobility of the dissolved Ca and −SiOH and therefore results in the slightly higher Ca/Si ratio. Additionally, the spot size of the X-ray is comparable to the

the carbonate is one of the key factors controlling the strength of the binder. It is worth noting that the matrix of C3S and βC2S is much denser than that of γ-C2S and C3S2, while CS matrix is made of loose products. This explains why γ-C2S generates more carbonates than β-C2S but gains less compressive strength in the end. Microstructure of Carbonated Matrix. The backscattered electron SEM images of polished carbonated calcium silicate matrices were collected at the same magnification (5000×) and are exhibited in Figure 11. Four distinctive zones based on the gray scale are noted. The unreacted mineral particles, the carbonated products CaCO3, the calciummodified silica gel, and the uncompacted voids are designated with a gradual increase in gray levels. The calcium-modified silica gel tends to be distributed at the periphery of the unreacted particles. A sSimilar phenomenon can also be found in Mg-bearing silicates carbonation.34 It has been suggested35 that the silica gel is a result of the migration of Ca and formation of a −SiOH group, followed by their connection and dehydration which forms oxide bonds. The dissociated Ca ions and dissolved CO2 precipitate as CaCO3 in the outer space. The formed inert layer of silica and carbonates acts as a barrier for further carbonation. The reaction progresses only when the calcium ions seep past the passivated layers to reach the carbonate ions. This is reflected by the I


Research Article


Figure 11. Typical backscattered electron images and Ca/Si atomic ratio of the polished surface of the five calcium silicates carbonated for 24 h (U, C, S, and P indicate unreacted calcium silicate, carbonate, calcium modified silica gel, and pore, respectively).

unreacted particles and the surrounding silica gels. Therefore, the mechanical performance of carbonated calcium silicate compacts is strongly dependent on the profile of the calcium carbonate areas. Small pore size, low porosity, abundant, and compact calcium carbonates are the essential traits of high mechanical strength. Furthermore, the polymorph of the calcium carbonate might also have some influence on the mechanical properties. The saturation porosity of carbonated calcium silicates compacts is illustrated in Figure 13. The porosity decreases with the extension of carbonation duration for each calcium silicate compact, demonstrating that carbonation of calcium silicate is a self-densifying process. This characteristic is beneficial for the enhancement of the resistance to erosion from the outside but also impedes CO2 penetration and subsequent carbonation toward the interior. γ-C2S compacts have the lowest porosity, attributed to the fact that γ-C2S is the most carbonation reactive and generates the greatest amount of calcium carbonates that fill the voids among the particles. The porosity of β-C2S is a bit higher than that of γ-C2S. However, β-C2S has the highest compressive strength and a much lower content of CaCO3 than γ-C2S. It is deduced that the carbonation process in compacted β-C2S does not occur as deep as in γ-C2S; thus, further carbonation densifies the surface of β-C2S. This densification would keep strengthening the

width of the silica gel (about a few micrometers); thus, the EDS point analysis results might be influenced by the closely located unreacted minerals of a high Ca/Si ratio. Similarly, the silica gel nearby explains why the Ca/Si ratio of the carbonated zone is not as high as the expected calcium carbonate. In particular, for carbonated CS the loose matrix composed of clavate aragonites has a lower Ca/Si ratio, while for C3S and βC2S with a tight carbonate region the Ca/Si ratio is much higher. To better illustrate the phase distribution of carbonated calcium silicates, the four phases were identified using gray level separation at a magnification of 2500. As exhibited in Figure 12, the green, red, blue, and white regions indicate unreacted minerals, calcium carbonates, calcium-modified silica gels, and pores, respectively. It should be pointed out that the presence of a thin red rim around the unreacted mineral is not an indicator of carbonate but is generated due to the fact that the gray level of medium-gray carbonates is the same as the transition area between light-gray unreacted minerals and dark-gray silica gel. Similarly, the blue rim around the pores indicates the transition zone between carbonates and pores. The distribution of the four phases demonstrates that the skeleton of the carbonated matrix is composed of the continuous calcium carbonates encapsulating the isolated J


Research Article


Figure 12. Distribution of unreacted calcium silicates (green), calcium-modified silica gels (blue), calcium carbonates (red), and pores (white).

sequestration while producing a sustainable nonhydraulic binder that has potential applications in building materials. In this paper, a comparative study was carried out on the carbonation characteristics of five calcium silicates of varied calcium/silica ratios and polymorphs (C3S, β-C2S, γ-C2S, C3S2, and CS). Calcium silicate compacts subjected to accelerated carbonation achieve a high early strength. At 24 h, carbonated β-C2S achieves the highest compressive strength of 80 MPa followed by γ-C2S, C3S2, and C3S, while only 20 MPa is reached for carbonated CS. The 1 h compressive strength accounts for 83%, 61%, 60%, 61%, and 62%, respectively, for γ-C2S, β-C2S, C3S, C3S2, and CS with respect to the strength after 24 h. The rapidity in the mechanical strength development is further demonstrated by the attainment of 50% degree of carbonation (DOC) in 24 h for γ-C2S, followed by C3S2, CS, β-C2S, and C3S. The CO2 reactivity of calcium silicates is evaluated as well by the normalized carbonation temperature increase, which indicates that C3S, γ-C2S, and C3S2 are more reactive than βC2S and CS. The phase assemblage of the carbonated samples by the Rietveld method reveals calcite and aragonite are the two exclusive polymorphs of calcium carbonate in the carbonated C3S, β-C2S, γ-C2S ,and C3S2, while generation of vaterite is also

Figure 13. Saturation porosity of carbonated calcium silicates compacts.

mechanical property but has little influence on the DOC of the whole sample, explaining the sluggish increase in DOC of βC2S after 5 h.

■

CONCLUSIONS Compacted calcium silicate minerals have been found to develop appreciable mechanical strength under a CO2-rich environment. This offers a promising route for CO 2 K


Research Article


(10) Ashraf, W.; Olek, J.; Tian, N. Multiscale characterization of carbonated wollastonite paste and application of homogenization schemes to predict its effective elastic modulus. Cem. Concr. Compos. 2016, 72, 284−298. (11) Ashraf, W.; Olek, J. Elucidating the accelerated carbonation products of calcium silicates using multi-technique approach. J. CO2 Util. 2018, 23, 61−74. (12) Mu, Y.; Liu, Z.; Wang, F.; Huang, X. Effect of barium doping on carbonation behavior of gamma-C2S. J. CO2 Util. 2018, 27, 405− 413. (13) Mu, Y.; Liu, Z.; Wang, F.; Huang, X. Carbonation characteristics of γ-dicalcium silicate for low-carbon building material. Constr. Build. Mater. 2018, 177, 322−331. (14) Swenson, E. G.; Thorvaldson, T. The alcohol-glycerol method for the determination of free lime. Can. J. Chem. 1951, 29 (2), 140− 153. (15) Berger, R. L.; Young, J. F.; Leung, K. Acceleration of Hydration of Calcium Silicates by Carbon Dioxide Treatment. Nature, Phys. Sci. 1972, 240 (97), 16−18. (16) Li, Z.; He, Z.; Shao, Y. Early Age Carbonation Heat and Products of Tricalcium Silicate Paste Subject to Carbon Dioxide Curing. Materials 2018, 11 (5), 730. (17) Hashin, Z. The Elastic Moduli of Heterogeneous Materials. J. Appl. Mech. 1962, 29 (1), 143−150. (18) Ashraf, W.; Tian, N. Nanoindentation assisted investigation on the viscoelastic behavior of carbonated cementitious matrix: Influence of loading function. Constr. Build. Mater. 2016, 127, 904−917. (19) Yang, C. C. Approximate Elastic Moduli of Lightweight Aggregate. Cem. Concr. Res. 1997, 27 (7), 1021−1030. (20) de Leeuw, N. H.; Parker, S. C. Surface Structure and Morphology of Calcium Carbonate Polymorphs Calcite, Aragonite, and Vaterite: An Atomistic Approach. J. Phys. Chem. B 1998, 102 (16), 2914. (21) Zhou, G. T.; Yao, Q. Z.; Fu, S. Q.; Guan, Y. B. Controlled crystallization of unstable vaterite with distinct morphologies and their polymorphic transition to stable calcite. Eur. J. Mineral. 2010, 22 (2), 259−269. (22) Gruver, R. M. Differential Thermal-Analysis Studies of Ceramic Materials: II, Transition of Aragonite to Calcite. J. Am. Ceram. Soc. 1950, 33 (5), 171−174. (23) Santos, R. M.; Ceulemans, P.; Van Gerven, T. Synthesis of pure aragonite by sonochemical mineral carbonation. Chem. Eng. Res. Des. 2012, 90 (6), 715−725. (24) Shang, D.; Wang, M.; Xia, Z.; Hu, S.; Wang, F. Incorporation mechanism of titanium in Portland cement clinker and its effects on hydration properties. Constr. Build. Mater. 2017, 146, 344−349. (25) Goto, S.; Suenaga, K.; Kado, T.; Fukuhara, M. Calcium Silicate Carbonation Products. J. Am. Ceram. Soc. 1995, 78 (11), 2867−2872. (26) Oelkers, E. H.; Declercq, J.; Saldi, G. D.; Gislason, S. R.; Schott, J. Olivine dissolution rates: A critical review. Chem. Geol. 2018, 500, 1−19. (27) Ashraf, W.; Olek, J. Carbonation behavior of hydraulic and nonhydraulic calcium silicates: potential of utilizing low-lime calcium silicates in cement-based materials. J. Mater. Sci. 2016, 51 (13), 6173−6191. (28) Perić, J.; Vučak, M.; Krstulović, R.; Brečević, L.; Kralj, D. Phase transformation of calcium carbonate polymorphs. Thermochim. Acta 1996, 277, 175−186. (29) Maciejewski, M.; Oswald, H. R.; Reller, A. Thermal transformations of vaterite and calcite. Thermochim. Acta 1994, 234 (5), 315−328. (30) Š auman, Z. Carbonization of porous concrete and its main binding components. Cem. Concr. Res. 1971, 1 (6), 645−662. (31) Chang, R.; Choi, D.; Kim, M. H.; Park, Y. Tuning Crystal Polymorphisms and Structural Investigation of Precipitated Calcium Carbonates for CO2 Mineralization. ACS Sustainable Chem. Eng. 2017, 5 (2), 1659−1667. (32) Küther, J.; Seshadri, R.; Knoll, W.; Tremel, W. Templated growth of calcite, vaterite and aragonite crystals onself-assembled

observed in the carbonated CS. The backscattered electron (BSE) imaging of polished carbonated calcium silicate compacts illustrates the calcium carbonates as the skeleton of the carbonated matrix with the encapsulation of unreacted grains peripherally bordered by a thin rim of calcium-modified silica gel taking the filling and strengthening effect. The difference in the compactness of the carbonate zone from BSE imaging and the variation in the relative proportion of crystalline and noncrystalline carbonation products may explain the noted disassociation between compressive strength and DOC. In conclusion, these results will facilitate the selection of calcium silicates for the fabrication of low carbon binder which enables CO2 absorption and uncompromised mechanical performance.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06841. XRD patterns of carbonated C3S, β-C2S, γ-C2S, C3S2, and CS with 10 wt % Al2O3 as internal standard (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 027 87227128. E-mail: [email protected]. ORCID

Fazhou Wang: 0000-0001-9376-5632 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Key Research and Development Program of China (Project 2017YFB0310001) and National Natural Science Foundation of China (No. 51672200) is gratefully acknowledged.

■

REFERENCES

(1) Scrivener, K. L.; Kirkpatrick, R. J. Innovation in use and research on cementitious material. Cem. Concr. Res. 2008, 38 (2), 128−136. (2) Gartner, E.; Hirao, H. A review of alternative approaches to the reduction of CO2 emissions associated with the manufacture of the binder phase in concrete. Cem. Concr. Res. 2015, 78, 126−142. (3) Wesselsky, A.; Jensen, O. M. Synthesis of pure Portland cement phases. Cem. Concr. Res. 2009, 39 (11), 973−980. (4) Taylor, H. F. 1 Portland cement and its major constituent phases. Cement chemistry; Thomas Telford: 1997. (5) Bukowski, J. M.; Berger, R. L. Reactivity and strength development of CO2 activated non-hydraulic calcium silicates. Cem. Concr. Res. 1979, 9 (1), 57−68. (6) Young, J. F.; Berger, R. L.; Breese, J. Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO2. J. Am. Ceram. Soc. 1974, 57 (9), 394−397. (7) Chang, J.; Fang, Y.; Shang, X. The role of β-C2S and γ-C2S in carbon capture and strength development. Mater. Struct. 2016, 49 (10), 4417−4424. (8) Bodor, M.; Santos, R. M.; Kriskova, L.; Elsen, J.; Vlad, M.; Van Gerven, T. Susceptibility of mineral phases of steel slags towards carbonation: mineralogical, morphological and chemical assessment. Eur. J. Mineral. 2013, 25 (4), 533−549. (9) Ashraf, W.; Olek, J. Carbonation activated binders from pure calcium silicates: Reaction kinetics and performance controlling factors. Cem. Concr. Compos. 2018, 93, 85. L


Research Article

ACS Sustainable Chemistry & Engineering monolayers of substituted alkylthiols on gold. J. Mater. Chem. 1998, 8 (3), 641−650. (33) De Silva, P.; Bucea, L.; Moorehead, D. R.; Sirivivatnanon, V. Carbonate binders: Reaction kinetics, strength and microstructure. Cem. Concr. Compos. 2006, 28 (7), 613−620. (34) Béarat, H.; McKelvy, M. J.; Chizmeshya, A. V.; Gormley, D.; Nunez, R.; Carpenter, R.; Squires, K.; Wolf, G. H. Carbon sequestration via aqueous olivine mineral carbonation: role of passivating layer formation. Environ. Sci. Technol. 2006, 40 (15), 4802−4808. (35) Huijgen, W. J.; Witkamp, G.-J.; Comans, R. N. Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process. Chem. Eng. Sci. 2006, 61 (13), 4242−4251. (36) Ashraf, W.; Olek, J.; Jain, J. Microscopic features of nonhydraulic calcium silicate cement paste and mortar. Cem. Concr. Res. 2017, 100, 361−372. (37) Daval, D.; Martinez, I.; Corvisier, J.; Findling, N.; Goffé, B.; Guyot, F. Carbonation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modeling. Chem. Geol. 2009, 265 (1−2), 63−78.

M


Comparative Study on the Carbonation-Activated Calcium Silicates as

Recommend Documents