Carbonation of a Synthetic CAF Compound by CO2 Absorption and Its Effect on Cement Matrix

In the field of construction materials, the development of fundamental technologies to reduce energy consumption and CO2 emissions, such as manufacturing process improvement and the expanded use of alternative materials, is required. Technologies for effectively reducing energy consumption and improving CO2 absorption and reduction that can meet domestic greenhouse gas reduction targets are also required. In this study, calcium–aluminate–ferrite (CAF), a ternary system of CaO·Al2O3·Fe2O3, was sintered at a low temperature (1100 °C) to examine the possibility of CO2 adsorption, and excellent CO2 absorption performance was confirmed, as the calcite content was found to be 11.01% after 3 h of the reaction between synthetic CAF (SCAF) and CO2. In addition, the physical and carbonation characteristics were investigated with respect to the SCAF substitution rate for cement (10%, 30%, 50%, 70%, and 100%). It was found that SCAF 10% developed a compressive strength similar to that of ordinary Portland cement (OPC 100%), but the compressive strength tended to decrease as the SCAF substitution rate increased. An increase in the SCAF substitution rate led to the rapid penetration of CO2, and carbonation was observed in all the specimens after 7 days. As carbonation time increased, the CO2 diffusion coefficient tended to decrease. This is because the diffusion of CO2 in the cement matrix follows the semi-infinite model of Fick’s second law. SCAF can contribute to reduced energy consumption and CO2 emissions because of the low-temperature sintering and can absorb and fix CO2 when a certain amount is substituted.


Introduction
Since the Paris Climate Change Accord in 2015, countries worldwide have set goals to reduce carbon emissions by >45% compared with 2010 and achieve net-zero by 2050 [1,2].South Korea declared "net-zero by 2050" and raised the nationally determined contribution (NDC) in 2030 to 40% compared with 2018.According to a press release from the Ministry of Environment (2022), the national greenhouse gas (GHG) emissions in 2021 are expected to be 679.6 million tons, which is an increase of approximately 23 million tons (3.5%) relative to the previous year.The representative industries that generate GHG emissions are the steel, petrochemical, oil refining, and cement industries.They exceed 79% of all industrial GHG emissions and are important for achieving net-zero targets.In particular, the cement industry emitted 0.87 tons of CO 2 per ton of cement as of 2000, and 0.53 tons of CO 2 are generated by the decarbonization of limestone [3,4].
In the field of construction materials, it is necessary to develop fundamental technologies to reduce energy consumption and CO 2 emissions, such as manufacturing process improvement and expanded use of alternative materials, in addition to technologies for effectively reducing energy consumption and improving CO 2 absorption and reduction that can meet domestic GHG reduction targets.Research has been actively conducted on the use of blast furnace slag, which is a byproduct of the steel industry, and fly ash, which is a byproduct of coal power plants, as cement substitutes [5,6]; however, they fail to develop compressive strength at early ages compared with cement.Concrete carbonation generally reduces the pH of concrete, causing corrosion of the steel reinforcement embedded in the concrete.Therefore, studies have been conducted on the prediction of the concrete carbonation depth and the inhibition of carbonation [7][8][9][10].
Concrete carbonation has effects such as pore-filling in concrete, densification, and compressive-strength improvement because the carbonation product, that is, CaCO 3 , has a higher molecular weight than hydrates [11][12][13].Therefore, positive effects of carbonation on concrete can be expected in ordinary concrete and precast concrete products.Studies have been conducted on the effects of CO 2 curing that artificially uses CO 2 .The representative effects of CO 2 curing include porosity reduction, performance-inhibitor blocking, compressive-strength improvement, shrinkage compensation by carbonation, and CO 2 reduction due to the reaction with CO 2 .Previous studies were limited to ordinary Portland cement (OPC) and focused on the rate of increase in the compressive strength, as they were conducted in the early stage of research on CO 2 curing [14][15][16].C 4 AF, a cement clinker product, underwent a phase change at a sintering temperature of 700-1200 • C and was converted into ferrite in a stable state at the time of cooling after being in an unstable solid solution state with various compositions, such as 2CaO [17].The CaO•Al 2 O 3 •Fe 2 O 3 ternary system is cubic, and its hydra- tion reaction is divided into the C 3 AH 6 -C 3 FH 6 series (garnet-hydrogarnet series) and the C 4 AH n -C 4 FH n series (hexagonal system); it is known that the hydration reaction by water and the carbonation reaction by CO or CO 2 is possible.
Therefore, in this study, we investigated the CO 2 absorption and fixation characteristics of synthetic calcium aluminoferrite (SCAF), which was synthesized at a temperature (1100 • C) lower than the cement sintering temperature (1450 • C), resulting from the carbonation reaction.We further examined the characteristics of the cement matrix and CO 2 diffusion characteristics with respect to the SCAF substitution rate for cement.

Materials
Table 1 presents the physical and chemical properties of the materials used in this study.For the cement, OPC was obtained from Sampyo cement (Samcheck-si, Republic of Korea) and had a density of 3.15 g/cm 3 and fineness of 3.475 cm 2 /g.For the sand used in the mortar, ISO standard sand with a SiO 2 content of ≥90 wt.%, a moisture content of ≤0.2 wt.%, and a fineness modulus of 2.65 was employed.Specific gravity (g/cm 3 ) 3.15 3.52 Figure 1 shows the CAF (mixed in molar ratio; CaO, Al 2 O 3 , Fe 2 O 3 ) sintering process and conditions.The mixture was heated at a rate of 10 • C/min until the temperature reached 1100 • C.After 2 h of sintering at a low temperature (1100 • C), the mixture was cooled in a furnace to room temperature and pulverized with a ball mill to produce SCAF.reached 1100 °C.After 2 h of sintering at a low temperature (1100 °C), the mixture was cooled in a furnace to room temperature and pulverized with a ball mill to produce SCAF.

Carbonation Characteristics of SCAF
A carbonation test was conducted to evaluate the CO2 reactivity of the SCAF.In the test, SCAF was dispersed in water (binder: water = 1:1) to convert it into a slurry state and subjected to a gas-liquid (wet method) reaction with CO2, as shown in Figure 2. The reaction time for CO2 was set at 30 min, 1 h, 3 h, and 6 h.Each time, samples for analysis were collected, and their crystal structures were examined via a qualitative X-ray diffractometer (Rigaku, D/Max-2500 V, Tokyo, Japan scan range: 5 °-80 ° accelerating voltage: 40 kV, 200 mA; scan speed: 2 °/min; target: Cu) analysis (5 °/min).In addition, the hydrates generated through carbonation were subjected to a thermal analysis using an STA409PC Luxx instrument (Netzsch, Selb, Germany).For the thermogravimetry differential thermal analysis (TG-DTA), the temperature was increased at 10 °C/min from room temperature to a maximum value of 1000 °C, and the calcite content was measured in a nitrogen (N2) atmosphere to calculate the amount of adsorbed CO2.

Effects of SCAF Carbonation on the Properties of Cement Matrix
A test was conducted to examine the material properties of the cement matrix and CO2 diffusion for the matrix with the substitution of SCAF by setting the SCAF substitution rate at 10%, 30%, 50%, 70%, and 100%, as shown in Table 2.The cement-to-sand ratio was set at 1:3, and the water-to-cement (W/C) ratio was set at 0.5.

Carbonation Characteristics of SCAF
A carbonation test was conducted to evaluate the CO 2 reactivity of the SCAF.In the test, SCAF was dispersed in water (binder: water = 1:1) to convert it into a slurry state and subjected to a gas-liquid (wet method) reaction with CO 2 , as shown in Figure 2. The reaction time for CO 2 was set at 30 min, 1 h, 3 h, and 6 h.Each time, samples for analysis were collected, and their crystal structures were examined via a qualitative X-ray diffractometer (Rigaku, D/Max-2500 V, Tokyo, Japan scan range: 5 • -80 • accelerating voltage: 40 kV, 200 mA; scan speed: 2 • /min; target: Cu) analysis (5 • /min).In addition, the hydrates generated through carbonation were subjected to a thermal analysis using an STA409PC Luxx instrument (Netzsch, Selb, Germany).For the thermogravimetry differential thermal analysis (TG-DTA), the temperature was increased at 10 • C/min from room temperature to a maximum value of 1000 • C, and the calcite content was measured in a nitrogen (N 2 ) atmosphere to calculate the amount of adsorbed CO 2 .ls 2023, 16, 7344 3 of 11 reached 1100 °C.After 2 h of sintering at a low temperature (1100 °C), the mixture was cooled in a furnace to room temperature and pulverized with a ball mill to produce SCAF.

Carbonation Characteristics of SCAF
A carbonation test was conducted to evaluate the CO2 reactivity of the SCAF.In the test, SCAF was dispersed in water (binder: water = 1:1) to convert it into a slurry state and subjected to a gas-liquid (wet method) reaction with CO2, as shown in Figure 2. The reaction time for CO2 was set at 30 min, 1 h, 3 h, and 6 h.Each time, samples for analysis were collected, and their crystal structures were examined via a qualitative X-ray diffractometer (Rigaku, D/Max-2500 V, Tokyo, Japan scan range: 5 °-80 ° accelerating voltage: 40 kV, 200 mA; scan speed: 2 °/min; target: Cu) analysis (5 °/min).In addition, the hydrates generated through carbonation were subjected to a thermal analysis using an STA409PC Luxx instrument (Netzsch, Selb, Germany).For the thermogravimetry differential thermal analysis (TG-DTA), the temperature was increased at 10 °C/min from room temperature to a maximum value of 1000 °C, and the calcite content was measured in a nitrogen (N2) atmosphere to calculate the amount of adsorbed CO2.

Effects of SCAF Carbonation on the Properties of Cement Matrix
A test was conducted to examine the material properties of the cement matrix and CO2 diffusion for the matrix with the substitution of SCAF by setting the SCAF substitution rate at 10%, 30%, 50%, 70%, and 100%, as shown in Table 2.The cement-to-sand ratio was set at 1:3, and the water-to-cement (W/C) ratio was set at 0.5.

Effects of SCAF Carbonation on the Properties of Cement Matrix
A test was conducted to examine the material properties of the cement matrix and CO 2 diffusion for the matrix with the substitution of SCAF by setting the SCAF substitution rate at 10%, 30%, 50%, 70%, and 100%, as shown in Table 2.The cement-to-sand ratio was set at 1:3, and the water-to-cement (W/C) ratio was set at 0.5.To investigate the compressive-strength characteristics with respect to the SCAF substitution rate, prismatic specimens with dimensions 40 × 40 × 160 mm were prepared in accordance with ASTM C 109 [18].The specimens were subjected to dry curing (20 ± 1 • C, 60% ± 5% RH) until their compressive strengths were measured.The flexural strength was first measured according to age, and then the compressive-strength test was conducted using the broken specimens.In addition, the porosity was measured by mercury intrusion porosimetry to examine the pore structures in the hydrates.The carbonation test was conducted in accordance with KS F 2584 [19] ("Standard Test Method for Accelerated Carbonation of Concrete"), and 40 × 40 × 160 mm specimens for carbonation were prepared by setting the water/binder ratio at 0.5, with a sand/binder ratio of 3. The specimens were subjected to water curing at 20 ± 2 • C for 28 d.They were then cured in a constant-temperature and humidity chamber with a humidity of 60% ± 5% and temperature of 20 ± 2 • C. The pouring surface, the bottom surface, and both sides were coated three times using epoxy resin to block CO 2 .The specimens were subjected to carbonation tests using two methods.First, they were cured for different ages (3, 28, and 91 days) in a vacuum desiccator with a humidity of 60% ± 5%, temperature of 20 ± 2 • C, and CO 2 gas concentration of 100%.Second, they were exposed to the atmosphere, and measurements were performed according to their age (3, 28, and 91 days) to examine the carbo reaction caused by the adsorption of CO 2 in the atmosphere.Regarding the test surface preparation and measurement method, the specimens were cut to a size of 40 × 40 × 10 mm.The cut surface was sprayed with a 1% phenolphthalein solution and the change in color of the surface to purple was measured in accordance with KS F 2596 [20] ("Method for Measuring Carbonation Depth of Concrete"). Figure 3 shows the accelerated-carbonation experimental setup.To investigate the compressive-strength characteristics with respect to the SCAF substitution rate, prismatic specimens with dimensions 40 × 40 × 160 mm were prepared in accordance with ASTM C 109 [18].The specimens were subjected to dry curing (20 ± 1 °C, 60% ± 5% RH) until their compressive strengths were measured.The flexural strength was first measured according to age, and then the compressive-strength test was conducted using the broken specimens.In addition, the porosity was measured by mercury intrusion porosimetry to examine the pore structures in the hydrates.The carbonation test was conducted in accordance with KS F 2584 [19] ("Standard Test Method for Accelerated Carbonation of Concrete"), and 40 × 40 × 160 mm specimens for carbonation were prepared by setting the water/binder ratio at 0.5, with a sand/binder ratio of 3. The specimens were subjected to water curing at 20 ± 2 °C for 28 d.They were then cured in a constant-temperature and humidity chamber with a humidity of 60% ± 5% and temperature of 20 ± 2 °C.The pouring surface, the bottom surface, and both sides were coated three times using epoxy resin to block CO2.The specimens were subjected to carbonation tests using two methods.First, they were cured for different ages (3, 28, and 91 days) in a vacuum desiccator with a humidity of 60% ± 5%, temperature of 20 ± 2 °C, and CO2 gas concentration of 100%.Second, they were exposed to the atmosphere, and measurements were performed according to their age (3, 28, and 91 days) to examine the carbo reaction caused by the adsorption of CO2 in the atmosphere.Regarding the test surface preparation and measurement method, the specimens were cut to a size of 40 × 40 × 10 mm.The cut surface was sprayed with a 1% phenolphthalein solution and the change in color of the surface to purple was measured in accordance with KS F 2596 [20] ("Method for Measuring Carbonation Depth of Concrete"). Figure 3 shows the accelerated-carbonation experimental setup.

Characteristics of SCAF
The SCAF had a density of 3.52 g/cm 3 , and its fineness after pulverization was 3117 cm 2 /g. Figure 4 shows the X-ray diffraction (XRD)-Rietveld analysis results for the SCAF,

Carbonation Characteristics of SCAF
Figure 5 shows the crystalline phase analysis results for the reaction hydrates at different reaction times (30 min, 1 h, 3 h, and 6 h) after the SCAF powder was dispersed in water to convert it into a slurry state and cause the gas-liquid (wet method) reaction with CO2.After 3 h of carbonation, numerous hydrates were generated by the reaction between SCAF and CO2, including calcite, calcium carboaluminate compounds (CAC(Ca4Al2O6CO3•11H2O) and CACH(Ca4Al2O6CO3•11H2O)), and calcium carboaluminoferrite compounds (CFC(Ca4Al2Fe2O12CO3(OH)2•22H2O)).In addition, the peak intensity of lime stopped decreasing as the carbonation reaction time increased.This appeared to be because the CaO that remained in the SCAF compounds contributed to the generation of CaCO3 by the carbonation reaction.In particular, the peak intensity of CaCO3 was strong after 3 h of carbonation.After 6 h of the carbonation reaction, the peak intensity was further increased.The intensities of the peaks corresponding to CAC and CFC crystals, i.e., calcium carbo compounds, increased with the reaction time.This indicated that SCAF generates calcite and calcium carbo compounds through the reaction with CO2 and that the carbo reaction becomes more active over time.Because SCAF generates calcite and calcium carbo compounds by adsorbing CO2 as a result of the carbo reaction, it is applicable as a CO2 absorption and CO2 fixation material.

Carbonation Characteristics of SCAF
Figure 5 shows the crystalline phase analysis results for the reaction hydrates at different reaction times (30 min, 1 h, 3 h, and 6 h) after the SCAF powder was dispersed in water to convert it into a slurry state and cause the gas-liquid (wet method) reaction with CO 2 .After 3 h of carbonation, numerous hydrates were generated by the reaction between SCAF and CO 2 , including calcite, calcium carboaluminate compounds (CAC(Ca 4 Al 2 O 6 CO 3 •11H 2 O) and CACH(Ca 4 Al 2 O 6 CO 3 •11H 2 O)), and calcium carboaluminoferrite compounds (CFC(Ca 4 A l2 Fe 2 O 12 CO 3 (OH) 2 •22H 2 O)).In addition, the peak intensity of lime stopped decreasing as the carbonation reaction time increased.This appeared to be because the CaO that remained in the SCAF compounds contributed to the generation of CaCO 3 by the carbonation reaction.In particular, the peak intensity of CaCO 3 was strong after 3 h of carbonation.After 6 h of the carbonation reaction, the peak intensity was further increased.The intensities of the peaks corresponding to CAC and CFC crystals, i.e., calcium carbo compounds, increased with the reaction time.This indicated that SCAF generates calcite and calcium carbo compounds through the reaction with CO 2 and that the carbo reaction becomes more active over time.Because SCAF generates calcite and calcium carbo compounds by adsorbing CO 2 as a result of the carbo reaction, it is applicable as a CO 2 absorption and CO 2 fixation material.
After SCAF was dispersed in water to convert it into the slurry state and cause the gas-liquid (wet method) reaction with CO 2 , sampling was performed according to the CO 2 reaction time, and TG-DTA was conducted to examine the adsorbed amount of CO 2 .Table 3 presents the weight loss for each of the three temperature sections.The first section corresponds to the weight lost by the decomposition of water, and the second section corresponds to the weight lost by Ca(OH) 2 .Finally, the third section corresponds to the weight loss in the 700-900 • C range, which is the typical thermal decomposition characteristic of calcite generated through the absorption of CO 2 by the hardened body of SCAF.In the first section, corresponding to the weight lost by the decomposition of H 2 O, the loss of water from the generated compound decreased as the carbonation reaction time increased.The amount of Ca(OH) 2 that decomposed in the second section also decreased.However, the amount of thermal decomposition in the third section increased.This indicates that the carbonation of SCAF interferes with the absorption of H 2 O or its reaction in the presence of CO 2 .The weight loss in the third section was caused by the thermal decomposition of CaCO 3 generated by the carbonation reaction, indicating that the amount of CaCO 3 generated increased with the carbonation reaction time.After SCAF was dispersed in water to convert it into the slurry state and cause the gas-liquid (wet method) reaction with CO2, sampling was performed according to the CO2 reaction time, and TG-DTA was conducted to examine the adsorbed amount of CO2.Table 3 presents the weight loss for each of the three temperature sections.The first section corresponds to the weight lost by the decomposition of water, and the second section corresponds to the weight lost by Ca(OH)2.Finally, the third section corresponds to the weight loss in the 700-900 °C range, which is the typical thermal decomposition characteristic of calcite generated through the absorption of CO2 by the hardened body of SCAF.In the first section, corresponding to the weight lost by the decomposition of H2O, the loss of water from the generated compound decreased as the carbonation reaction time increased.The amount of Ca(OH)2 that decomposed in the second section also decreased.However, the amount of thermal decomposition in the third section increased.This indicates that the carbonation of SCAF interferes with the absorption of H2O or its reaction in the presence of CO2.The weight loss in the third section was caused by the thermal decomposition of CaCO3 generated by the carbonation reaction, indicating that the amount of CaCO3 generated increased with the carbonation reaction time.
According to Duguid et al., supercritical CO2 (scCO2) and water under certain conditions form a two-phase system composed of wet scCO2, a phase rich in scCO2, and a waterrich phase (CO2-saturated aqueous solution) because they can dissolve in each other without being mixed [21].Therefore, cementitious materials can be exposed to CO2-saturated aqueous solutions, wet scCO2, or their combination.In this case, CO2 is dissolved in water to form carbonic acid, which forms calcium carbonate in cementitious materials.The CAF used in this study was a cementitious material, and it was determined that calcium carbonate was generated by the carbonation reaction of SCAF owing to exposure to the CO2dissolved aqueous solution in the carbonation reaction, even though it was not supercritical CO2.According to Duguid et al., supercritical CO 2 (scCO 2 ) and water under certain conditions form a two-phase system composed of wet scCO 2 , a phase rich in scCO 2 , and a water-rich phase (CO 2 -saturated aqueous solution) because they can dissolve in each other without being mixed [21].Therefore, cementitious materials can be exposed to CO 2saturated aqueous solutions, wet scCO 2 , or their combination.In this case, CO 2 is dissolved in water to form carbonic acid, which forms calcium carbonate in cementitious materials.The CAF used in this study was a cementitious material, and it was determined that calcium carbonate was generated by the carbonation reaction of SCAF owing to exposure to the CO 2 -dissolved aqueous solution in the carbonation reaction, even though it was not supercritical CO 2 .

General Properties
The compressive strengths at each age (3, 7, and 28 days) were measured and compared for the reference OPC mixture (P) and SCAF substitution rates of 10%, 20%, 30%, 50%, 70%, and 100%, as shown in Figure 6.As the SCAF substitution rate increased, the compressive strength at each age tended to decrease.SCAF 10% developed a similar strength to 100% OPC (P).However, at substitution rates of 20%, 50%, and 70%, the compressive strength ranged from 50% to 2.3% of that of 100% OPC at 3 days of age and from 70% to 2.3% at 28 days.In the case of SCAF 100%, almost no strength developed at 3 and 28 days of age.Thus, the compressive strength decreased rapidly as the SCAF content increased.This indicates that the strength development due to the hydration reaction is insignificant for SCAF and that SCAF has no hardening characteristics due to the hydration reaction.To evaluate the pore structure characteristics with respect to the SCAF content, the pore-size distribution at 28 days of age was analyzed for the 100% OPC, SCAF 10%, and SCAF 100% specimens.The porosity of the cement was 23.76% at 28 days, whereas those of the SCAF 10% and SCAF 100% specimens were higher (40.12% and 52.84%, respectively).
Figure 7 shows the pore-size distributions for each specimen.SCAF 100% contained more capillary pores (≥50 nm) and fewer mesopores (4.5-50 nm) and gel micropores (3.0-4.5 nm) than the 100% OPC and SCAF 10% specimens.SCAF 100% exhibited a high porosity owing to the evaporation of a large amount of residual water that could not contribute to the hydration reaction.The presence of such pores facilitates the penetration or diffusion of CO2 during the carbonation reaction.As the SCAF substitution rate increased, the compressive strength at each age tended to decrease.SCAF 10% developed a similar strength to 100% OPC (P).However, at substitution rates of 20%, 50%, and 70%, the compressive strength ranged from 50% to 2.3% of that of 100% OPC at 3 days of age and from 70% to 2.3% at 28 days.In the case of SCAF 100%, almost no strength developed at 3 and 28 days of age.Thus, the compressive strength decreased rapidly as the SCAF content increased.This indicates that the strength development due to the hydration reaction is insignificant for SCAF and that SCAF has no hardening characteristics due to the hydration reaction.To evaluate the pore structure characteristics with respect to the SCAF content, the pore-size distribution at 28 days of age was analyzed for the 100% OPC, SCAF 10%, and SCAF 100% specimens.The porosity of the cement was 23.76% at 28 days, whereas those of the SCAF 10% and SCAF 100% specimens were higher (40.12% and 52.84%, respectively).
Figure 7 shows the pore-size distributions for each specimen.SCAF 100% contained more capillary pores (≥50 nm) and fewer mesopores (4.5-50 nm) and gel micropores (3.0-4.5 nm) than the 100% OPC and SCAF 10% specimens.SCAF 100% exhibited a high porosity owing to the evaporation of a large amount of residual water that could not contribute to the hydration reaction.The presence of such pores facilitates the penetration or diffusion of CO 2 during the carbonation reaction.

Penetration and Diffusion of CO 2 in Cement Matrix Containing SCAF Compounds
After the carbonation test, the area of the specimen was largely divided into two areas: (1) a colorless area in which carbonation occurred along with the consumption of hydroxyl ions when Ca(OH) 2 reacted with CO 2 to form CaCO 3 , and (2) a purple area without a carbonation reaction in which hydroxyl ions that did not react with existing CO 2 .The carbonation depth by age was measured for each specimen, and the results are presented in Table 4. Carbonation was performed using an accelerated method.Carbonation hardly occurred in the 100% OPC (P) specimen and in the cases where a small amount of SCAF was added.As the SCAF content increased, rapid carbonation occurred at an early stage.For the ordinary cement with no SCAF and the cement matrix containing a small amount of SCAF compounds, it appeared that the penetration or diffusion of CO 2 due to accelerated carbonation was hindered because the density of the matrix was increased by the generation of calcium silicate hydrates, calcium aluminate hydrate, ettringite, or monosulfate owing to the active hydration of cement.This formed a two-phase system composed of wet scCO 2 , a phase rich in scCO 2 , and a water-rich phase (CO 2 -saturated aqueous solution), as reported by Duguid et al. [21].Therefore, according to the theory that cementitious materials can be exposed to a CO 2 -saturated aqueous solution, wet scCO 2 , or both, in which CO 2 is dissolved in water to form carbonic acid, and that carbonic acid forms calcium carbonate in cementitious materials, it is judged that hardened cement resists the penetration and diffusion of CO 2 in the early stage of carbonation, but occur through the pores in hardened cement after a certain period of time.When the SCAF substitution rate was ≥30%, CO 2 penetrated more rapidly as the substitution rate increased, and all the specimens were carbonated at 7 days of age.In the case of accelerated carbonation, carbonation occurred rapidly because the CO 2 concentration was high.In general, cement hydrates are carbonated through reactions with CO 2 in the atmosphere, and the types of reactions are presented in Table 5.For these reactions, carbonation occurs after the hydration of cement, indicating that the penetration and diffusion of CO 2 in the cement matrix proceed after the occurrence of hydration at a certain age.

CO 2 Diffusion Coefficient Calculation
The theory of diffusion follows Fick's law.Fick's second law describes the changes in the concentration of the diffusing solute at a position over time based on the first law.Therefore, it considers changes in the concentration of a sample at a given position over time.As for diffusion in the cement matrix where carbonation proceeds, the concentration increase in the unit area over time can be explained by the difference between the inflow and outflow volume fluxes according to Equation (1).Here, F in represents the mass of incoming CO 2 , and F out represents the mass of outgoing CO 2 .∆C represents the amount of calcium carbonate (excluding CaO), combined during carbonation.
Fick's second law quantifies diffusion in a specimen as the change in concentration over time.The concentration of the diffusion component decreases as the distance from the surface (x) increases, and increases over time at a given position of X.The mathematical solution to Fick's second law can be expressed using Equation (2), which is limited by the boundary condition.The concentration at time t(C(x, t)) at a given distance x can be calculated using Equation (3), which uses the error function for diffusion in a semiinfinite solid.
When the surface concentration C 0 and the concentration at distance X (C X ) are known, the diffusion coefficient D can be calculated using Equation (4).
Table 6 presents the diffusion coefficients according to the CO 2 penetration depth.As shown in the figure, the diffusion coefficient was large in the early stage of carbonation but decreased over time.This indicates that the diffusion of CO 2 in the cement matrix closely follows a semi-infinite model of Fick's second law.Thus, the diffusion of CO 2 due to penetration is easy in the early stage of carbonation, but it is slowed by the filling of pores by carbonation products over time.According to these research results, SCAF can contribute to a reduction in CO 2 emissions through energy savings because it can be sintered at a lower temperature (1100 • C) than cementitious compounds or cement (1450 • C).SCAF is also expected to contribute to the absorption and fixation of CO 2 when a certain amount is substituted.The use of CAF compounds may cause degradation of properties, such as the compressive strength, but this can be improved through optimal mix design.

Figure 5 .
Figure 5. XRD patterns for carbonation with the wet method.

Figure 6 .
Figure 6.Compressive strength with respect to the SCAF substitution rate.

Figure 6 .
Figure 6.Compressive strength with respect to the SCAF substitution rate.

Materials 2023, 16 , 7344 8 of 11 Figure 7 .
Figure 7. Pore-size distribution of paste with SCAF.3.3.2.Penetration and Diffusion of CO2 in Cement Matrix Containing SCAF Compounds After the carbonation test, the area of the specimen was largely divided into two areas: (1) a colorless area in which carbonation occurred along with the consumption of hydroxyl ions when Ca(OH)2 reacted with CO2 to form CaCO3, and (2) a purple area without

Table 1 .
Chemical and physical properties of materials.

Table 3 .
Weight loss for different temperature sections according to the carbonation time.

Table 3 .
Weight loss for different temperature sections according to the carbonation time.