Effect of Pre-Curing Time and Residual Water–Cement Ratio on CO₂ Curing of Recycled Concrete

Abstract

Using recycled concrete (RC) created from building debris to capture, utilize, and sequester CO₂ is a green and sustainable development strategy. Before CO₂ curing, pretreatment can provide a suitable environment for the carbonation reaction of the RC, accelerate the carbonation rate of the RC, and enhance its performance. The effects of the pre-curing time and residual water–cement ratio (Re) on the carbon sequestration rate, carbon sequestration, carbonation depth, and mechanical strength of RC were investigated and validated through X-ray diffraction (XRD) and scanning electron microscopy (SEM). The study demonstrated optimal carbon sequestration properties at a pre-curing time of 5 days. The corresponding carbon sequestration rate, unit carbon sequestration, carbonation depth, and compressive strength were 23.17%, 19.88 g/kg, 15.79 mm, and 28.7 MPa, respectively. Optimal carbon sequestration performance occurred at a Re of 0.26. The measured values were 20.15% (carbon sequestration rate), 17.38 g/kg (unit carbon sequestration), 12.55 mm (carbonation depth), and 31.1 MPa (compressive strength). According to the XRD and SEM results, the effects of pre-curing time and Re were mainly seen in the conversion rate of CaCO₃ and a denser microstructure. This implies that improving the CO₂ curing effect by controlling the pre-curing time and Re can both alleviate the pressure of greenhouse gas emissions and increase the utilization efficiency of RC.

1. Introduction

Construction waste is accumulating and continuously increasing due to growing urbanization. According to previous studies, 35% of construction waste is not used globally [1]. Construction waste has a significant negative environmental impact. Currently, crushing it into recycled aggregate (RA) is the primary method for recycling. However, the defects of RAs itself (old mortar [2]) limit its application in engineering [3,4]. Numerous researchers have worked on improving the performance of recycled concrete (RC) [5,6,7], and accelerating the carbonation of RC is considered a sustainable and economical method to improve the performance of RC [8]. Particularly, the improvement effect is in terms of mechanical strength [9], drying shrinkage [10], and resistance to chloride permeability [11].

CO₂ must be dissolved in water to react with other chemical products [12]; therefore, the carbonation reaction of concrete requires water inside the concrete to provide a moist environment [13]. However, too much or too little moisture inside the concrete is not conducive to the carbonation reaction of concrete, which is necessary in finding a critical point to provide an optimal environment for CO₂ curing of RC. This not only accelerates the carbonation rate of RC [14] but also promotes the formation of a denser microstructure of RC [15,16].

The pre-curing time is defined as the period after casting until the specimens are cured using CO₂. Controlling the initial moisture content and pore structure of the specimens during this period plays an important role in the subsequent CO₂ curing performance [17], which effectively improves the compressive strength of cement mortar by controlling the combination of the pre-curing and carbonation times [18]. However, Shi et al. investigated the effect of pre-curing time on the CO₂ curing of lightweight concrete blocks and found that the length of the pre-curing time was not important for the CO₂ curing process [19]. There are differences in the effects of the pre-curing time on different specimens.

Meanwhile, the carbon sequestration and mechanical strength of concrete can be effectively increased by controlling the water loss during concrete pretreatment [19]. Shi C and He, F et al. (2010) [20] proposed the concept of the residual water–cement ratio (Re), which is a key factor influencing the carbonation rate of cement mortar. A high Re leads to excessive water accumulation in the internal pores of concrete, which hinders the penetration of gases such as CO₂ from the outside to the inside of the pore structure [21]. A Re that is too low can lead to a weakened or impossible carbonation reaction. Therefore, concrete can not only improve the diffusion ability of CO₂ in concrete [22,23], but also increase the solubility of CO₂ [24,25] and accelerate the generation of hydration products, such as calcium carbonate, by pretreating and controlling the water–cement ratio in the appropriate range before CO₂ curing.

The length of pre-curing time is related to the degree of hydration reaction inside the concrete, which affects the relative humidity (RH) and porosity inside the concrete. Meanwhile, the Re is a short period of time to achieve a certain RH inside the concrete, which has almost no relationship with the degree of hydration reaction inside the concrete. In practical terms, managing these parameters influences RC’s application in construction. For structural applications requiring high strength and durability, fine-tuning the pre-curing time and controlling the Re can significantly enhance performance. Conversely, for non-structural applications where less stringent performance criteria apply, these parameters can be adjusted to optimize cost-effectiveness without compromising the functional integrity of the concrete.

Considering the unique properties of RC (the presence of old mortar on the aggregate surface, high porosity, and high water absorption, among others), the effects of pre-curing time and Re on RC may be more significant. Whether it is possible to effectively accelerate the carbonation rate and enhance the mechanical properties by controlling the pre-curing time and Re must be further investigated.

Previous studies on the effects of pre-curing time and Re on carbon sequestration in RC remain limited. Thus, this study examines their influence on carbon sequestration rate, unit carbon sequestration, carbonation depth, mechanical properties, and microstructure. We aim to evaluate the effectiveness of pretreatment methods in terms of pre-curing time and Re to improve the carbon sequestration efficiency of RC and determine the optimal pre-curing time and optimal Re that affect the carbon sequestration properties of RC.

2. Materials and Experimental Details

2.1. Raw Materials

Recycled coarse aggregate (RCA) and natural coarse aggregates (NCAs) (Figure 1A) were obtained from Chengdu Polymerization New Building Material Co., Ltd., and their detailed indices are listed in

Table 1. Physical properties of coarse aggregate.

No.	Apparent Density (kg/m3)	Bulk Density (kg/m3)	Crushing Index (%)	Rate of Water Content (%)	Water Absorption Rate (%)	Mud Content (%)
RCA	2610.20	1221.95	17.6	2.5	3.9	1.7
NCA	2708.82	1518.41	11.4	2.0	1.7	0.8

. The RCA can be categorized into the following four types due to different levels of attachment to the old mortar (Figure 1B): full (No gravel), partial (Gravel volume > Mortar volume), bulk (Gravel volume < Mortar volume), and a little (Gravel volume >> Mortar volume). The fine aggregates are composed of natural river sand (medium sand). Portland Ordinary (P.O.) 42.5 Lafarge Cement was used as the cementitious material, and the composition of the cement is listed in

Table 2. Chemical composition of cement.

Oxide	CaO	SiO2	Al2O3	P2O5	SO3	K2O	Fe2O3	Na2O	MgO
Content (%)	62.23	21.63	5.77	0.02	2.17	0.72	3.34	0.20	1.53

. The CO₂ gas was produced by Chengdu Wangrui Gas Sales Co., Ltd., and its purity was 99.5%. All of the above raw materials come from Chengdu, Sichuan, China.

2.2. Specimen Preparation and Curing

In this study, RC blocks made of all RCA were used as the research objects, and natural concrete (NC) prepared using NCA was set as the control to study the effects of different pre-curing times and Res on the carbon sequestration properties, mechanical properties, and microstructures of the specimens after CO₂ curing. The concrete prepared in this test was calculated based on the provisions of the “Specification for mix proportion design of ordinary concrete” (JGJ 55-2011) [26].

Table 3. Mixture proportions of concrete.

No.	Cement (kg/m3)	Water (kg/m3)	Water–Cement Ratio	Sand (kg/m3)	Coarse Aggregate (kg/m3)
					5~10 mm	10~15 mm	15~20 mm
NC	460	184	0.4	529	338.1	676.2	112.7
RC

lists the calculation of the proportions. The selection of the proportions of the individual particle sizes of the RCA was made with reference to the literature [27,28]. The particle size distribution ratio was 3:6:1 for 5–10 mm, 10–15 mm, and 15–20 mm fractions, respectively. The dosage of cement was determined through the literature [29] and preliminary pre-experiments.

RC and NC were cast into cubes with dimensions of 100 mm × 100 mm × 100 mm. This study used a pressurized carbonization method, which can achieve a high degree of carbonization in a short period of time [30]. The test setup for CO₂ curing is shown in Figure 2.

This study is mainly divided into the following two aspects. (1) To examine the effect of pre-curing time on RC, demolded specimens were first cured in a thermostatic chamber (20 ± 2 °C, 80 ± 5% RH) for 1, 2, 3, 5, and 7 days. Subsequently, they were transferred to a vacuum vessel (0.2 MPa, 99.5% CO₂) for a 2 h carbonation process (based on the literature [31] and numerous pre-experiments). Then, it was moved to a room temperature of 23 ± 2 °C and RH of 60 ± 5% for 28 d, and its performance was measured. (2) To study the effect of Re on RC after the test blocks were demolded, the initial mass of the test blocks was weighed, and the water–cement ratio was adjusted by using an exhaust fan in an environment of 26 ± 2 °C and RH of 40% ± 5%. The specimen mass was measured at 1 min intervals, and the Re was calculated using Equation (5) (Section 3.4), yielding values of 0.34, 0.31, 0.26, and 0.24. Then, they were placed into a vacuum vessel with an air pressure of 0.2 MPa, CO₂ concentration of 99.5% of the vacuum container maintenance for 2 h, and finally moved to room temperature 23 ± 2 °C and RH of 60% ± 5% of the environment maintenance 28 d (28 days including pre-curing time). The total maintenance time is 28 days. The carbon sequestration and carbonation depth were determined after 2 h of CO₂ curing, and the specimens were taken for 28 days for mechanical properties, X-ray diffraction (XRD), and scanning electron microscope (SEM) measurements. A flowchart is shown in Figure 3.

3. Testing Methods

3.1. Carbonization Depth Test

According to the specimen carbonation depth test with reference to GB/T 50082-2009 “Standard for test methods of long-term performance and durability of ordinary concrete” [32], the carbonization depth of the test piece was obtained by measuring 5 equidistant points on the end faces of the two sides of the test piece, for a total of 10 points, and taking the average value of the distances between the ten measurement points. Figure 4 shows the alcohol phenolphthalein test solution at 1% concentration.

3.2. Unit Carbon Sequestration Test

The carbon sequestration capacity of concrete, involving atmospheric CO₂ capture and storage within the material, must be quantified to evaluate its environmental sustainability. This study introduces the unit carbon sequestration parameter (φ), calculated as

$$M_c=△M_w+△M_s$$

(1)

$$\phi ={\displaystyle \frac{M_c}{M_i}}$$

(2)

where $M_c$ is the mass of CO₂ absorbed by the sample, g; $△M_w$ is the moisture loss of the sample during CO₂ curing and can be determined by weighing the CO₂ conditioning drum before and after CO₂ curing, g; $△M_s$ is the mass increase in the sample after CO₂ curing, g$; \phi$ is the unit carbon sequestration, g/kg; and $M_i$ is the mass of the specimen after injection molding, g.

3.3. Carbon Sequestration Rate Test

The carbon sequestration rate (α) is given by [33]:

$$\alpha ={\displaystyle \frac{M_c}{M\times {CO}_{2}max\%}}$$

(3)

where $\alpha$ is the carbon sequestration rate of samples, %; $M$ is the mass of the cementitious material contained in the sample, g; ${CO}_{2}max\%$ is the maximum theoretical CO₂ absorbed by the cementitious material. This is mainly determined by the proportions of cement, fly ash, and slag in the specimen, which can be calculated using the following formulas:

$${CO}_{2}max\%=42.97\%\times {\beta }_{cement}+14.13\%\times {\beta }_{flyash}+28.64\%\times {\beta }_{slag}$$

(4)

where β_cement is the cement dosage, %; β_{fly ash} is the fly ash dosage, %; β_slag is the slag dosage, %.

3.4. Residual Water–Cement Ratio Test

In this study, the Re was calculated as follows [20]:

$$R_e={\displaystyle \frac{M_w{-(M}_0-M_f)}{M_b}}$$

(5)

where $R_e$ is the residual water–cement ratio; $M_0$ is the mass of the specimen before drying, g;$M_f$ is the mass of the specimen after drying, g;$M_w$ is the initial water content of the specimen, g; and $M_b$ is the mass of the initial binders in the specimen, g.

3.5. Compressive Strength Test

According to the compressive strength test with reference to the standard GB/T 50081-2002 “Standard for test methods of concrete physical and mechanical properties” [34], the concrete specimen that reached the planned age was taken out, and the Shanghai Hualong Automatic Pressure Tester was used to carry out the compressive test at the rate of 0.5 MPa/s (Figure 5). The calculation results are accurate to 0.1 MPa.

3.6. XRD Test

In this study, a DX-2700BH ray diffractometer from Dandong Haoyuan Instrument Co., Ltd. in Dandong City, Liaoning Province, China was used to scan the powdered samples between 5° and 70° at 2θ increments of 0.02° per step with a scanning speed of 0.5 s per step to characterize the specimens in physical phase. The results were processed using Jade 6.5.

Specimens without CO₂ curing were obtained from the internal uncarbonated area of the concrete specimen block. After CO₂ curing, the specimens were taken from the outer surface carbonated area of the concrete specimen with the removal of the coarse aggregate. The test process is shown in Figure 6.

3.7. SEM Test

In this study, a Prisma E scanning electron microscope (SEM; Thermo Scientific, Waltham, MA, USA) was used to observe and analyze the microstructure in each specimen. The equipment is manufactured in Shanghai, China. The test process is shown in Figure 7. The sampling method of the SEM is the same as that of XRD.

4. Results and Discussion

4.1. Effect of Pre-Curing Time

4.1.1. Effect of Pre-Curing Time on Carbon Sequestration Efficiency

Figure 8 shows a graph of the effect of pre-curing time on the carbon sequestration efficiency of concrete. When pre-curing for 1 day, the carbon sequestration efficiency of RC is similar to that of NC. The carbon sequestration rate of the RC specimen is 5.17%, the unit carbon sequestration is 4.46 g/kg, and the carbonization depth is 3.41 mm, while the carbon sequestration rate of NC specimen is 6.15%, the unit carbon sequestration is 5.3 g/kg, and the carbonization depth is 2.77 mm. Both RC and NC specimens have poor carbon sequestration efficiency, and the carbon sequestration efficiency of both increases as the pre-curing time increases. Subsequently, with an increase in the pre-curing time, the carbon sequestration efficiency of both specimens showed an upward trend, and the carbon sequestration efficiency of RC significantly increased. The carbon sequestration rate of the RC specimens was 23.17% on the 5th day, when the unit carbon sequestration reached 19.98 g/kg, and the carbonation depth was 15.79 mm. On the other hand, the carbon sequestration rate of the NC specimens was 17.84%, the unit carbon sequestration was 15.39 g/kg, and the carbonation depth was 11.78 mm, which were lower than that of RC specimens. The carbon sequestration rate, unit carbon sequestration, and carbonation depth of the RC specimens decreased when the pre-curing time was 7 days.

At the beginning of the pre-curing, the pores of the RC specimens were almost filled with free water not involved in the reaction, preventing the penetration of CO₂ [35]. This explains why the lowest carbon sequestration efficiency of RC was observed when the pre-curing time was 1 day. With the prolongation of the pre-curing time, the free water inside the specimens was mainly divided into three (see Figure 9): (i) evaporation; (ii) participation in the hydration reaction and consumption; and (iii) stagnation in the pores. On the fifth day, the interior of the specimens reached an appropriate RH, which facilitated the dissolution of CO₂ in the pores and accelerated the carbonation reaction [6,24]. This corresponds to the superior carbon sequestration efficiency of RC with a pre-curing time of 5 days. However, if the pre-curing time is too long, the generated hydration products will precipitate on the capillary pore walls of the cement paste, increasing the compactness of the concrete and preventing the penetration of CO₂. For example, RC with a pre-curing time of 7 days. Second, in a wet environment, the longer the pre-curing time, the generated hydration products, which increase the capillary pores, impede the diffusion of CO₂ to the interior [36]. Third, water evaporation and hydration reactions represent the primary mechanisms through which free water is lost from concrete [37]. During the initial phase of the curing process, RC exhibits a high water-to-cement ratio, harboring an abundance of internal moisture that gravitates towards the surface [38], consequently expediting the evaporation of water. Conversely, with the progression of the curing duration, the water–cement ratio in RC abates, and a greater volume of water is assimilated into the hydration process [39], culminating in a reduction in the internal RH. As the water in RC evaporates and the hydration reaction advances, the internal moisture content diminishes, thereby decelerating the hydration process to some degree. Additionally, compared with NC, there are more micropores and cracks inside the RC specimen, increasing the contact area of CO₂ with the cement matrix [5]. The permeability of CO₂ increases and then decreases with increasing pre-curing time (i.e., there exists an optimal pre-curing time that effectively promotes carbonation efficiency in RC).

4.1.2. Effect of Pre-Curing Time on Compressive Strength

Figure 10 shows the effect of pre-curing time on the mechanical properties of RC carbonation. For a pre-curing time of 1 to 5 days, the compressive strength of both RC and NC showed an increasing trend, and the compressive strength of RC significantly increased. The compressive strength of the RC specimens with a pre-curing time of 5 days increased by 6 MPa (28.7 MPa) compared to that with one day of pre-curing, which is close to that of the NC that is attributed to the good carbon sequestration efficiency of the RC (Section 4.1.1). However, the compressive strength of the specimens after seven days of pre-curing exhibited a decreasing trend. The trend of the compressive strength was similar to that of the carbonation effect of the concrete specimens, which shows that the change in mechanical properties is positively correlated with the carbon sequestration effect of concrete. The presence of the old RA mortar and the porous nature of the RC allow better penetration of CO₂ gas into the interior of the specimen, accelerating the carbonation rate but leading to a higher water content at the beginning of the pre-curing of the RC. By regulating the pre-curing time, a relatively suitable environment is first formed to provide favorable conditions for the reaction of CO₂, accelerating the generation of hydration products, such as CaCO₃, which not only fill the pores but also improve the microstructure of the interface transition zone [40] and the mechanical properties of RC. Second, hydration products other than CaCO₃ were generated inside the RC during the pre-curing period, which changed the pore structure and enhanced its mechanical strength to a certain extent. However, when the pre-curing time is excessively long, reactants, such as Ca(OH)₂ (C-H) and calcium silicate hydrate (C-S-H), are surrounded by dense carbonation products [41], reducing the diffusion and dissolution of CO₂ and are not conducive to the strength development of RC [42].

4.1.3. Effect of Pre-Curing Time on Microstructure

Given the proximity of pre-curing time, variations in the hydration products within the concrete were minimal. Therefore, this study selected RC with pre-curing times of 1 day and 5 days for comparison. RC that had not undergone CO₂ curing was also chosen as a control group.

XRD was used to analyze the physical composition and hydration products of the cement paste samples before and after carbonation and to further investigate the characteristics and mechanism of carbonation, as shown in Figure 11. The hydration age of all non-CO₂ cured RC in this study is 28 days, which is consistent with the other specimens. We investigated whether the pre-curing time, in addition to affecting the production of CaCO₃, also affected the production of other new products. As shown in Figure 11, there is a significant difference between the XRD plots of the specimens without CO₂ curing and those with CO₂ curing, manifesting as a change in the intensity of the diffraction peaks. For the specimens without CO₂ curing, the main hydration products were C-H, CaCO₃, ettringite (AFt), and unreacted C₂S and C₃S. The diffraction peaks of C-H, AFt, C₂S, and C₃S weakened or even disappeared in the specimens after CO₂ curing with a change in pre-curing time. The intensities of the CaCO₃ diffraction peaks significantly increased. Simultaneously, the diffraction peaks of different crystal styles of calcium carbonate (i.e., calcite, vaterite and aragonite) appear. For the generation of CaCO₃ in different crystalline forms, scholars proposed Equation (5) [43,44,45]. The above results indicate that the product composition and crystal form of harden pastes before and after carbonation change, i.e., C-H, AFt, C₃S, and C₂S are converted to the more crystalline CaCO₃ [46]. This is consistent with the findings of the literature [24]. Comparing the specimens cured with carbonization after one day of curing and those cured with carbonization after 5 days, the specimens cured with CO₂ after 5 days of pre-curing showed a significant increase in the diffraction peaks of SiO₂. The specimens undergoing a 5-day pre-curing facilitated the formation of a three-dimensional mesh of Si-gel while generating CaCO₃ [46]. The combined filling effect of CaCO₃ and Si-gel enhanced the densification and mechanical strength of the RC [47].

C-S-H (C-H) + CO₂ → CaCO₃ (nuclei) → CaCO₃ (vaterite/aragonite) → CaCO₃ (calcite)

(6)

Moreover, the effect on the micromorphology of RC before and after carbonation and at different pre-curing times was tested through SEM analysis, as shown in Figure 12. As shown in Figure 12, there was a significant effect on the microsamples of RC before and after carbonation. The specimens without CO₂ curing can be observed as needle-like or rod-like AFt, fibrous and flocculent C-S-H and flaky C-H with poor surface uniformity, looseness, and porosity. And there are significant cracks. The RC that underwent CO₂ curing after pre-curing showed that AFt, C-H, and C-S-H were almost absent, the pores were filled with granular CaCO₃ (vaterite) and bulk CaCO₃ (calcite) (Figure 12b,c), and the microstructure was denser. This is consistent with the phenomenon observed by XRD. A comparison of Figure 12b,c showed that Figure 12b is almost always filled with granular CaCO₃, and although dense, the particles show loose stacking. This indicates that the calcium carbonate crystals produced by the carbonation reaction alone are not sufficient to compensate for the cementation defects in the RA [48]. In contrast, Figure 12c shows the presence of not only granular CaCO₃ but also large flocculent hydration products, which were identified as a combination of CaCO₃ (vaterite and calcite) and Si-gel, as well as a larger pore volume compared to that in Figure 12b. This was attributed to C-S-H carbonation, which formed additional silica gel, increasing the average chain length of the silica gel and resulting in a smaller volume and increased pore volume (decalcification shrinkage) [49].

In addition, because of the smooth and hydrophobic nature of SiO₂, gaps often form between hydration products and SiO₂. These gaps can act as permeation channels for CO₂ gas, enhancing the gas permeability of RC that have undergone 5 days pre-curing. After CO₂ curing (Figure 12c), carbonation products bond the hydration products and SiO₂ to create a continuous product layer that strengthens the interfacial transition zone. The filling effect of carbonation products enhances the densification of the RC.

For RC, the pre-curing time affects the CO₂ curing effect, which depends on moisture dissipation and may also be related to the residual reactants and hydration products produced during the pre-curing period.

4.2. Effect of Residual Water–Cement Ratio

4.2.1. Effect of Residual Water-Cement Ratio on the Carbon Sequestration Efficiency

The effect of the Re on the carbonation of RC was investigated, as shown in Figure 13. When the Re was higher than 0.34, the carbon sequestration rate, unit carbon sequestration, and carbonation depth of RC were 11.29%, 9.74 g/kg, and 8.84 mm, respectively, which were close to the carbon sequestration efficiency of NC. As the Re decreases, the carbon sequestration efficiency of RC shows an increasing and then decreasing trend and reaches a peak at a Re of 0.26, with a carbon sequestration rate, unit carbon sequestration, and carbonation depth of 20.15%, 17.38 g/kg, and 12.55 mm, respectively. In contrast, the carbon sequestration efficiency of NC showed an increasing trend in the range of 0.34–0.24 of the tested Re.

When the Re of the test block was high, the internal pores were filled with free water, which blocked the permeation path of CO₂, and the carbonation area remained in the surface layer of the test block, resulting in poor carbon sequestration performance. For example, the carbon sequestration performance at a Re of 0.34. As the Re decreases, it provides a channel for the penetration of CO₂ and the internal moisture, providing a favorable environment for the reaction of CO₂. The RH of RC reaches the optimal state when the Re is 0.26, enhancing the carbon sequestration efficiency of RC. However, a decreasing trend was observed when the Re exceeded 0.26. This was because the RH inside the specimen was low, which was not conducive to the dissolution of CO₂, decreasing the rate of the reaction and weakening the carbon sequestration efficiency of RC.

Notably, when the Re of NC is in the range of 0.34–0.24 with the decrease in the Re, the carbon sequestration efficiency has an upward trend (i.e., the carbon sequestration rate is increased from 10.27% to 18.28%), and the unit carbon sequestration is elevated from 8.86 g/kg to 15.76 g/kg. There is no peak such as that of RC, which is probably due to the fact that the RA surface with the presence of aged mortar leads to higher internal porosity of RC and higher water requirement for RC to undergo hydration reaction.

4.2.2. Effect of Residual Water-Cement Ratio on Compressive Strength

For the compressive strength, the carbonation effect of RA concrete has also proved the change rule of its mechanical properties, and the test results (Figure 14) show that the mechanical properties of the optimal group of carbonation efficiency with a Re of 0.26 also reached 31.1 MPa, which is close to the compressive strength of NC, and the strength of RC with a Re of 0.34 increased by 7.3 MPa. When the Re was greater than 0.26, the compressive strength of the RC specimens began to decrease, whereas the ordinary concrete maintained its increasing trend. When the Re was greater than 0.26, the compressive strength of RC specimens began to decrease, whereas that of the NC specimens continued to increase.

The trend of the mechanical properties of concrete after carbonation with the Re is similar to the trend of the carbon sequestration efficiency with the Re. In other words, the strength increase in the concrete is positively correlated with the carbon sequestration efficiency. This is because in the process of CO₂ curing, the appropriate Re promotes the generation of hydration products, such as CaCO₃, which fills the pores of the RC and increases its densification. Moreover, CaCO₃ is not easy dissolve in water and has a high strength, further improving the mechanical properties of the concrete. Therefore, in the process of decreasing the Re from 0.34 to 0.24, RC specimens with a Re of 0.26 showed a higher compressive strength and degree of CO₂ conservation.

4.2.3. Effect of Residual Water-Cement Ratio on Microstructure

XRD analysis of specimens with a Re of 0.34 cured by CO₂, specimens without CO₂ curing, and specimens with a Re of 0.26 cured by CO₂ were carried out in this study to investigate the changes in the crystal morphology of concrete components during CO₂ curing of RC. The changes in the internal compositions of the specimens before and after curing by CO₂, as well as under the conditions of different Res, were compared, and the results are shown in Figure 15.

The effect of CO₂ curing on the concrete specimens was reflected in the changes in the components of the specimens. Based on the XRD patterns, the main hydration products of the specimens without CO₂ curing were C-H, CaCO₃, and AFt compared to those after CO₂ curing, while the diffraction peaks of C₂S and C₃S were stronger than those of the specimens after carbonation. After CO₂ curing, the C-H, C₂S, and C₃S in the specimens reacted with CO₂ to form carbonate and Si-gel [50]. This conclusion is also verified in the literature [51].

To clearly understand the effect of Re on the carbonation of RC, the microstructure of specimens without CO₂ curing and specimens of RC with Re of 0.34 and 0.26 after CO₂ curing were observed by SEM plots (Figure 16).

As shown in Figure 16, the main hydration products of RC without CO₂ curing were needle-like AFt and fibrous C-S-H, while unreacted C₂S and C₃S were also present. Its microstructure exhibits large and numerous pores. After the RC with a Re of 0.34 was cured by CO₂, small granular spherulites (vaterite, etc.) and bulks (calcite, etc.) were loosely accumulated. A considerable amount of fibrous C-S-H gels was distributed not only along the edges of the foil gels and granular products (vaterite, etc.) but also widely distributed in the gaps between the products. These observations may be due to the depletion of calcite by reacting with calcium aluminate hydrate in the hydrated cement to form calcium hemialuminate or calcium monoaluminate [52]. Notably, further analysis of the calcite diffraction peaks in the XRD patterns showed that the calcite content of the samples remained virtually unchanged regardless of the subsequent water curing process. Additionally, no peaks of calcium hemialuminate or calcium monoaluminate were found in the XRD patterns. Therefore, it can be surmised that the calcite formed during the CO₂ curing process was incorporated into the newly formed C-S-H gel during the subsequent water curing process. Further studies are required to verify this hypothesis [53]. In addition, the spherulites were bonded using Si-gel, and some of the gaps were filled with a small amount of calcite, which had a much denser microstructure.

Particular attention is warranted as the heterogeneity in internal pore distribution in RC generates variable pressures across different regions, influencing the physical state of the hydration products. This variability is likely a contributing factor to the formation of diverse crystal types within the hydration products of the RC.

5. Discussion

5.1. EDS and FTIR Discussion

CO₂ penetration alters the chemical composition of hydration products in RC and modifies the SiO₂ bonding state, as evidenced by Energy Dispersive Spectrometer (EDS) and Fourier Transform Infrared Spectrometer (FTIR) analyses. These are discussed below.

From the EDS perspective, this is mainly attributed to two factors: first, a decrease in the Ca/Si ratio: the carbonation reaction consumes Ca²⁺ in both C-H and C-S-H phases, forming CaCO₃ and consequently reducing the Ca/Si ratios in these hydration products [54]. Second, the distribution state of Si and C elements: Uncarbonized regions of Si are uniformly distributed (C-S-H gels). Following carbonation, Si demonstrates localized aggregation, attributed to C-S-H structural contraction from Ca²⁺ depletion [55]. Concurrently, a new carbon signal emerges at ~0.28 keV [56], corresponding to CaCO₃ formation.

From the FTIR perspective, this is mainly attributed to two factors: first, the Si-O vibrations of C-S-H gels (900~1000 cm⁻¹): Uncarbonized, the main C-S-H peak is at ~970 cm⁻¹ (unit Q², chain silicate) [57]. Following carbonation: (i) The C-S-H peak shifts to higher frequency [58], indicating modification of the Si-O bonding environment. This shift corresponds to SiO₂ polymerization state transition from Q² to Q³/Q⁴ [59], consistent with observed Si enrichment in EDS analysis. (ii) C-S-H structural disordering results in peak intensity reduction and broadening [60]. Second, relative intensity of CaCO₃ and Si-O peaks: the higher the degree of carbonation, the stronger the CaCO₃ peaks and the wider the Si-O peaks (structural disruption) [61,62].

Combining EDS and FTIR characterization, the following highlights can be observed: (i) Spatial correlation: EDS shows that the carbonized region (low Ca, high C) corresponds to the displacement of CaCO₃ peaks and Si-O peaks in FTIR, which proves that the SiO₂ bonding environment is changed due to carbonization. (ii) Reaction mechanism validation: Early stage of carbonation: C-H and C-S-H peaks decrease rapidly in FTIR, and EDS shows Ca loss. Late stage of carbonation: Si-O peaks move to high frequency, and EDS shows that Si-enriched areas coexist with CaCO₃, suggesting that SiO₂ is repolymerized after C-S-H decomposition.

5.2. Discussion of pH Changes Before and After Carbonization

CO₂ penetrates into RC and the internal pH changes accordingly, which is critical for the long-term performance of RC. Therefore, the changes in pH before and after carbonation are discussed in this section.

Change in pH before and after carbonization: in uncarbonated concrete, the pore solution exhibits strong alkalinity (pH ≈ 12.5) [63], primarily attributed to portlandite (C-H) formation and minor alkali hydroxide (NaOH/KOH) dissolution during cement hydration. Carbonation consumes these alkaline phases, forming neutral CaCO₃ and reducing pH to 8–9 [64,65].

Chemistry in the Carbonation Reaction Process:

(i) CO₂ dissolution and diffusion. CO₂ diffuses through the concrete pores and dissolves in the pore solution to form carbonic acid (H₂CO₃).

CO₂ + H₂O → H₂CO₃

(7)

(ii) Neutralization reaction

C-H + H₂CO₃ → CaCO₃ + 2 H₂O

(8)

(iii) Decrease in pH

As C-H is consumed, the alkalinity of the solution in the pores diminishes and the pH gradually decreases to the neutral range.

The change in pH value, while bringing positive effects, will also be accompanied by the generation of negative effects.

(i) Negative effects: Reinforcing steel depassivation and corrosion (destruction of passivation film at pH < 11.5) [66]; triggering volume contraction, leading to microcracks.

(ii) Positive effects: The carbonation product CaCO₃ partially fills the pores [67], increasing the hardness of the surface layer and the resistance to chemical attack in the short term.

6. Conclusions

In this study, the effect of pre-curing time and Re on the carbonation properties of RC is investigated as follows:

(1) The carbon sequestration rate, unit carbon sequestration, carbonation depth, and compressive strength of RC were optimal at a pre-curing time of 5 days and were 23.17%, 19.88 g/kg, 15.79 mm, and 28.7 MPa, respectively. The carbon sequestration efficiency at this point is much better than that of NC, and the strength is minimally different from that of NC.

(2) The carbon sequestration rate, unit carbon sequestration, carbonation depth, and compressive strength of RC were optimal at a Re of 0.26, which were 20.15%, 17.38 g/kg, 12.55 mm, and 31.1 MPa, respectively.

(3) From XRD, it can be seen that controlling the pre-curing time or Re can increase the conversion rates of C₂S, C₃S, AFt, C-H, and C-S-H, etc., and promote the production of CaCO₃.

(4) From SEM, it can be seen that the optimal pre-curing time or the Re formed a microstructure with flocculent associations, which resulted in larger pores. However, the overall structure was denser.

In summary, it can be seen that there exists an optimum pre-curing time and Re for the best carbon sequestration performance of RC. This implies that by controlling the pre-curing time and Re, not only can the greenhouse effect be effectively mitigated, but also low-cost RC applications can be realized, reducing the dependence on natural resources. However, there are certain shortcomings in this study, such as the lack of exploration of durability and certain limitations in practical engineering applications. We will also carry out deeper research in future work.