Effects of Different Carbonation Treatment Methods for Recycled Concrete Aggregate

Abstract

Compared to natural aggregates, recycled concrete aggregates (RCAs) which are derived from construction and demolition (C&D) waste exhibit inferior properties, such as lower density and higher water absorption. Accelerated carbonation was an effective approach to enhance the properties of RCA. This study conducted a comparative analysis on the property enhancement of both coarse recycled concrete aggregate (CRCA) and fine recycled concrete aggregate (FRCA) by utilizing four carbonation approaches: conventional carbonation, CH spraying with conventional carbonation, wet carbonation, and two-step wet carbonation. Scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) analysis, and CO₂ uptake comparison were used to investigate the microstructural properties of the RCA. Furthermore, we also evaluated the compressive strength of mortar specimens with four different replacement ratios and the density and water absorption values of different carbonation-treated aggregates. The experimental findings revealed the following: (1) All of the accelerated carbonation approaches were more effective for FRCA than CRCA due to a higher adhered mortar content. (2) The pretreatment of CH spray provided external Ca²⁺ which improved the carbonation efficiency and therefore significantly enhanced the property of FRCA. (3) Liquid–solid phase carbonation achieved higher carbonation efficiency in the inner pore areas of the adhered mortar, resulting in a high CO₂ uptake and microstructure densification compared with conventional gas–solid phase carbonation.

1. Introduction

Due to the rapid urbanization and development of the construction industry, construction and demolition waste (CDW) became not only a major source of solid waste but also a significant contributor to environmental degradation [1]. In China, where urbanization continued to accelerate, the value of the construction sector was expected to reach 9.0 trillion (CNY) [2]. Data from China’s National Bureau of Statistics (NBSC) revealed that the industry’s construction area and completed building area totaled 15.3 billion and 3.9 billion square meters, respectively, in 2023 [2]. Globally, according to the “Building-GSR” report released by the United Nations Environment Programme and the Global Alliance for Construction and Building, the construction industry made a significant contribution to global climate change by accounting for approximately 21% of global greenhouse gas emissions [3]. In 2022, buildings accounted for 34% of global energy demand and 37% of CO₂ emissions related to energy and processes. To address these challenges, strategies such as reusing CDW in new construction by partially replacing natural aggregates (NAs) with recycled concrete aggregates (RCAs) in concrete production have emerged as one of the effective solutions to reduce waste and curb emissions [4].

Relative to natural aggregates (NAs), recycled concrete aggregates (RCAs) display compromised physical–mechanical performance, characterized by diminished bulk density, amplified porosity, heightened water uptake capacity, and inferior strength metrics [5]. These are mainly due to the porous adhered mortar [6]. For instance, the crush values of RCA with a particle size of 4.75–10 mm were 45% higher than those of NA [7]. The water absorption rate of FRCA was approximately 6%, while that of its corresponding natural counterpart (FNA) was less than 1% [8]. The water absorption of coarse RCA (CRCA) is approximately fourfold that of coarse NA (CAN) [8]. The substitution of NA with RCA in concrete production adversely impacted the performance of RAC. Studies demonstrated that the 28-day compressive strength of RAC specimens declined progressively as the RCA replacement ratio increased [9]. A parallel trend was observed in mortar samples, where both 28-day compressive and flexural strength diminished with higher RCA incorporation levels [10]. These findings underscore the necessity for effective treatment methods to mitigate the inherent drawbacks of RCA and enhance its viability in structural applications.

To enhance the properties of RCA, researchers have proposed various treatment methods. The modification techniques were primarily categorized into two groups: the removal and strengthening of the adhered mortar [11]. The methods of removing adhered mortar mainly include pre-soaking in acid, mechanical grinding, and pre-soaking in water, while the strengthening approaches include polymer emulsion, filler lime power, pozzolanic solution, sodium silicate, and carbonation [12,13,14,15,16,17,18]. Dimitriou et al. (2018) applied thermal processing to eliminate adhered mortar from RCA, enhancing their mechanical properties [19]. However, the employed equipment required a large investment and it commonly generates additional fine particles, altering the original RCA gradation [19]. Qiu et al. (2014) utilized microbial metabolic activity to produce carbonates, which were then deposited on the surface of RCA, thereby improving its physicochemical properties, particularly water absorption and bonding strength with the new concrete matrix [16]. The effectiveness of microbial carbonate precipitation (MCP) is influenced by multiple factors, including pH, temperature, calcium concentration, and bacterial concentration. These factors need to be precisely controlled to achieve optimal precipitation. At present, this method is still at the experimental stage and is difficult to apply in practical engineering [16].

Accelerated carbonation proves an efficient and economical approach for enhancing (RCA) performance [20]. Hu et al. (2025) explored the net CO₂ of RCA after carbonation by considering the life-cycle CO₂ emissions from material production, confirming the possibility of positive net CO₂ in practical engineering design by a machine learning model [21]. Goh et al. (2025) produced an alternative sand (AS) product through the carbonation of RCA to explore the feasibility of integrating CO₂ capture and sequestration, revealing the economic viability with a cost of 11.02 USD/t AS at the smallest scale and 8.02 USD/t AS at the largest scale [22]. Fang et al. (2021) employed soaking or spraying treatments with a saturated Ca(OH)₂ solution and ready-mix concrete plant wastewater to introduce supplementary Ca²⁺ and OH⁻ ions into recycled concrete aggregates (RCAs), thereby enriching carbonation products [23]. Sallehan Ismail et al. (2017) treated recycled concrete aggregates (RCAs) by initially soaking them in 0.5 M hydrochloric acid (HCl) for 24 h, followed by impregnation with a wollastonite (calcium metasilicate) solution [24]. The results demonstrated that acid pretreatment significantly improved RCA performance, and the compressive strength of RAC containing treated RCA increased notably [24]. Aojoy Kumar Shuvo et al. (2024) comprehensively reviewed the effects of four accelerated carbonation methods, standard, pressurized, flow-through, and wet carbonation, on the physical properties of RCA, respectively [25].

Previous studies explored the effects of different carbonation methods on the RCA enhancement [26,27,28,29,30]. However, the comparison of the carbonation efficiency of different carbonation methods on different types of aggregates is still limited. This study therefore assesses four carbonation approaches for enhancing the properties of both CRCA and FRCA. Water absorption, apparent density, compressive strength of mortar specimens, and CO₂ uptake through carbonation were compared and evaluated. The microscopic properties were also analyzed by scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD) analysis.

2. Materials and Methods

2.1. Recycled Concrete Aggregate (RCA)

Both CRCA and FRCA originated from laboratory-designed concrete specifically for RCA production. The mix design featured 460 kg/m³ of cement content, with a 0.45 water–cement ratio, locally sourced river sand, and granitic coarse aggregates (5–10 mm and 10–20 mm fractions). The cement of the original concrete was a commercial cement of type 53.5R. The details of the mixture proportion of the original concrete are shown in

Table 1. Mix proportions of the original concrete.

Unit (kg/m3)	OPC	Water	Sand	5–10 mm Aggregate	10–20 mm Aggregate
	460	205	700	530	430

. Following 28-day standard curing, concrete specimens were crushed and stored in sealed containers (>12 months) before processing into aggregates. The particle size range of CRCA and FRCA was 5–10 mm and 0.15–5 mm, respectively. The particle size distribution of FRCA is listed in

Table 2. Particle size distribution of river sand.

Particle Size (mm)	0.15–0.3	0.3–0.6	0.6–1.18	1.18–2.36	2.36–5
Passing (%)	35.12	19.38	25.00	15.45	5.05

, which was the same as the gradation of river sand.

2.2. Cement

The new concrete is prepared by ASTM Type I ordinary Portland cement (OPC) (density 3.11 g/cm³; specific surface area 3570 cm²/g).

Table 3. Chemical compositions of OPC (% by mass).

CaO	SiO2	Al2O3	Fe2O3	MgO	Na2O	SO3	Others
63.51	20.82	4.48	3.33	2.82	0.56	2.25	2.23

shows the chemical composition of OPC determined by X-ray fluorescence (XRF).

2.3. Chemical Agent

The reaction process of accelerated carbonation needs some chemical agents, including Na₂CO₃ solution, Triethanolamine (TEA), and Ca (OH)₂ solution. AR-grade Na₂CO₃ powder was dissolved in deionized water to prepare the Na₂CO₃ solution. The TEA was added to the Na₂CO₃ solution (0.5wt%) to chelate Ca²⁺. The CO₂ gas (>99.9%) injected into the system was commercially procured. AR-grade Ca (OH)₂ powder was dissolved in deionized water to prepare the saturated Ca(OH)₂ solution in the laboratory.

2.4. Treatment Method for RCA and FRCA

This study adopted four types of accelerated carbonation treatment, including conventional carbonation (I-B and II-B), conventional carbonation with CH pretreatment (I-C and II-C), wet carbonation (I-D and II-D), and two-step wet carbonation (II-E). The conventional carbonation utilized a carbonation chamber controlled at conditions with 20 °C, 50% RH, and 98% CO₂ concentration for 3 h. To better investigate the influence of liquid–solid carbonation of FRCA, this study exclusively employed two-step wet carbonation for FRCA. The CH pretreatment spraying set-up was placed above the samples (Figure 1). To ensure the samples were soaked, we distributed the particle samples evenly to form a single layer and turned them gently after each spray. The flow diagrams illustrating the two-step wet carbonation process and the wet carbonation process are presented in Figure 2 and Figure 3, respectively. And the wet carbonation and two-step carbonation used in this study are described in the previous study with CO₂ 99.9% [30]. All aggregates used in this study are listed in

Table 4. Group classification.

Group	Type of Aggregate	Method of Carbonation Treatment	Particles of Aggregates (mm)
I-A	Raw aggregate		5–10
I-B		Conventional carbonation for 3 h
I-C		(1) CH spray (2) Conventional carbonation for 3 h
I-D		Wet carbonation (10 min)
II-A	River sand		0.15–5
II-B		Conventional carbonation for 3 h
II-C		(1) CH spraying (2) Conventional carbonation for 3 h
II-D		Wet carbonation (10 min)
II-E		Two-step wet carbonation

.

3. Testing Methods

3.1. Density and Water Absorption

After different carbonation treatments, the RCA samples used in this study were subjected to 105 °C/24 h drying and 24 h water immersion. After that, the characterizations, including the density and water absorption of the CRCA and FRCA, were tested according to BS EN 1097-6 [33].

3.2. Measurement of CO₂ Uptake

According to thermogravimetric methods, the mass loss of RCA samples between 550 °C and 850 °C is mainly regarded as the mass of CO₂, which released due to the decomposition of calcium carbonates. In this study, in order to compare the CO₂ uptake of RCA treated by different carbonation methods, the weight loss of the CRCA and FRCA between 550 and 850 °C was determined and normalized over the initial powder mass. The CRCA and FRCA were crushed and ground into powder with a particle size smaller than 150 $\mathsf{\mu }\mathrm{m}$ after oven-drying at 105 °C for 24 h.

3.3. Scanning Electron Microscopy

In order to investigate the microstructural and morphological changes in samples, scanning electron microscope (SEM) analysis was conducted. Specimens underwent gold sputtering prior to morphological characterization via COXEM EM-30AX+ SEM.

3.4. X-Ray Diffraction (XRD) Analysis

XRD was performed using a Rigaku Smart-Lab diffractometer with Cu-Kα radiation (λ = 1.54 Å) operated at 45 kV and 200 mA. The results of XRD focus on the crystalline compositions, especially portlandite (CH) and calcium carbonates (CCs). Prior to testing, the samples were dried at 60 °C for 24 h, then ground and sieved through a 75 μm mesh. The 10% fluorite (CaF2) powder was doped into the samples for a semi-quantitative analysis of CH and CC content by using the reference intensity ratio (RIR) method according to Equation (1) [34]. The samples were scanned over a range of 10–70°, with a step size of 0.02° and a scanning speed of 5° per minute.

$${\mathrm{X}}_{\mathrm{i}}={\mathrm{X}}_{\mathrm{s}}·({\displaystyle \frac{ks}{ki}}·{\displaystyle \frac{Ii}{Is}}){\displaystyle \frac{1}{1-Xs}}$$

(1)

3.5. Compressive Strength

To investigate the influence of different carbonation methods and replacement ratios on the mechanical properties, compressive strength tests were conducted on mortar specimens measuring 40 mm × 40 mm × 40 mm. The mixture proportions for the mortar specimens are summarized in

Table 5. Mixture proportions for the mortar specimens (kg/m³).

Replacement Ratio	Cement	Water	River Sand	FRCA	Carbonated FRCA	Flowability (mm)
0%	600	300	1800	0		185 ± 10
25%	600	300	1350	450
50%	600	300	900	900
75%	600	300	450	1350
100%	600	300	0	1800
25%	600	300	1350		450
50%	600	300	900		900
75%	600	300	450		1350
100%	600	300	0		1800

. The effective water-to-cement ratio was fixed at 0.5, and the cement-to-aggregate ratio was fixed at 1:3. To compensate for the high water absorption of the fine aggregate and attain consistent flowability, the water content was adjusted accordingly for each mixture group. In this study, three identical specimens were prepared for each test.

4. Results and Discussion

4.1. Water Absorption and Density

The modification of water absorption of CRCA and FRCA by four carbonation methods is shown in

Table 6. Density and water absorption of CRCA and FRCA.

Size Range		Treatment Methods	Density (kg/m3)	Water Absorption (%)
5–10 mm	I-A	N/A	2690.0	4.65
	I-B	Conventional carbonation for 3 h	2796.7	3.92
	I-C	(1) CH spray (2) Conventional carbonation for 3 h	2741.7	4.15
	I-D	Wet carbonation	2772.0	3.64
0.15–5 mm	II-A	N/A	2749.14	12.37
	II-B	Conventional carbonation for 3 h	2864.3	9.22
	II-C	(1) CH spray (2) Conventional carbonation for 3 h	2811.6	10.94
	II-D	Wet carbonation	2933.0	7.66
	II-E	Two-step wet carbonation	2820.9	7.68

. The water absorption of both CRCA and FRCA was significantly reduced after carbonation. Compared to the other three carbonation methods, the wet carbonation method was proven to be very effective, particularly for fine aggregates, achieving a maximum water absorption reduction of 38%. This might mainly be due to the reason that when CO₂ was dissolved in water, it generated carbonated substances (CO₃²⁻/HCO₃⁻/H₂CO₃), which rapidly react with the available Ca(OH)₂ (CH). After the depletion of available CH, the stably dissolved CO₂ gradually reacted with C-S-H and released additional Ca²⁺ and OH⁻ from the interior of the particles [35]. Moreover, this process significantly enhanced the transport efficiency of CO₂ within the pore areas of the RCA. In addition, it was also seen from the figure that carbonation was more effective for reducing water absorption of fine aggregates compared to coarse aggregates.

The density values of CRCA and FRCA, both prior to and following carbonation, are displayed in Table 6. The density of both CRCA and FRCA increased after carbonation. Meanwhile, all carbonation methods showed comparable enhancement for the apparent density of RCA. Given that carbonation efficiency tended to decline with increasing depth—due to the densification of the surface layer, which impeded the further inward diffusion of CO₂—the maximum carbonation efficiency was thus limited [36]. Moreover, FRCA showed greater improvement in density compared with CRCA because of a higher level of adhered mortar content [5]. The density of carbonated FRCA could even be close to that of river sand, suggesting a potential substitute material for fine aggregates.

4.2. Compressive Strength

Figure 4 illustrates the compressive strength values of mortar specimens where river sand was replaced at different ratios with four types of carbonated RCA. It can be observed that the compressive strength was influenced by both the carbonation methods and the river sand replacement ratio. For each carbonation method, the control specimen containing 100% river sand exhibited the highest compressive strength, whereas the specimen with 100% carbonated FRCA had the lowest. Overall, the compressive strength showed a decreasing tendency as the river sand replacement ratio increased, which can be attributed to the fact that carbonated FRCA has inferior properties compared to river sand. This phenomenon arises from the cracks and fissures formed in recycled aggregates during processing, which render the aggregates more prone to fluid permeation, diffusion, and absorption [37]. Different carbonation methods showed varying degrees of compensation in relation to the compressive strength of mortar specimens. Compared to conventional carbonation and CH spray, wet carbonation and two-step wet carbonation resulted in better enhancements of compressive strength. For instance, as can be seen from Figure 4, at a 50% replacement ratio, wet carbonation and two-step wet carbonation showed compressive strength improvements of 11.16% and 12.40%, respectively, over conventional carbonation. This is mainly because RCA achieves a higher degree of carbonation and faster morphological enhancement during the wet carbonation process. At 100% replacement of river sand, mortar specimens with two-step wet carbonated FRCA exhibited a 26.5% growth in compressive strength when compared with the original FRCA.

4.3. Microscopic Observation

Figure 5 shows the morphological characteristics of carbonated RCA before and after carbonation with different carbonation treatments. As FRCA had a higher content of adhered mortar and therefore was easier for observation, Figure 5 utilized FRCA for SEM images analysis. Typical hydration products, including CH, Aft, and C-S-H gel, were noticed prior to carbonation, which formed a loose and multi-cracked microstructure at the surface of the RCA samples. After carbonation, it was observed (Figure 5c–f) that the hydration products transformed into calcite (CC) during all carbonation processes. A large quantity of rhombic CC crystals were stacked and filled the pore areas of the adhered mortar, resulting in a denser microstructure and reducing the porosity. Moreover, CC was proven to enhance the formation of C-S-H of new cement paste due to the nucleation effect and therefore contributed to the improvement of porosity of the ITZ between carbonated RCA and the new paste when used in the new concrete [10,38,39]. All four carbonation methods obtained CC with a similar crystalline phase, indicating that the effect on the form of CC by different carbonation approaches was limited. SEM examinations further confirmed that two-step wet carbonation generated finer calcite crystals with homogeneous surface deposition on fine recycled concrete aggregates (FRCAs). The main reason was that the CO₃²⁻ penetrated into the inner pore areas of the FRCAs in an aqueous environment.

4.4. Hydration Profiles

The TG and DTG results of CRCA and FRCA after different carbonation treatments are presented in Figure 6. Judging from the figure, the increase in CC and decrease in CH were notably observed in both CRCA and FRCA after carbonation. Generally, the carbonation effect of FRCA was better than that of CRCA in terms of CH consumption and CC production due to a greater content of hydration products [40]. For CRCA, carbonation with CH spray showed the highest increase in the content of CC compared with conventional carbonation and wet carbonation, which suggested that the pretreatment method of CH spraying contributed to the production of CC, as it introduced an external source of CH for carbonation [41]. The effect of conventional carbonation was marginal, probably due to a low content of the adhered mortar of CRCA and a limited carbonation efficiency under ambient pressure [42]. On the other hand, the formed CC particles during wet carbonation might precipitate into the solution due to the relatively large pore size of CRCA and therefore reduce the content of CC in the carbonated CRCA [43].

For FRCA, the two-step wet carbonation exhibited the highest increase in the content of CC as the particles were in the CO₃²⁻-rich environment and the carbonation efficiency at the liquid–solid phase was high [30,44]. Moreover, although CH spray and wet carbonation have achieved an equivalent content of CC when enhancing the FRCA, the 10 min wet carbonation process was slightly more effective. This was mainly due to the relatively short duration of the wet carbonation process and the generated CC maintained in the internal pore areas, while the CH spray resulted in a fast densification of the surface as the produced CC accumulated and prevented CO₂ gas from penetrating into inner pore areas [20,45]. Meanwhile, the conventional carbonation for FRCA was the least effective approach. Therefore, the two-step carbonation was proven most effective for enhancing FRCA in terms of the production of carbonation products.

4.5. X-Ray Diffraction Analysis

The XRD patterns of the samples are depicted in Figure 7. Quartz (SiO₂) and CC were the primary sources of the most prominent diffraction peaks, alongside fluorite (CaF₂). The CC content of FRCA subjected to different carbonation methods calculated by the RIR method is summarized in

Table 7. CC content in FRCA by different carbonation approaches according to the RIR method.

	Conventional Carbonation	CH Spray	Wet Carbonation	Two-Step Wet Carbonation
CC%	7.3	8.9	11.1	16.1

. A comparison of the XRD patterns for the four carbonation methods (Figure 7) revealed the presence of characteristic peaks attributable to portlandite (CH) at approximately 20.1° and 34.1° for the wet carbonation, conventional carbonation, and CH spray methods. In contrast, these CH peaks were not observed in the sample treated by the two-step wet carbonation method. This absence indicates a high carbonation efficiency and suggests complete carbonation of CH by the two-step wet carbonation process compared to the other three methods. This finding agreed with the TGA results. Meanwhile, the semi-QXRD results for CC content in FRCA (Table 7) exhibited a higher CC concentration introduced by two-step wet carbonation than by other carbonation methods. However, the relatively lower CC contents observed in the present study compared with the previous study were likely attributable to the substantially shorter carbonation duration of 3 h [20].

4.6. CO₂ Uptake

The CO₂ uptake of CRCA and FRCA during the carbonation treatments is shown in Figure 8. The variation of CO₂ absorptions by the CRCA and FRCA during the treatment processes led to the different property enhancement effect resulting from microstructural densification efficiency [46]. Compared to conventional carbonation, CRCA treated by wet carbonation exhibited a CO₂ uptake increase of 42.9%, as the direct introduction of CO₂ gas into the liquid phase promoted ionic reactions that substantially enhanced carbonation efficiency [32]. The pretreatment of CH spray achieved 3.0% CO₂ uptake, demonstrating a notably improved CO₂ absorption capacity compared to the other two carbonation methods. The reason was that the CH sprayed on the surface provided external Ca²⁺, which is prone to react with CO₂ during carbonation [47]. A large amount of calcium carbonate was generated and eventually densified the surface of the adhered mortar. However, forming a dense surface layer would also prevent further carbonation [48]. Therefore, although spraying CH on CRCA led to a relatively high CO₂ uptake value, the improvement in water absorption and density was inferior to that of wet carbonation.

For FRCA, liquid–solid carbonation treatment was more effective in terms of CO₂ uptake. As shown in Figure 8, the CO₂ uptake of the two-step wet carbonation was 9.2%, which was 475% higher than that of gas–solid carbonation. This was due to the high concentration environment of carbonate ions, which promoted the carbonation reaction both on the surface and inner areas of FRCA [30]. The CO₂ uptake of CH spraying was similar to that of wet carbonation, which was 10 min, as the surface densification of the surface layer caused by CH spraying hindered a further carbonation reaction by preventing CO₂ from permeating into the inner pore areas of the aggregate.

5. Conclusions

In order to investigate the effect of different carbonation methods on enhancing the quality of CRCA (5–10 mm) and FRCA (0.15–5 mm), a series of experimental tests were conducted. The physical properties, including water absorption and density, and the chemical properties of CRCA and FRCA with different carbonation treatments were compared. The main conclusions were drawn as follows:

• All carbonation methods significantly reduced the water absorption and improved the apparent density of both CRCA and FRCA. This was because carbonation products like calcite (CC) filled the pores and cracks near the surface of CRCA and FRCA particles, making the microstructure denser. In addition, FRCA exhibited particularly notable enhancements because of a higher level of adhered mortar content.
• After carbonation, the hydration products, including CH, C-S-H gel, and Aft, were largely carbonated, and the pores and cracks near the surface areas of the FRCA particles were filled by carbonation products, making the microstructure denser.
• Different carbonation treatments impacted the formation of CC in CRCA and FRCA. For CRCA, the pretreatment of CH spray provided more Ca²⁺ and resulted in the highest production of CC. For FRCA, liquid–solid wet carbonation was the most effective method for the generation of CC due to a high reaction efficiency.
• Although the pretreatment of CH spray was effective for both CRCA and FRCA and exhibited the highest enhancement in CO₂ uptake capacity for CRCA, the reduction in water absorption and improvement in density were inferior to those achieved by wet carbonation, as the reaction primarily concentrates on the quickly densified surface area.
• For FRCA, the two-step wet carbonation achieved a significantly high CO₂ uptake of 9.2% compared with conventional carbonation, as the internal pores and cracks were filled instead of just surface accumulating, contributing to the creation of a denser matrix. Meanwhile, in terms of compressive strength, the two-step wet carbonation method had the best enhancement, which was almost close to the conditions of river sand.