Synergy of Carbon Sequestration and Solid Waste Resource Utilization: A Review on Carbonation Behavior of Fly Ash Concrete

Abstract

In recent years, the application of fly ash concrete (FAC) has witnessed a remarkable expansion worldwide. Compared with ordinary Portland cement (OPC), the incorporation of fly ash (FA) reduces the consumption of cement, realizes solid waste resource utilization, and concurrently cuts down carbon emissions from cement production, thus yielding notable environmental benefits. With the gradual popularization of concrete carbon sequestration technology, the research focus of academic circles on concrete carbonation behavior has shifted from the traditional orientation of “optimizing carbonation resistance” to the new direction of “enhancing carbon sequestration efficiency”. Nevertheless, current research on the mechanical properties, durability, and other behaviors of FAC after carbonation remains scarce, lacking systematic and in-depth exploration, and the mechanism underlying the impacts of carbonation on material properties still requires further systematic collation and generalization. Consequently, research on the carbonation behavior of FAC holds profound academic significance and promising application value. This paper reviews the microscopic mechanisms and influencing factors of FAC carbonation; summarizes and analyzes the effects of FAC carbonation on its various properties and microscopic pore structure; introduces the innovative breakthroughs in FAC technology in recent years; and finally, prospects future research directions. It is anticipated to provide a valuable reference for subsequent relevant studies.

1. Introduction

With the continuous advancement of global infrastructure construction, the cement industry, as the largest source of carbon emissions from carbonate decomposition, has experienced a more than 30-fold increase in output from 1950 to 2018. Carbon dioxide (${\mathrm{CO}}_2$) emissions generated during cement production account for 8% of the global total emissions [1], emerging as one of the key driving factors exacerbating the greenhouse effect [2,3,4]. The Special Report on Global Warming of 1.5 $°\mathrm{C}$ released by the Intergovernmental Panel on Climate Change (IPCC) in 2018 clearly put forward the crucial goal of limiting the global temperature rise to 1.5 $°\mathrm{C}$ above the pre-industrial level and achieving global carbon neutrality around 2050 [5]. As a core supporting field of the construction industry, the concrete sector possesses stable carbon sequestration potential. Studies have estimated that the annual carbon absorption capacity of concrete can neutralize up to 16% of the carbon emissions from cement production [6].

As a core ${\mathrm{CO}}_2$ emission reduction technology with both strategic value and application potential, carbon sequestration technology is based on the core principle of utilizing alkali metal ions (such as ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$) contained in natural minerals or industrial solid wastes. Through carbonation reaction with ${\mathrm{CO}}_2$, gaseous ${\mathrm{CO}}_2$ is converted into inorganic carbonate minerals with stable thermodynamic properties and excellent environmental compatibility, thereby realizing long-term and safe sequestration of ${\mathrm{CO}}_2$. Cement-based cementitious materials, which contain a large amount of hydration products such as calcium hydroxide (${\mathrm{Ca}\left(\mathrm{OH}\right)}_2$) and calcium silicate hydrate (C-S-H), can undergo carbonation reaction with ${\mathrm{CO}}_2$, exhibiting significant carbon sequestration potential [7,8]. Notably, calcium carbonate (${\mathrm{CaCO}}_3$) generated during the concrete carbonation process can effectively fill internal pores and optimize the microscopic structure [9,10,11]. Therefore, concrete carbon sequestration has increasingly become a hot topic attracting widespread attention in academic circles [12]. At present, relevant research in the field of concrete carbon sequestration has accumulated abundant achievements, and diversified technical paths have been developed to meet the carbon sequestration demands throughout the whole life cycle of concrete. Table 1 summarizes the carbon sequestration effects and property impacts of concrete under different carbon sequestration methods. It is clear that concrete carbon sequestration is mainly realized via carrier modification, cementitious component synergy, pore structure regulation, and solid waste recycling for ${\mathrm{CO}}_2$ mineralization and stabilization. Among various strategies, biochar-based core–shell aggregates simultaneously exhibit superior carbon sequestration performance and favorable structural stability. Meanwhile, carbon sequestration technology relying on construction waste recycling possesses promising application potential in large-scale low-carbon engineering applications.

Meanwhile, the discharge of industrial and waste incineration residues represented by fly ash (FA) has been increasing year by year, imposing significant pressure on ecological environment governance [13,14]. Against this backdrop, the utilization of such industrial solid wastes to replace part of cement for preparing construction concrete is widely recognized as an innovative approach with integrated environmental, resource, and carbon reduction benefits [15,16]. As a commonly used admixture for cement-based cementitious materials in the construction industry, FA offers several advantages, including low hydration heat and improvement of mixture workability via its glass bead structure. After secondary hydration (pozzolanic effect), its strength, impermeability and other properties can reach or even surpass those of ordinary Portland cement (OPC) [15,17,18]. However, the pozzolanic reaction of FA exhibits kinetic hysteresis characteristics [19], which may lead to the deterioration of early mechanical properties of concrete, the increase of matrix porosity and the coarsening of average pore size [20,21,22]. Meanwhile, the reduction in cement dosage results in a decrease in the total amount of ${\mathrm{Ca}\left(\mathrm{OH}\right)}_2$ generated by hydration and a drop in the pH value of pore solution [23], which intensifies the permeation and transmission of ${\mathrm{CO}}_2$. Coupled with pore loosening, it significantly increases the risk of steel bar corrosion [24]. Studies have shown that the incorporation of FA can alter the mechanical and durability properties of concrete [5,25,26], especially causing prominent deterioration in frost resistance and carbonation resistance [27,28]. In addition, municipal solid waste incineration fly ash (MSWIFA) can achieve heavy metal immobilization, improve volume stability and realize harmful substance resource utilization through ultra-high-performance concrete (UHPC) [29,30,31], yet problems such as high pretreatment cost, pore coarsening and reduced mechanical properties still need to be solved urgently.

Table 1. Concrete carbon sequestration efficiency and performance impacts under different carbon sequestration methods.

Carbon Sequestration Method	Carbon Sequestration Efficiency	Performance Impact	References
Utilizing the internal pore space of porous coarse aggregates as carbon dioxide (${\mathrm{CO}}_2$) sequestration carriers and realizing ${\mathrm{CO}}_2$ fixation by virtue of cations in alkaline slurry	20 kg/${\mathrm{m}}^3$	No adverse effect on strength development and pH value	[12]
Synergistic carbon sequestration method based on blast furnace slag and CaO	69.32 kg/${\mathrm{m}}^3$	Increases the specific surface area, pore volume and pore size of samples	[32]
Synergistic carbon sequestration method based on biochar-enabled core–shell aggregates (BCSA) (concrete with high BCSA dosage)	247.1 kg/${\mathrm{m}}^3$	Density of 1778 kg/${\mathrm{m}}^3$ and compressive strength of 35.8 MPa (maintaining the structural performance of concrete)	[33]
Replacing partial cement with waste rice straw biochar (WRSB) in recycled concrete followed by ${\mathrm{CO}}_2$ curing	24.66 kg/${\mathrm{m}}^3$	Compressive strength of 37.74 MPa; frost–thaw strength loss rate of only 8.68%; sulfate resistance coefficient up to 90.7%	[34]
Preparing recycled cement, recycled aggregates and recycled concrete using construction and demolition wastes for carbon sequestration	An estimated annual ${\mathrm{CO}}_2$ sequestration of 140–308 million tons if ordinary Portland cement is completely replaced		[35]
Foamed concrete was prepared via chemical foaming using cement, solid waste-based materials and 30% hydrogen peroxide, followed by curing for carbon sequestration	66.35 kg/${\mathrm{m}}^3$	The pore structure was optimized; under the optimal mix proportion conditions, the specific surface area, pore volume and average pore diameter reached 25.412 ${\mathrm{m}}^2$/g, 0.144 ${\mathrm{cm}}^3$/g and 12.840 nm, respectively	[36]

Table 1. Concrete carbon sequestration efficiency and performance impacts under different carbon sequestration methods.

Carbon Sequestration Method	Carbon Sequestration Efficiency	Performance Impact	References
Utilizing the internal pore space of porous coarse aggregates as carbon dioxide (${\mathrm{CO}}_2$) sequestration carriers and realizing ${\mathrm{CO}}_2$ fixation by virtue of cations in alkaline slurry	20 kg/${\mathrm{m}}^3$	No adverse effect on strength development and pH value	[12]
Synergistic carbon sequestration method based on blast furnace slag and CaO	69.32 kg/${\mathrm{m}}^3$	Increases the specific surface area, pore volume and pore size of samples	[32]
Synergistic carbon sequestration method based on biochar-enabled core–shell aggregates (BCSA) (concrete with high BCSA dosage)	247.1 kg/${\mathrm{m}}^3$	Density of 1778 kg/${\mathrm{m}}^3$ and compressive strength of 35.8 MPa (maintaining the structural performance of concrete)	[33]
Replacing partial cement with waste rice straw biochar (WRSB) in recycled concrete followed by ${\mathrm{CO}}_2$ curing	24.66 kg/${\mathrm{m}}^3$	Compressive strength of 37.74 MPa; frost–thaw strength loss rate of only 8.68%; sulfate resistance coefficient up to 90.7%	[34]
Preparing recycled cement, recycled aggregates and recycled concrete using construction and demolition wastes for carbon sequestration	An estimated annual ${\mathrm{CO}}_2$ sequestration of 140–308 million tons if ordinary Portland cement is completely replaced		[35]
Foamed concrete was prepared via chemical foaming using cement, solid waste-based materials and 30% hydrogen peroxide, followed by curing for carbon sequestration	66.35 kg/${\mathrm{m}}^3$	The pore structure was optimized; under the optimal mix proportion conditions, the specific surface area, pore volume and average pore diameter reached 25.412 ${\mathrm{m}}^2$/g, 0.144 ${\mathrm{cm}}^3$/g and 12.840 nm, respectively	[36]

It is worth noting that coal-fired FA can realize ${\mathrm{CO}}_2$ fixation by leaching metal ions via wet process or serving as adsorbent carrier via dry process, a characteristic that has been verified in previous studies [37,38,39]. As a supplementary cementitious material, FA also demonstrates significant carbon sequestration potential—FAC (Unless otherwise specified in this paper, FAC refers to concrete with a FA replacement ratio of no more than 40%. High-volume fly ash concrete with a replacement ratio exceeding 40% will be explicitly indicated in the text.) can act as a ${\mathrm{CO}}_2$ “scavenger”, realizing solid waste resource utilization while accomplishing carbon sequestration [40,41,42]. Furthermore, studies have indicated that under accelerated carbonation conditions, MSWIFA can not only capture ${\mathrm{CO}}_2$, but also adsorb and immobilize heavy metal ions [19,43]; however, the application of this technical path in the field of carbon sequestration for supplementary cementitious materials of concrete still has certain limitations. The application of FAC carbon sequestration technology can not only enhance the mechanical properties and long-term durability of materials, but also realize the resource utilization of FA. It provides support for the “dual-carbon” goal through ${\mathrm{CO}}_2$ mineral sequestration and ultimately constructs a synergistic efficiency system of “solid waste resource utilization—carbon sequestration—material property optimization” (Figure 1), laying a foundation for the research, development and application of low-carbon high-performance concrete.

At present, relevant academic research mostly focuses on the carbon sequestration characteristics and resource utilization of FAC. This paper presents a narrative literature review that aims to discuss the carbonation behavior of FAC. It will deeply explain the carbonation mechanism and regulatory factors of FAC, analyze the evolution laws of pores and various properties caused by FAC carbonation, introduce the innovative breakthroughs in FAC carbon sequestration technology throughout the whole life cycle, and discuss and prospect future research directions, so as to provide reference for subsequent relevant studies.

To obtain valid literature related to “carbon sequestration of fly ash concrete”, CNKI, Wanfang and other Chinese databases as well as Web of Science, PubMed and other foreign databases were selected for retrieval. The retrieval time was set from January 2019 to April 2026, with the retrieval deadline of 27 April 2026; in addition, some earlier but classic and high-quality studies were also included to ensure the comprehensiveness of the retrieval. “Fly Ash Concrete, Carbonation, Carbon sequestration” were taken as the main search terms, and “Carbonation curing, Carbonation Depth, Pore structure, Durability, Low-Carbon, in-situ carbon sequestration, sodium carbonate, mechanical properties, influencing factors, mechanism, stress state, curing conditions, accelerated carbonation and natural carbonation” as the secondary search terms. The retrieval formula was constructed by combining AND/OR/NOT Boolean logic. Studies included were formally published, relevant in content, up to standard in quality, and belonged to EI, Chinese Core Journals and SCIE, including Chinese and English journal papers and dissertations. Irrelevant, low-quality, duplicate and non-academic studies were excluded to ensure that the retrieval results accurately meet the research needs.

2. Factors Influencing the Carbonation of Fly Ash Concrete

Figure 2 presents the correlation framework of key influencing factors in the carbonation reaction system of fly ash concrete (FAC). It can be observed that the direct factors affecting FAC carbonation include ${\mathrm{CO}}_2$ concentration, calcium content of raw materials, pore structure, pore water content, etc. The external influencing factors are mainly humidity, type of cementitious materials, water-to-binder ratio (w/b), FA dosage, temperature [44], and so forth. In addition, the carbonation of FAC is also affected by factors such as stress state [45], ultraviolet radiation [46], and curing conditions [47].

2.1. Fly Ash Content

FA replacement level is one of the core factors influencing the carbonation performance of FAC. Lu et al. [48] reported that, under identical accelerated carbonation conditions, the carbonation depth of concrete with a 45% FA replacement ratio was approximately twice that of concrete with a 15% FA replacement ratio. Zhang et al. [49] indicated that, within the same carbonation duration, the average increases in carbonation depth for concrete incorporating 5%, 10%, 20%, and 30% FA were 6.4%, 14.9%, 59.8%, and 73.5%, respectively, compared with concrete without FA. It can be observed that, at the same carbonation age, the carbonation rate of FAC accelerates and its carbonation depth increases significantly with an increasing FA replacement ratio. However, such an increase in carbonation extent is not inevitable, but can be controlled by the w/b. Under low w/b between 0.16 and 0.2, a high FA replacement ratio can optimize the pore structure [50], thereby enhancing the carbonation resistance of concrete. When the fly ash content ranges from 10% to 30%, the proportions of micropores and mesopores in concrete are significantly increased, exhibiting a pore structure refinement effect. Within this dosage range, all three types of characteristic pore diameters decrease with the increase in fly ash content [51]. From the perspective of the action mechanism, the influence of FA on concrete carbonation can be divided into two aspects: indirect and direct effects. On the one hand, the pore characteristics of FAC show different variation trends with changes in the FA replacement ratio [52,53]; thus, the FA replacement ratio can indirectly affect the diffusion behavior of ${\mathrm{CO}}_2$ by altering the pore characteristics, thereby regulating the carbonation reaction process. On the other hand, the FA replacement level directly modulates the carbonation reaction process by controlling the pH value and the content of low-calcium C-S-H [54]. With the gradual increase in FA replacement level, the supply of calcium ions inside the paste becomes increasingly limited; at this point, C-S-H will participate more extensively in the carbonation reaction [55], which in turn leads to an increase in paste porosity and a further expansion of carbonation depth [56].

2.2. Water-to-Binder Ratio

For concrete with the same fly ash content, a lower w/b results in a smaller carbonation coefficient. This is primarily attributed to the densification of the concrete’s pore structure induced by a low w/b, which impedes the diffusion of ${\mathrm{CO}}_2$ [33]. An increase in the w/b elevates the content of free water in the cementitious system, thereby reducing its compactness [57]. Subsequent relevant studies [58,59] have further confirmed the inhibitory effect of low-w/b concrete microstructures on ${\mathrm{CO}}_2$ diffusion. In addition, existing studies [60] have demonstrated that when the w/b values are 0.65, 0.50 and 0.35, the carbonation coefficients (n = 4) are 6.621, 6.216 and 2.986, respectively. It can be observed that the carbonation coefficient of FAC presents an overall decreasing trend with the reduction of w/b, which further verifies the aforementioned theoretical analysis. More importantly, a lower w/b weakens the influence of fly ash content on fly ash concrete (i.e., leading to a lower carbonation coefficient) [33]. Notably, in concrete mix design, achieving a low water–binder ratio generally requires a higher dosage of cementitious materials, which is an established objective fact. Therefore, FAC with a low water–binder ratio does not only provide beneficial effects: The increased cementitious material consumption associated with a low w/b may offset part of the ${\mathrm{CO}}_2$ emission reduction benefits derived from FA incorporation, and these impacts warrant a comprehensive evaluation.

2.3. Carbon Dioxide Concentration

Cui et al. [52] observed a remarkably steep porosity gradient at the carbonation front, which indicates that all carbonation processes in this study were diffusion controlled. According to Fick’s first law of diffusion, a higher ${\mathrm{CO}}_2$ concentration gradient corresponds to a faster ${\mathrm{CO}}_2$ ingress rate into concrete and a more rapid carbonation reaction. However, ${\mathrm{CO}}_2$ diffusion exhibits distinct behaviors at different concentration levels due to the progressive carbonation process. As illustrated in Figure 3, under the supply of high-concentration ${\mathrm{CO}}_2$ (20–100%), pores in the outermost layer of concrete are filled and blocked by carbonation products. Consequently, the permeability to ${\mathrm{CO}}_2$ diffusion is significantly reduced, and the concrete structure undergoes pronounced densification. This mechanism has also been reported in subsequent studies [6]. The reduction in pore space under high ${\mathrm{CO}}_2$ concentrations is also seen in FAC; however, it leads to a more pronounced coarsening of the pore size distribution [61,62]. When the ${\mathrm{CO}}_2$ concentration ranges from 0 to 20%, carbonation results are mainly found close to the pore surfaces, making some pore filling and letting more ${\mathrm{CO}}_2$ move into the concrete body. Cui et al. [63] reported that the carbonation depth initially increases and then tends to stabilize with rising ${\mathrm{CO}}_2$ concentration. Under the conditions of 28-day carbonation curing, the carbonation depths were measured as 1.76 mm, 8.23 mm, 14.60 mm, 17.90 mm, and 17.90 mm at ${\mathrm{CO}}_2$ concentrations of 2%, 10%, 20%, 50%, and 100%, respectively.

Houst et al. [64] said that the link between ${\mathrm{CO}}_2$ level and carbonation speed is not set but instead shows changing variation features. Den et al. [62] proposed the moisture-induced pore blocking effect when conducting experimental investigations on the exposure of high-volume fly ash (HVFA) concrete to different ${\mathrm{CO}}_2$ concentrations. Meanwhile, the carbonation reaction rates of different phases show significant differences in response to ${\mathrm{CO}}_2$ concentrations [58]. Some studies have indicated that [65] high ${\mathrm{CO}}_2$ concentrations can enhance the carbonation degree of C-S-H. On the other hand, at a constant temperature, ${\mathrm{CO}}_2$ concentration can promote carbonation by increasing its solubility in the pore solution [66].

2.4. Curing Conditions

Studies have indicated [67,68] that prolonged curing ages can significantly reduce the carbonation depth of FAC. This is attributed to the fact that extended curing periods accelerate and intensify the pozzolanic reaction of FA; the resultant hydration products, such as C-S-H and calcium-aluminum-silicate hydrate (C-A-S-H), fill the internal pores, impede the diffusion of ${\mathrm{CO}}_2$, and thereby inhibit the progression of the carbonation reaction.

The effects of different curing methods and temperatures on the carbonation behavior of FAC show significant differences. Cui et al. [69] indicate that after 28 days of carbonation curing, the carbonation depths are 12.74 mm (25% replacement rate, 35 °C), 14.38 mm (25% replacement rate, 20 $°\mathrm{C}$), 14.76 mm (40% replacement rate, 35 $°\mathrm{C}$), and 17.04 mm (40% replacement rate, 20 $°\mathrm{C}$), respectively. It can be seen that under the same FA content and curing duration, the carbonation depth of concrete cured at 35 $°\mathrm{C}$ is lower than that of concrete cured at 20 $°\mathrm{C}$. On the other hand, Luo Guo et al. [70] conducted indoor and film-covered curing at different temperatures and got the opposite finding (The carbonation depths under film-covered curing at 20 °C and 35 °C are 14.96 mm and 17.04 mm, respectively). This discrepancy can be mainly attributed to the difference in curing methods: 24 h high-temperature (like 35 $°\mathrm{C}$) film-covered curing does not sufficiently support the pozzolanic reaction, and a tight pore structure cannot be produced, yielding good conditions for ${\mathrm{CO}}_2$ spread and carbonation inside the concrete. Also, after form removal and moving to a 30 $°\mathrm{C}$ setting, the fairly high temperature may cause the production of linked pores inside the concrete [71]. In contrast, long-term high-temperature water bath curing can yield a faster and fuller pozzolanic reaction of FA, leading to a tighter pore structure [72,73] that effectively stops ${\mathrm{CO}}_2$ spread. Studies have indicated [74] that higher temperatures can speed up ion breakdown and so help carbonation; however, temperatures above 30–40 $°\mathrm{C}$ often lower ${\mathrm{CO}}_2$ dissolving and weaken later hydration reactions.

2.5. Stress State

In real engineering cases, the weight of concrete and added loads can cause changes in internal stress. So, looking at how stress state affects the carbonation of FAC is very important. The effect of stress state on FAC carbonation clearly depends on stress type: Under pulling stress, the carbonation depth goes up steadily with the stress level. Under pushing stress, when the stress level range is big enough (0–0.9 fc), the carbonation depth shows a drop-then-rise pattern, with the best stress level found to be 0.3 fc [44]. Old carbonation depth models do not fully account for the stress effect. Wang et al. [75] introduced a stress impact factor to the concrete carbonation depth model, built a math link between this factor and the concrete stress ratio, and put forward a changed carbonation depth guess model. The correctness of this model was checked using test data and real-world notes. For FAC, its carbonation behavior is naturally greatly controlled by hydration steps (e.g., ${\mathrm{Ca}\left(\mathrm{OH}\right)}_2$ use and pore structure improvement). The modeling plan given by Wang et al. offers a key guide for measuring the combined effects of stress and FA amount on carbonation depth. In short, the stress state controls the ${\mathrm{CO}}_2$ spread and carbonation reaction process by changing the micro-pore and crack structure of FAC: Pulling stress always speeds up carbonation, while pushing stress shows a stop-then-speed-up change feature.

2.6. Effects of Various Factors on Mechanical Performance of Carbonated Concrete

Table 2. Carbonation curing parameters and performance of fly ash concrete.

Test Procedure *	Carbonation Conditions	Fly Ash Content (%)	w/b	Compressive Strength After Normal Curing (MPa)	Compressive Strength After Carbonation (MPa)	Carbon Sequestration Efficiency (kg/${\mathbf{m}}^3$)	Reference
(1) Demolding → Standard curing for 28 d → Carbonation curing for 26 d (2) Demolding → Standard curing for 28 d	Temperature: (20 ± 3) $°\mathrm{C}$ Relative humidity: (75 ± 5)% ${\mathrm{CO}}_2$ concentration: (70 ± 5)%	10	0.4	38.46	43.99	4.2	[78]
(1) Demolding → Standard curing for 28 d → Carbonation curing for 26 d (2) Demolding → Standard curing for 28 d	Temperature: (20 ± 3) $°\mathrm{C}$ Relative humidity: (75 ± 5)% ${\mathrm{CO}}_2$ concentration: (70 ± 5)%	20	0.4	39.9	44.5	5.2	[78]
(1) Pre-curing for 8 h (20 $°\mathrm{C}$, 60% RH) → Carbonation curing for 12 h → Standard curing for 148 h (2) Standard curing for 7 d	Temperature: 20 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 20%	30 (High-calcium)	0.35	52.22	49.72	–	[76]
(1) Pre-curing for 8 h (20 $°\mathrm{C}$, 60% RH) Carbonation curing for 12 h → Standard curing for 148 h (2) Standard curing for 7 d	Temperature: 20 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 20%	30 (Low-calcium)	0.35	32.25	34.33	–	[76]
(1) Pre-curing for 8 h (20 $°\mathrm{C}$, 60% RH) Carbonation curing for 12 h → Standard curing for 148 h (2) Standard curing for 7 d	Temperature: 20 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 20%	10 (Low-calcium)	0.35	49.04	49.06	–	[76]
(1) Demolding → Carbonation curing for 3 d (2) Demolding → Water curing for 3 d	Temperature: 35 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 10%	10	0.45	12.33	25.84	21.86	[42]
(1) Demolding → Carbonation curing for 3 d (2) Demolding → Water curing for 3 d	Temperature: 35 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 10%	20	0.45	18.67	24.17	10.01	[42]
(1) Demolding → Carbonation curing for 3 d (2) Demolding → Water curing for 3 d	Temperature: 35 $°\mathrm{C}$ Relative humidity: 70% ${\mathrm{CO}}_2$ concentration: 10%	30	0.45	16.29	22.09	20.55	[42]
(1) Water curing for 28 d → Drying for 24 h → Carbonation curing for 28 d (2) Water curing for 28 d	Temperature: 30–35 $°\mathrm{C}$ Relative humidity: 60–70% ${\mathrm{CO}}_2$ concentration: 5%	30	0.35	35.6	36.16	–	[60]
(1) Water curing for 28 d → Drying for 24 h → Carbonation curing for 28 d (2) Water curing for 28 d	Temperature: 30–35 $°\mathrm{C}$ Relative humidity: 60–70% ${\mathrm{CO}}_2$ concentration: 5%	30	0.5	25.23	28.15	–	[60]
(1) Water curing for 28 d → Drying for 24 h → Carbonation curing for 28 d (2) Water curing for 28 d	Temperature: 30–35 $°\mathrm{C}$ Relative humidity: 60–70% ${\mathrm{CO}}_2$ concentration: 5%	30	0.65	19.97	24.14	–	[60]

* (1) carbonation curing; (2) conventional curing (control group).

shows the carbonation curing parameters and performance of FAC. As indicated in Table 2, the mechanical properties of carbonated FAC are generally superior to those of the non-carbonated control group. This is mainly attributed to the filling effect of ${\mathrm{CaCO}}_3$ generated by carbonation reaction, which effectively densifies the internal microstructure of concrete [76]; meanwhile, ${\mathrm{CaCO}}_3$ acts as an internal skeleton and adhesive phase inside concrete, further improving the structural integrity [77]. It should be noted that the above rule is not applicable to all working conditions. The research conducted by Yao et al. [78] demonstrated that carbonation may lead to early strength degradation of FAC, which can be explained by differences in curing regimes. Existing studies have verified that stable strength growth of FAC can be achieved when sufficient conventional early curing is implemented prior to carbonation curing [60]. In contrast, if carbonation curing is carried out with inadequate early conventional curing, the strength variation of FAC becomes unpredictable [76]. This phenomenon is presumably due to the inhibitory effect of carbonation on the early pozzolanic reaction of concrete [68]. Further analysis of the data in Table 2 shows that the replacement ratio of FA exerts an uncertain effect on the compressive strength of carbonated FAC. In comparison with ordinary low-calcium FAC, mixtures incorporating high-calcium FA exhibit better mechanical performance after carbonation. Meanwhile, under reasonable curing conditions, increasing the ${\mathrm{CO}}_2$ concentration within a short term can significantly enhance the compressive strength of FAC. Nevertheless, considering that the carbonation of hydration product C-S-H can cause the deterioration of concrete pore structure [79], the long-term stability of such strength enhancement still requires further investigation and verification. In addition, the compressive strength of carbonated FAC presents a distinct declining trend with the increase of w/b. A higher w/b contributes to the formation of a large number of interconnected pores inside concrete, weakens structural compactness, and consequently reduces the compressive capacity of FAC.

3. Mechanism of Fly Ash Concrete Carbonation

3.1. Hydration Mechanism of Fly Ash Concrete

The hydration of FAC is a step-by-step process, split into the cement–water reaction-led part and the pozzolanic reaction-led part. In the first part, cement–water reaction comes first, making C-S-H gel, $\mathrm{Ca}(\mathrm{OH}{)}_2$, and ettringite (AFt), which build the early-strength frame of the concrete. In the later part, the pozzolanic reaction takes over, making low-calcium C-S-H with a tobermorite-like shape. This low-calcium C-S-H has many flaws focused at the bridging tetrahedral spots. At the same time, the making of low-calcium C-S-H raises the aluminum uptake in the C-S-H phase, leading to the making of C-A-S-H [55]. The calcium amount of FA can control the water reaction results and their tiny structures, thereby boosting the long-term strength and lasting quality of the concrete.

It is widely thought that the main parts of FA, that is silicon dioxide (${\mathrm{SiO}}_2$) and aluminum oxide (${\mathrm{Al}}_2{\mathrm{O}}_3$), react with $\mathrm{Ca}(\mathrm{OH}{)}_2$ and water, the water reaction results of cement, to make C-S-H and calcium-aluminum hydrate (C-A-H). The reactions are shown in Equations (1) and (2) below:

$$\begin{array}{cc}\hfill {\mathrm{SiO}}_2+{\mathrm{Ca}\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2\mathrm{O}& \to \mathrm{C}\text{-}\mathrm{S}\text{-}\mathrm{H}\hfill \end{array}$$

(1)

$$\begin{array}{cc}\hfill {\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{Ca}\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2\mathrm{O}& \to \mathrm{C}\text{-}\mathrm{A}\text{-}\mathrm{H}\hfill \end{array}$$

(2)

Given the significant hysteretic characteristics of the FA pozzolanic reaction and the structural features of hollow FA particles, in the later stage of hydration, as the pozzolanic reaction proceeds continuously, the hollow FA particles react with $\mathrm{Ca}(\mathrm{OH}{)}_2$ to form hydration products, leaving residual cavities inside the particles, which then transform into ink-bottle-shaped pores. Simultaneously, in the blended cement paste system, these cavities can provide additional accommodation space for reaction products, including both cement hydration products and FA pozzolanic reaction products [80].

3.2. Carbonation Mechanism and Products of Fly Ash Concrete

The carbonation of FAC is an extremely complex process, which can be generalized as the carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ and C-S-H to form polymorphic ${\mathrm{CaCO}}_3$. Most of the time, the reaction between unhydrated cement and ${\mathrm{CO}}_2$ is not important [55]. Many studies have shown [63,81] that when put next to natural carbonation, the quick carbonation process under dry conditions shows two clear features: (1) a ${\mathrm{CaCO}}_3$ coating layer is on the surface of $\mathrm{Ca}(\mathrm{OH}{)}_2$ crystals; (2) the making of metastable and amorphous ${\mathrm{CaCO}}_3$, together with the creation of bicarbonate ions (${\mathrm{HCO}}_3^{-}$) under certain conditions. From a thermodynamic point of view, $\mathrm{Ca}(\mathrm{OH}{)}_2$, with a bigger solubility product constant ($K_{sp}$), should react with ${\mathrm{CO}}_2$ in the carbonation system first, and the carbonation of C-S-H would not start until $\mathrm{Ca}(\mathrm{OH}{)}_2$ is used up. However, this thermodynamic-led reaction order does not fully explain the microstructural differences and the speed of carbonation, so it has clear limits. Studies have confirmed [82] that at the initial stage of carbonation, the carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ indeed shows a significant thermodynamic advantage with a higher reaction priority than that of C-S-H; however, as the carbonation reaction proceeds to the middle and late stages, the carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ and C-S-H no longer follows a sequential pattern but instead occurs synchronously. It is noteworthy that the incorporation of FA results in a $\mathrm{Ca}(\mathrm{OH}{)}_2$ content much lower than that in ordinary Portland cement paste, leading to the carbonation of a larger proportion of C-S-H. Under such circumstances, the microstructural evolution induced by carbonation in concrete with high FA replacement ratios is primarily governed by the carbonation of complex mixtures of multi-type C-S-H [55]. Based on laboratory research scenarios, this section systematically summarizes the reaction mechanisms and product characteristics of accelerated carbonation tests from the perspective of different hydration products in cementitious materials.

3.2.1. Carbonation of Calcium Hydroxide

The carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ refers to the process in which ${\mathrm{CO}}_2$ and $\mathrm{Ca}(\mathrm{OH}{)}_2$ dissolve in the pore solution and react to form carbonate compounds, followed by the precipitation of ${\mathrm{CaCO}}_3$ under supersaturated conditions, as illustrated in Equation (3). Studies have revealed [83] that the final crystalline form of ${\mathrm{CaCO}}_3$ derived from $\mathrm{Ca}(\mathrm{OH}{)}_2$ carbonation is consistently calcite, while the existence of intermediate products depends on the dominant relationship between thermodynamic and kinetic factors within the reaction system: When the reaction is dominated by kinetic factors, metastable ${\mathrm{CaCO}}_3$ (e.g., aragonite and vaterite) precipitates first; when governed by thermodynamic factors, ${\mathrm{CaCO}}_3$ directly precipitates in the form of stable calcite.

$${\mathrm{Ca}\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(3)

The carbonation curve of $\mathrm{Ca}(\mathrm{OH}{)}_2$ exhibits a gradual pattern without an obvious reaction front [82]. Figure 4 presents the typical $\mathrm{Ca}(\mathrm{OH}{)}_2$ depth distribution curves after 14 and 28 days of accelerated carbonation. It can be observed that the carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ can be divided into three regions: (1) Uncarbonated zone: The $\mathrm{Ca}(\mathrm{OH}{)}_2$ content in this zone remains at the initial level (typically with pH > 12); (2) Transition zone: The characteristic of this zone is that as the distance from the specimen surface increases, the degree of ${\mathrm{CO}}_2$ penetration gradually decreases and the carbonation reaction weakens, resulting in less $\mathrm{Ca}(\mathrm{OH}{)}_2$ consumption; (3) External carbonated zone: The carbonation reaction in this zone is relatively complete, yet residual $\mathrm{Ca}(\mathrm{OH}{)}_2$ is detected. This is because $\mathrm{Ca}(\mathrm{OH}{)}_2$ exists as micron-sized coarse crystals, and the ${\mathrm{CaCO}}_3$ formed at the initial stage of carbonation generates a dense coating layer on the crystal surface, which hinders the transport of ions (${\mathrm{Ca}}^{2+}$, ${\mathrm{OH}}^{-}$) and ${\mathrm{CO}}_2$, thereby slowing down the reaction and preventing complete $\mathrm{Ca}(\mathrm{OH}{)}_2$ consumption. Consequently, the carbonation rate of $\mathrm{Ca}(\mathrm{OH}{)}_2$ gradually decelerates in the later stage and eventually stabilizes.

3.2.2. Carbonation of Calcium Silicate Hydrate

The carbonation of C-S-H refers to a process where the hydrated product C-S-H reacts with ${\mathrm{CO}}_2$ to undergo decalcification, thereby forming silica gel and polymorphic ${\mathrm{CaCO}}_3$ [84]. Studies have indicated that each kilogram of C-S-H is capable of absorbing 301 g of ${\mathrm{CO}}_2$ [85]. Similar to $\mathrm{Ca}(\mathrm{OH}{)}_2$, C-S-H also lacks a distinct reaction front, whereas its reaction rate remains relatively constant [82]. The reaction mechanisms are expressed in Equations (4)–(6) [72].

$$3\mathrm{CaO}·2{\mathrm{SiO}}_2·3{\mathrm{H}}_2\mathrm{O}+3{\mathrm{CO}}_2\to 3{\mathrm{CaCO}}_3·2{\mathrm{SiO}}_2·3{\mathrm{H}}_2\mathrm{O}$$

(4)

$$3\mathrm{CaO}·{\mathrm{SiO}}_2+\gamma {\mathrm{H}}_2\mathrm{O}+3{\mathrm{CO}}_2\to 3{\mathrm{CaCO}}_3+{\mathrm{SiO}}_2·\gamma {\mathrm{H}}_2\mathrm{O}$$

(5)

$$2\mathrm{CaO}·{\mathrm{SiO}}_2+\gamma {\mathrm{H}}_2\mathrm{O}+2{\mathrm{CO}}_2\to 2{\mathrm{CaCO}}_3+{\mathrm{SiO}}_2·\gamma {\mathrm{H}}_2\mathrm{O}$$

(6)

During the carbonation process, the calcium-silicon ratio (Ca/Si) and relative humidity conditions exert a significant influence on the collapse and reconstruction processes of the C-S-H structure; specifically, a higher Ca/Si corresponds to a more pronounced degree of carbonation [84,86]. Figure 5 presents a schematic diagram of the C-S-H carbonation process, from which it can be observed that the carbonation process is divided into three stages [87]: (1) Dissolution stage: Calcium ions between the silicate layers dissolve preferentially and react with carbonate ions (${\mathrm{CO}}_3^{2-}$) to form a ${\mathrm{CaCO}}_3$ layer. As the ${\mathrm{CaCO}}_3$ layer thickens, the rate of ${\mathrm{CaCO}}_3$ formation decreases, with the C-S-H framework remaining structurally intact during this period. (2) Diffusion stage: With the progression of the carbonation reaction, the C-S-H framework initiates decomposition, and silica gel is gradually generated. (3) Slow and ongoing reaction stage: Nearly complete carbonation of C-S-H is achieved in this final stage. Notably, the formation of carbonation products during the entire process exerts a significant detrimental effect on the micromechanical properties of the material.

As can be seen from

Table 3. Carbonation products of silicate hydrate (C-S-H).

Research Content	Carbonation Conditions	Carbonation Degree	Main Products	References
Accelerated carbonation test of synthetic calcium C-S-H	20 $°\mathrm{C}$, atmospheric pressure (101 kPa), ${\mathrm{CO}}_2$ concentration of 99%, flow rate of 2 L/min (through C-S-H solution in the reactor), solution stirred at 500 rpm	100%	68.4% calcite, 31.6% silica gel	[85]
Accelerated carbonation test	21 ± 1 $°\mathrm{C}$, ${\mathrm{CO}}_2$ concentration of 50% ± 5%, relative humidity (RH) of 53% (14 days)		Calcite, aragonite, vaterite, amorphous calcium carbonate ${\mathrm{CaCO}}_3$, silica gel	[82]
Accelerated carbonation test of synthetic calcium C-S-H	0.2 MPa, ${\mathrm{CO}}_2$ concentration of 99.5%, duration of 2 h, RH ≥ 95%	71.5–78%	33.98% calcite, 17.13% aragonite, 18.74% vaterite, 30.15% amorphous phase	[88]
Accelerated carbonation test of synthetic calcium C-S-H	Temperature of 20 ± 0.3 $°\mathrm{C}$, RH of 75% ± 0.5%, ${\mathrm{CO}}_2$ concentration of 3% ± 0.2%; C-S-H powder vacuum-dried at 45 $°\mathrm{C}$ to constant weight before carbonation	60%	Calcite, vaterite, a small amount of aragonite (classified by high and low calcium-silicon ratio (Ca/Si))	[87]
Accelerated carbonation test of synthetic calcium C-S-H	Ambient temperature (no additional temperature control, approx. 25 $°\mathrm{C}$), ${\mathrm{CO}}_2$ concentration of 3 vol%, RH of 40% and 75%, carbonation duration of 1 day and 7 days	75%	Calcite	[89]

(Data on C-S-H carbonation products under different carbonation conditions), the primary carbonation products of C-S-H are polymorphic ${\mathrm{CaCO}}_3$ and silica gel. It is evident that the specific crystalline forms of ${\mathrm{CaCO}}_3$ are not singular and fixed, but rather jointly regulated by multiple factors including the Ca/Si and carbonation degree. Studies have found that [87] high-Ca/Si conditions tend to facilitate the formation of calcite, a thermodynamically stable phase, while under low-Ca/Si conditions, the carbonation products are dominated by vaterite, a metastable phase.

4. Properties of Fly Ash Concrete After Carbonation

4.1. Influence of Pores

The pores in concrete can be categorized into gel pores (10 nm), fine capillary pores (10–50 nm), medium capillary pores (50–100 nm), coarse capillary pores (100 nm–10 µm), and macropores (10 µm) [90]. The carbonation of C-S-H is the dominant factor governing porosity variation during paste carbonation, accounting for more than 70% of the total porosity change [79]. Figure 6 illustrates the variation law of pore volume with different pore sizes under carbonation. Generally, the accelerated carbonation process tends to reduce capillary porosity while increasing gel porosity, with the peak value of pore size distribution decreasing and shifting toward the macropore direction [53,55,91].

Studies have found [92,93] that with the continuous progression of carbonation in FAC, the porosity of the FAC system exhibits a distinct decreasing trend. Shen et al. [79] observed that volume expansion occurs in the early stage of carbonation; upon completion of the carbonation process, volume shrinkage takes place, and ${\mathrm{CaCO}}_3$ content reaches its maximum value. However, Cui et al. [52] investigated the evolution characteristics of pore structure under carbonation for concrete with different FA replacement ratios. The results showed that after carbonation, the porosity of all concrete specimens with varying FA replacement ratios presents a significant increasing trend. In summary, the evolution law of pore structure induced by the carbonation of FAC requires systematic analysis based on multi-dimensional influencing factors, making it difficult to draw a simplistic conclusion. Essentially, this change in pore structure can be attributed to the regulatory effect of the carbonation behavior of concrete hydration products ($\mathrm{Ca}(\mathrm{OH}{)}_2$ and C-S-H gel) on the pore system. Studies have indicated [6,55] that the carbonation reaction of $\mathrm{Ca}(\mathrm{OH}{)}_2$ leads to a decrease in the porosity of cement paste. The essential driving force for this phenomenon stems from the molar volume difference between the carbonation product ${\mathrm{CaCO}}_3$ and the reactant $\mathrm{Ca}(\mathrm{OH}{)}_2$ ($V_{\mathrm{mCH}}=33.07{\mathrm{cm}}^3/\mathrm{mol}$; $V_{{\mathrm{mCaCO}}_3}=36.93{\mathrm{cm}}^3/\mathrm{mol}$). The larger molar volume of ${\mathrm{CaCO}}_3$ generates a filling effect on the original pores during the reaction process, thereby reducing the porosity. Nevertheless, different types of C-S-H gels exhibit significant differences in their inherent properties, and both the carbonation reaction process and the properties of the resulting silica gel are characterized by uncertainty. This results in distinct discrepancies in the influence laws of different C-S-H gel types on the microstructure of cement paste (especially porosity and pore size distribution) after carbonation.

To systematically achieve qualitative analysis and quantitative characterization of the pore structure evolution characteristics of FAC under carbonation, the academic community has carried out extensive targeted research on this topic. Morandeau et al. [56] established a model for solid volume change under carbonation, the expression of which is shown in Equation (7). $\Delta \varphi$ denotes the volume change before and after carbonation; $\Delta {\varphi }_{\mathrm{CH}}$ and $\Delta {\varphi }_{\mathrm{CSH}}$ represent the volume changes of $\mathrm{Ca}(\mathrm{OH}{)}_2$ and C-S-H before and after carbonation, respectively; $n_{\mathrm{CH}}^{\mathrm{CC}}$ and $n_{\mathrm{CSH}}^{\mathrm{CC}}$ stand for the molar amounts of ${\mathrm{CaCO}}_3$ generated from the carbonation of $\mathrm{Ca}(\mathrm{OH}{)}_2$ and C-S-H, respectively; $V_{\mathrm{CC}}^{\mathrm{CH}}=V_{\mathrm{CC}}^{\mathrm{CSH}}=36.93{\mathrm{cm}}^3/\mathrm{mol}$ and $V_{\mathrm{CH}}=33.07{\mathrm{cm}}^3/\mathrm{mol}$ refer to the molar volumes of ${\mathrm{CaCO}}_3$ and $\mathrm{Ca}(\mathrm{OH}{)}_2$, respectively; $V_{\mathrm{CSH}\left(t\right)}$ is the molar volume of C-S-H before carbonation; C/S($t_0$) and C/S(t) represent the Ca/Si of C-S-H before and after carbonation, respectively. On the basis of previous research findings, Wu et al. [55] further constructed a linear relationship between the Ca/Si and the molar volume of C-S-H gel when the volume change of C-S-H gel is in a critical state under carbonation, the expression of which is shown in Equation (8). $V_{\mathrm{SHt}\left(\mathrm{gel}\right)}$ denotes the molar volume of silica gel.

$$\Delta \varphi =\Delta {\varphi }_{\mathrm{CH}}+\Delta {\varphi }_{\mathrm{CSH}}=n_{\mathrm{CC}}^{\mathrm{CH}}\left(V_{\mathrm{CC}}^{\mathrm{CH}}-V_{\mathrm{CH}}\right)+n_{\mathrm{CC}}^{\mathrm{CSH}}\left[V_{\mathrm{CC}}^{\mathrm{CSH}}+{\displaystyle \frac{V_{\mathrm{CSH}}\left(t\right)-V_{\mathrm{CSH}}\left(t_0\right)}{C/S\left(t_0\right)-C/S\left(t\right)}}\right]$$

(7)

$$V_{\mathrm{CSH}}\left(t_0\right)=36.93\times C/S\left(t_0\right)+V_{\mathrm{SHt}\left(\mathrm{gel}\right)}$$

(8)

4.2. Influence of Carbonation on the Mechanical Properties of Fly Ash Concrete

The mechanical properties of FAC after carbonation represent a critical index for the widespread application of low-carbon building materials. To date, scholars have conducted systematic investigations via various carbonation methods and modification techniques, yielding pivotal findings. The combined application of accelerated carbonation and alkali activation technology can improve the early-stage compressive strength [77,94]; however, the absence of an alkali buffering system in the later stage leads to substantial long-term strength loss [95]. Assaggaf et al. [96] explored the mechanical properties of FAC after carbonation by means of accelerated carbonation curing. The results showed that the compressive strength of the specimens exceeded 20 MPa at 10 h; after 7 days of air exposure, the strength increased by 60%, and the long-term (90-day) strength loss was lower than that of ordinary concrete. It can be concluded that the incorporation of FA can enhance the early strength of concrete while achieving relatively low strength loss. To investigate the effect of carbonation of FA lightweight aggregate (FULA) on concrete structures, Liu et al. [97] prepared FULA concrete by subjecting FULA to pre-alkali immersion followed by carbonation treatment. The results indicated that the 28-day compressive strength of the concrete reached 46.79 MPa, which was 5.74% higher than that of ordinary FULA concrete, and the microhardness of the interfacial transition zone (ITZ) was significantly higher than that of the cement paste matrix. Silva et al. [98] performed accelerated carbonation curing on recycled coarse aggregate-FA composite systems. The results demonstrated that carbonation resulted in lower early compressive and tensile strengths of the concrete compared with the reference concrete; nevertheless, the strength of the group with 30% FA content gradually approached that of the reference concrete in the long term, with the difference in carbonation depth being less than 5% relative to the reference concrete. Silva et al. [99] adopted plastic film-wrapped carbonation curing to study concrete with 0% and 30% FA content. The results showed that the 7-day compressive strength of the 30% content group was 15% higher than that under conventional curing, the 28-day compressive strength decreased slightly by 1.9%, while the 28-day splitting tensile strength increased by 5.5%. Morandeau et al. [56] focused on the regulatory mechanism of different FA contents on the mechanical properties of FAC after carbonation. Their study revealed that the total porosity exhibited a significant decreasing trend after carbonation; meanwhile, with the increase in FA content, the pores gradually evolved from uniformly distributed pores to reconstructed large capillary pores. This reconstruction phenomenon is associated with the synergistic effect of decalcification from C-S-H carbonation and ${\mathrm{CaCO}}_3$ deposition. Studies have pointed out [23,53] that the lower the compressive strength, the higher the carbonation rate coefficient, and there exists a good correlation between the carbonation rate coefficient and compressive strength.

In summary, the underlying mechanism by which carbonation enhances the mechanical properties of FAC lies in the pore-filling effect of carbonation products. The enhancement effect of carbonation curing on the early compressive strength of FAC is universal, which is attributed to the synergistic action of efficient cementitious material hydration and carbonation processes. Alkali agents can activate the hydration activity of FA, induce the formation of multiple hydration products (e.g., C-S-H, sodium aluminosilicate hydrate N-A-S-H), and provide sufficient precursors for the carbonation reaction. It is noteworthy that the incorporation of FA reduces the alkali reserve level of concrete compared with OPC concrete. Therefore, considering the structural deterioration caused by the susceptibility of hydration products to decalcification constitutes an indispensable aspect in mix proportion design.

4.3. Effect of Fly Ash Concrete Carbonation on Durability

As a key evaluation index of concrete after carbonation, durability can effectively characterize its complex composite deterioration behavior in actual service environments, which holds significant academic research value and practical engineering significance. This section analyzes some existing studies and cases. Studies have confirmed [100,101,102,103] that the frost resistance of concrete is mainly dominated by capillary porosity and has a significant correlation with pores with diameters ranging from 40 to 2000 nm; in addition, the sanding resistance is closely related to the pore volume of pores larger than 75 nm [53] (as illustrated in Figure 7). Therefore, when exposed to carbonation, the evolution of pore volume and pore size distribution induced by carbonation plays a crucial role in the subsequent durability changes of concrete. At present, relevant research in this field is relatively scarce and has not yet formed a systematic framework.

Ding et al. [53] conducted accelerated carbonation tests and freeze–thaw tests to investigate the post-carbonation durability of air-entrained (AE) and non-air-entrained (Non-AE) concrete with FA contents ranging from 0% to 25%. The results demonstrated that the frost resistance and sanding resistance of AE concrete were not affected by FA content or carbonation, and the high air content could mitigate freeze–thaw damage, and that in Non-AE concrete, an increase in FA content led to a significant decline in frost resistance and sanding resistance. However, carbonation did not alter the gel/capillary pore ratio, resulting in no obvious change in freeze–thaw resistance, while it reduced the volume of pores larger than 75 nm, thereby making the improvement of sanding resistance more pronounced with the increase in FA content. Zhang et al. [103] adopted ${\mathrm{CO}}_2$ curing combined with freeze–thaw cycle tests to focus on the sanding resistance of concrete with 20% FA content. The results indicated that ${\mathrm{CO}}_2$ curing reduced the surface sanding mass of FAC to 0.02 kg/${\mathrm{m}}^2$, decreased the total pore volume by 26%, and refined the critical diameter of capillary pores from 50 nm to less than 10 nm.

In summary, the evolution of pore structure and the filling effect of carbonation products are the core causes for the changes in the durability of FAC after carbonation. ${\mathrm{CO}}_2$ curing significantly enhances the freeze–thaw and sanding resistance by optimizing the pore size distribution and reducing the proportion of harmful pores; in contrast, the coupling effect of freeze–thaw and carbonation affects durability by altering pore connectivity (i.e., the fluctuation of ink bottle pore volume).

5. Applications and Innovative Breakthroughs in Carbonation of Fly Ash Concrete

Early work on FAC mostly tried to make it better at resisting carbonation, with studies looking at changing the mix to improve carbonation resistance [104,105,106]. Lately, with the growth of carbon capture technology in building materials, how people see FAC has moved from just dealing with carbonation problems to using its ability to carbonate for capturing ${\mathrm{CO}}_2$. The main way to capture ${\mathrm{CO}}_2$ in FAC is through FAC carbonation curing. Looking at current studies and how carbon capture works over concrete’s life, the carbon capture methods can be split into two types: The first is single-stage carbon sequestration, which means conducting carbonation at one specific time (mixing, curing, use, or recycling), with the steps shown in Figure 8; the second is multi-step carbon sequestration, which means conducting carbonation at two or more different times to get better carbon capture across several stages. Current studies and uses of concrete carbon capture technology are mostly in the mixing and curing stages.

5.1. Single-Stage Carbon Sequestration

5.1.1. Carbon Sequestration in the Preparation Stage

Carbon sequestration during the mixing stage refers to the active capture and storage of carbon prior to concrete hardening, which is primarily implemented through the following three approaches: (1) subjecting cementitious materials to pre-carbonation treatment during the material preparation stage, before their use; (2) introducing ${\mathrm{CO}}_2$ into concrete during mixing to enable sufficient reaction between ${\mathrm{CO}}_2$ and the cementitious materials, thereby producing carbonated ready-mixed concrete (CRC); (3) incorporating carbonate additives simultaneously with cementitious materials, aggregates, water, and other components during the concrete mixing stage. The additives chemically react with active components (e.g., ${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$) in the cementitious materials to generate stable carbonates in situ, thus achieving the dual goals of carbon sequestration and concrete performance improvement.

Wei et al. [111] pointed out that a sintering-free cementation technology based on FA carbonation enables direct reaction between calcium-rich fly ash and ${\mathrm{CO}}_2$-containing flue gas under wet and low-temperature conditions. The core mechanism is that active calcium and magnesium components in FA react with ${\mathrm{CO}}_2$ to form a cementitious network dominated by ${\mathrm{CaCO}}_3$, thereby achieving simultaneous strength development and ${\mathrm{CO}}_2$ sequestration, which leads to a 7-day compressive strength of approximately 35 MPa and a ${\mathrm{CO}}_2$ uptake of 9%. In addition, a novel feasible approach—direct utilization of power plant flue gas without prior ${\mathrm{CO}}_2$ capture—has been proposed for the preparation of low-carbon building materials. From previous studies [112,113], it is confirmed that biochar-based ${\mathrm{CO}}_2$ sequestration technology, which can significantly enhance ${\mathrm{CO}}_2$ transport and surface reaction kinetics, accelerates the carbonation process and improves ${\mathrm{CO}}_2$ sequestration efficiency by modifying the pore structure and active site distribution of cementitious materials. In addition, this technique provides new insight into ${\mathrm{CO}}_2$ uptake by concrete incorporating supplementary cementitious materials. Jaworska et al. [114] adopted a high-pressure carbonation pre-treatment for calcareous fly ash and applied it as a supplementary cementitious material. The results show that this approach cannot only sequester approximately 3% ${\mathrm{CO}}_2$ but also extend the service life of concrete by more than 20 times under chloride attack, without causing obvious adverse impacts on reinforcing steel bars. Regarding the risks induced by excessive carbonation, such as reduced pH and steel corrosion, Mi et al. [12] proposed a novel strategy for ${\mathrm{CO}}_2$ sequestration in concrete utilizing the pores of porous coarse aggregates. Experimental results indicated that the ${\mathrm{CO}}_2$ uptake capacity of pre-soaked coral aggregate concrete reached approximately 20 kg/${\mathrm{m}}^3$. This approach can maintain the normal pH of the concrete pore solution while ensuring steady strength development, providing an important concept for the investigation of ${\mathrm{CO}}_2$ sequestration technologies for FAC.

At present, the research on FAC during the mixing stage remains relatively limited. Nevertheless, based on insights into ${\mathrm{CO}}_2$ sequestration in concrete at the mixing stage, effective ${\mathrm{CO}}_2$ sequestration strategies can be formulated for fly ash concrete, combining with existing knowledge regarding carbon uptake by FA. Jaworska et al. [114] presented a comprehensive review of CRC technology, which not only improves the early-age strength and freeze–thaw resistance of concrete, but also reduces its life-cycle carbon footprint and enhances its carbon-related performance. Monkman et al. [115] applied a method involving direct ${\mathrm{CO}}_2$ injection into concrete during mixing, combining with mix proportion optimization to reduce cement content. The fundamental mechanism lies in utilizing the carbonation reaction to accelerate early strength development of the material, which can lead to decreasing the consumption of cementitious binders. Therefore, this method can achieve a lower carbon footprint for concrete while guaranteeing the concrete’s performance, which can facilitate the transition of the concrete industry toward a low-carbon circular economy. He et al. [108] achieved early carbonation of cement paste using carbonation mixing technology, which can increase ${\mathrm{CO}}_2$ absorption during the mixing stage and can effectively control the process of strength development. The experimental results indicate that this technique significantly improves the ${\mathrm{CO}}_2$ sequestration capacity of concrete, and the concrete retains mechanical properties after optimization of the mixing procedure.

Portland cement materials have been extensively studied for carbon sequestration via in situ carbonation using carbonate additives, which can accelerate the cement hydration process [116,117] while the in situ-formed nano-sized ${\mathrm{CaCO}}_3$ effectively refines and fills the pore structure. It is reported that the composite use of ${\mathrm{NaHCO}}_3$ and aluminum sulfate as additives can increase the compressive strength of cement paste by 86.9% compared to the control group, with no strength regression observed within 90 days [118]. However, investigations into this method in the field of fly ash concrete are limited. Mao et al. [77] used fly ash as the starting material and prepared cement-free concrete via the synergy of sodium carbonate activation and in situ carbonation. The results demonstrated that this method can effectively sequester ${\mathrm{CO}}_2$ and enhance the mechanical properties and structural densification of the material. Indeed, the addition of ${\mathrm{Na}}_2$ ${\mathrm{CO}}_3$ not only accelerates the hydration of fly ash but also promotes the incorporation of Al into C-S-H, forming C-A-S-H), which exhibits superior mechanical properties [119]. However, although the dual skeleton is composed of ${\mathrm{CaCO}}_3$ crystals and C-A-S-H gel, the compressive strength remains below 20 MPa [77]. It has not been subjected to rigorous quantitative evaluation for its carbon sequestration efficiency, long-term performance, or practical engineering applicability. Therefore, the application of fly ash as a sole cementitious material still exhibits limitations in practical production, and exploring high-efficiency in situ carbonation curing technologies for FA-based multiphase cementitious material systems (e.g., FAC) holds substantial practical significance.

In summary, active carbonation during concrete mixing has been proven to be an effective approach for achieving ${\mathrm{CO}}_2$ sequestration while simultaneously improving material performance. Carbonation can densify the pore structure and thereby improve the compressive strength and durability of concrete through minor adjustments to the mixing procedure or mixture proportions. However, carbon sequestration at this stage still faces several challenges. The uneven contact between injected ${\mathrm{CO}}_2$ and cementitious materials leads to premature hardening and non-uniform pore structure distribution—issues that remain unresolved. Further exploration is needed as to the potential value of integrating in situ carbonation sequestration using carbonate additives (e.g., ${\mathrm{NaHCO}}_3$, ${\mathrm{Na}}_2$ ${\mathrm{CO}}_3$) with industrial CCS/CCU technologies in order to realize large-scale carbon sequestration. Although basic research on carbon sequestration in ordinary concrete is relatively mature, investigations into the carbon sequestration mechanism of FAC—especially the reactivity differences of fly ashes with diverse characteristics during carbonation, along with the multiphase interfacial reaction mechanism and carbon sequestration performance—still require further in-depth exploration.

5.1.2. Carbon Sequestration During the Curing Stage

Carbon sequestration during the curing stage refers to the artificial introduction of ${\mathrm{CO}}_2$ for carbonation intervention after the demolding of FAC. Currently, carbon sequestration during the curing stage is one of the main carbon sequestration methods for concrete, and it has been proven to be an effective means to optimize the microstructure and enhance long-term durability [120].

Studies have found [42] that rational optimization of the mix ratio of cementitious material admixtures can achieve the dual goals of strength development and carbon sequestration in accelerated carbonation curing. It is worth noting that carbonation curing can optimize the pore structure, enhance ${\mathrm{CO}}_2$ resistance during the service stage, and prevent steel bar corrosion. Liu et al. [94] achieved the complete replacement of cement with FA and constructed a FA-based cementitious system with a denser microstructure by virtue of carbonation-induced cementation constraints. This system exhibits superior mechanical properties; however, this technology relies on mechanical pressing, which consequently leads to additional energy consumption. Zhang et al. [68] introduced ${\mathrm{CO}}_2$ at the early curing stage of FAC and synergized the carbonation and hydration processes, thereby realizing the refinement of the pore structure of FAC while enhancing its alkalinity and improving its durability. Belayneh et al. [121] indicated that a rational FA content enables the carbon sequestration capacity to reach the level of 52.9 g of ${\mathrm{CaCO}}_3$ generated after 28-day carbonation curing. Furthermore, this study revealed that the incorporation of FA can significantly enhance the belite reaction and improve the ${\mathrm{CO}}_2$ binding capacity of the system.

In conclusion, carbonation intervention during the curing stage can efficiently fix ${\mathrm{CO}}_2$ and, at the same time, bring a denser pore structure to the cementitious system, improving early strength and long-term durability. It is worth noting that carbonation curing can serve as a solution to the problem of steel bar corrosion. The carbonation curing technology of cement-based concrete has strongly promoted the transformation of the construction industry towards low-carbon and high-performance development. However, current research is still limited, failing to form a systematic technical pathway for carbon sequestration during the curing stage, and relevant specifications are still incomplete.

5.1.3. Carbon Sequestration During the Service Stage

Carbon sequestration of FAC during the service stage refers to the process in which hydration products of cementitious materials react with ${\mathrm{CO}}_2$ in the environment to form ${\mathrm{CaCO}}_3$, permanently fixing ${\mathrm{CO}}_2$ inside the concrete during the normal service period of building structures or components (i.e., the service stage) [122]. Compared with the mixing and curing stages, due to the low concentration of ${\mathrm{CO}}_2$ in the air and the limited diffusion capacity in dense pores, the ${\mathrm{CaCO}}_3$ generated at the same time produces a “blocking effect” that further hinders the carbon sequestration process. Therefore, carbon sequestration during the service stage is relatively slow compared with that in the mixing and curing stages [6]. Curing conditions are a key factor affecting the carbon sequestration efficiency of concrete: Good curing can stabilize the internal hydration and carbonation reactions, resulting in a steady growth trend of carbonation depth [67]. Therefore, high-quality curing can ensure the reliable long-term service of concrete while achieving effective carbon sequestration [123].

Aiming at the natural carbonation behavior of HVFA concrete, Philip et al. [61] carried out a 16-week experimental study. The results showed that after carbonation, the concrete exhibits degradation phenomena such as significant depletion of alkali reserve, carbonation products dominated by metastable ${\mathrm{CaCO}}_3$ crystal forms, and slight coarsening of pore structure. Carević et al. [124] conducted accelerated carbonation tests and natural carbonation tests (exposure for 21 months and 48 months), deeply analyzed the kinetic laws of ${\mathrm{CO}}_2$ concentration on the carbonation process of high-calcium FA concrete (HVFAC), recycled aggregate concrete (RAC) and natural aggregate concrete (NAC) and, based on relevant data, made targeted modifications to the Tuutti model and the fib 2010 model, forming an optimized model scheme that can accurately predict the carbonation depth of HVFAC. Rathnarajan et al. [83] conducted a 5-year natural exposure test and a 112-day accelerated exposure test on 34 types of concrete including FAC. Based on the test data, they proposed the “A-to-N” model, a conversion model between accelerated carbonation and natural carbonation coefficients. This model provides important theoretical support for subsequent concrete carbonation depth prediction, durability performance evaluation and protective layer thickness design and also has positive significance for the quantitative evaluation of carbon sequestration efficiency.

In summary, natural carbonation of concrete, as a passive carbon sequestration technology, can realize the normalized progress of carbon sequestration and has important research value and broad application prospects. It is worth noting that carbon sequestration in this stage needs to fully consider the adverse effects of key factors such as curing conditions and FA content on the concrete structure after carbonation so as to avoid the risk of structural degradation after carbon sequestration.

It is noteworthy that a certain correlation exists between the pozzolanic reaction and carbonation process. When carbonation and hydration proceed simultaneously in FA-OPC systems, the carbonation process at the early reaction stage inhibits the progression of the pozzolanic reaction [68], while nano-scale ${\mathrm{CaCO}}_3$ formed in the later stage promotes the hydration of FA [76]. Existing studies have indicated that the incorporation of FA can improve the ${\mathrm{CO}}_2$ absorption capacity. Furthermore, the addition of fly ash can significantly enhance the belite reaction and elevate the ${\mathrm{CO}}_2$ binding capacity [121]. Overall, compared with pure cement paste, FA-OPC paste exhibits higher reaction activity with carbon dioxide [68]. Meanwhile, ${\mathrm{CaCO}}_3$ generated by carbonation can remarkably reduce the quantity of connected pores and decrease the total porosity of cementitious matrices [53,79].

5.1.4. Carbon Sequestration During the Secondary Utilization Stage

Carbon sequestration of FAC during the secondary utilization stage refers to the process in which, after the end of its first service life, concrete is crushed and processed into recycled aggregates or powder, and in the process of recycling and reuse, it actively or passively captures and permanently sequesters ${\mathrm{CO}}_2$ in the atmosphere or surrounding environment through a series of physical and chemical reactions. Studies have pointed out that the pores and cracks in recycled aggregates are the optimal adsorption sites for ${\mathrm{CO}}_2$ during the carbon sequestration process [112].

Zhan et al. [125] systematically revealed that material properties such as water content, water–cement ratio, recycled aggregate content, density and cementitious material type of recycled aggregate concrete blocks significantly affect carbonation efficiency and mechanical properties during ${\mathrm{CO}}_2$ curing. Through micro-mechanism analysis, it was confirmed that the porous structure of recycled aggregates and old cement paste can effectively promote ${\mathrm{CO}}_2$ absorption and reaction, providing a key process basis for realizing low-carbon strengthening and carbon sequestration of building materials. Miyata et al. [109] proposed a resource utilization method based on microwave heating and sintering of waste concrete and FA, successfully converting it into carbon-containing structural materials. This method reduces the volume of raw materials by about 21% and achieves a strength of up to 40 MPa, providing a promising solution for the sustainable development of solid waste resource utilization in the construction field. Yao et al. [78] prepared recycled concrete using recycled coarse aggregates and recycled fine aggregates as aggregates and partially replacing cement with FA as cementitious materials, and they carried out accelerated carbonation tests to systematically investigate the effects of accelerated carbonation on the properties and microstructure of recycled concrete. The study found that accelerated carbonation can increase the compressive strength of recycled concrete by up to 13%. This achievement provides an important reference for improving concrete carbon sequestration efficiency and promoting the resource utilization of recycled concrete.

In summary, as an active carbon sequestration pathway, carbon sequestration during the secondary utilization stage mainly relies on the porous structure of recycled aggregates to provide a natural structural basis for carbon sequestration. It simultaneously achieves the dual goals of solid waste resource utilization, ${\mathrm{CO}}_2$ capture and permanent sequestration, and realizes the optimization and improvement of material properties after mix proportion optimization.

5.2. Progressive-Stage Carbon Sequestration

As an advanced low-carbon technology, progressive-stage carbonation curing for FAC carbon sequestration is based on the core logic of integrating and synergizing two or more single-stage carbon sequestration modes described in Section 4.1 to maximize carbon sequestration efficiency. On the basis of large-scale replacement of cement with FA (industrial solid waste), this technology permanently sequesters ${\mathrm{CO}}_2$ as a raw material inside concrete, simultaneously achieving the three core goals of solid waste resource utilization, carbon capture and sequestration, and material performance enhancement. Through the scientific design of the combination form and process parameters of carbon sequestration stages, it gives full play to the unique advantages of each single-stage carbon sequestration technology, makes up for the inherent limitations of a single carbon sequestration stage, and then realizes efficient ${\mathrm{CO}}_2$ sequestration throughout the life cycle and synergistic optimization of concrete performance.

6. Future Prospects

In recent years, against the backdrop of the continuous advancement of the global strategic goals of carbon peaking and carbon neutrality and the deepening consensus on energy conservation and emission reduction, the research paradigm in the field of civil engineering materials has inevitably shifted toward “solid waste resource utilization” and “low-carbon emission reduction”. As a crucial pathway to realize the synergistic development of solid waste recycling and ${\mathrm{CO}}_2$ sequestration, carbon sequestration technology for FAC has achieved phased research outcomes. It is worth noting that multiple uncertainties still exist in the carbon sequestration performance of concrete. ${\mathrm{CO}}_2$ leakage during carbonation reduces the actual carbon sequestration efficiency, and the indirect carbon emissions from raw material transportation and on-site construction are commonly overlooked. Furthermore, existing studies have presented conflicting explanations for the carbon sequestration mechanism of cementitious materials, and a unified and systematic theoretical system has not yet been formed. Meanwhile, the long-term effects of carbonation on concrete durability and structural stability remain insufficiently understood. Hence, the development of a synergistic theoretical system for solid waste concrete that couples carbon emission reduction and mechanical performance improvement still faces considerable technical bottlenecks and challenges. Future research priorities are proposed as follows.

• Currently, comprehensive assessments of the total life-cycle carbon emissions, carbon sequestration, and ${\mathrm{CO}}_2$ emission rates associated with FAC are still lacking, and related research can be regarded as a promising direction for future studies.
• Further exploration should be conducted into the synergistic carbon sequestration mechanisms of FA combined with other solid waste materials (e.g., recycled concrete aggregates, steel slag). Carbon sequestration technologies for multi-solid waste composite systems should be developed to expand the channels for solid waste resource utilization and the capacity for ${\mathrm{CO}}_2$ sequestration.
• The carbon sequestration potential of cementitious materials should be thoroughly exploited to break the technical dependence on traditional cement-based materials. By leveraging the carbonation reaction and the physicochemical properties of solid waste materials, the development of novel cement-free cementitious systems should be pursued.
• Current research primarily focuses on improving carbon sequestration efficiency, while the discussion on how carbonation reactions enhance the structural performance and durability of concrete remains insufficient. Therefore, future studies need to delve into the micro-characteristics and action mechanisms of carbonation behavior, clarify the enhancement pathways of carbonation on key properties of concrete (e.g., strength, impermeability, corrosion resistance), and provide theoretical support for the long-term service reliability of carbon-sequestered concrete.
• Efforts should be made to reduce the carbon emission intensity of auxiliary processes such as ${\mathrm{CO}}_2$ capture, transportation and injection so as to realize the low-carbon industrialization of the entire carbon sequestration process. Meanwhile, systematic collaboration between carbon sequestration technologies and related industries should be strengthened to promote the industrial development of the ${\mathrm{CO}}_2$ capture–transportation–sequestration industrial chain.
• Future design of concrete carbon sequestration technologies should deeply integrate the dual orientations of “low-carbonization” and “resource utilization”. While keeping pace with industry development trends, a three-dimensional balance among carbon sequestration efficacy, structural performance and economic cost should be achieved.
• Strengthen industry policy guidance, establish a carbon sequestration efficacy evaluation index system and carbon emission accounting methods, and standardize the carbon sequestration process parameters, equipment technical requirements and engineering application procedures.