As an energy-intensive industry, cement manufacturing is a significant contributor to CO₂ emissions. Simultaneously, the increasing volume of construction waste has also exerted significant pressure on the environment. Recycling demolished concrete has been proposed as a strategy to address the depletion of aggregate resources. In addition, it facilitates the disposal of construction waste and reduce carbon emissions. A particularly promising approach is the recycling of waste concrete powders (WCP) as a supplementary cementitious material (SCM), which has garnered significant attention. However, the high absorption and low reactivity of WCP pose a challenge to its direct use as SCM. Accelerated carbonation of WCP produces calcium carbonate (CaCO₃) and amorphous silica gel. These products enhance the reactivity of WCP and facilitate the effective sequestration of CO₂. This process not only promotes the high-quality utilization of WCP but also helps to mitigate the environmental and energy burdens associated with cement production.

This study reviews the properties of carbonated waste concrete powder (cWCP) and provides an in-depth analysis of the carbonation process of WCP. It covers various carbonation methods, the regulation of carbonation degree and the prperties of carbonation products. Furthermore, the review evaluates the influence of cWCP on the performances of cement-based materials, including rheology, hydration behavior, mechanical properties, and impermeability.

The carbonation products of cWCP are influenced by the carbonation conditions. In turn, cWCP influences the properties of cement-based materials through the properties of these carbonation products. The primary products of cWCP are CaCO₃ and amorphous silica gel. Their composition, morphological characteristics, chemical properties, and relative distribution depend significantly on the carbonation method and conditions used. The reaction environments and carbonation kinetics of dry and wet carbonation processes differ significantly, resulting in variations in both the degree of carbonation and the polymorphs of products. By optimizing these carbonation conditions, it is possible to improve the degree of carbonation of cWCP while also controlling the polymorphs of CaCO₃ and polymerization degree of silica gel. Furthermore, the influence exerted by these carbonation products on cement-based materials primarily stems from their surface characteristics combined with various effects. Surface properties include both geometrical and surface electrochemical characteristics of CaCO₃ in diverse polymorphs, as well as the hydrophilicity of the silica gel. The effects of carbonation products include the filler effect, nucleation effect, chemical reaction with C₃A, and the pozzolanic effect attributed to silica gel. Notably, improvements in rheological properties are not significantly influenced by either the filler effect or surface electrochemical characteristics of CaCO₃. Instead, more pronounced negative influences arise from the hydrophilicity of silica gel and the fibrous geometry of aragonite. These factors contribute to a significant deterioration in rheological properties, with hydrophilicity being the primary mechanism behind this deterioration. When the degree of carbonation is sufficiently high, the combined positive effects of the filler effect, nucleation effect, chemical reaction, and pozzolanic effect can surpass the negative influence of dilution effect. As a result, the incorporation of cWCP accelerates the hydration, improves the microstructure, and enhances the compressive strength and impermeability.

Existing studies have shown that the incorporation of cWCP accelerates cement hydration, improves microstructure, enhances compressive strength, and strengthens mortar impermeability. However, it may also result in a degradation in rheology.

Base on these findings, further advancements are essential to fully understand the properties of cWCP-based cementitious materials and to support their widespread application. Key areas for development include: 1) establishing a model that correlates carbonation conditions with carbonation products to precisely control their performance, 2) addressing the contradiction between compressive strength and rheological properties, 3) deepening our understanding of the durability of cement-based materials containing cWCP, particularly concerning corrosion of steel reinforcement, and 4) overcoming challenges related to improving the degree of carbonation in cWCP due to an accumulation of carbonation products. These efforts will lay the groundwork for the industrial adoption of cWCP and drive the concrete industry toward more sustainable and environmentally friendly practices.

CO₂ curing technology has been extensively studied due to its dual benefits of carbon sequestration and enhancement in the properties of cementitious materials. During CO₂ curing, the carbonatable binders react with CO₂ and form carbonation products, such as calcium carbonate and silica gel, leading to matrix densification and rapid strength development. The development of this technology enables the use of low lime calcium silicate minerals, such as wollastonite (CS), rankinite (C₃S₂) and γ-dicalcium silicate (γ-C₂S) with very low hydration reactivity to produce low CO₂ footprint binders through CO₂ curing. The carbonation activity, carbonation products and mechanical properties of the carbonated matrix of calcium silicate minerals have been widely studied. However, the relationship between the mechanical properties and microstructure of carbonated calcium silicate minerals remains unclear, which potentially limits the application of CO₂ cured cement and concrete as well as low-calcium cementitious materials. In this work, the microstructure and mechanical properties of CO₂ cured Portland cement, β-C₂S, γ-C₂S, C₃S₂ and CS were investigated, and the correspondence between the structure and properties of CO₂ cured calcium silicate minerals and the carbonation process was analyzed.

The raw materials used in this study include Portland cement (PC, PI 42.5), β-dicalcium silicate (β-C₂S), γ-dicalcium silicate (γ-C₂S), rankinite (C₃S₂) and wollastonite (CS). Pastes were prepared with a water-to-binder ratio of 0.18 for microstructural and phase analysis, while mortars were prepared with a constant water-to-binder ratio of 0.25 and sand-to-binder ratio of 2 for mechanical and porosity analysis. Specimens were prepared by compaction molding, with a pressure of approximately 10 MPa for pastes and 25 MPa for mortars. After molding and pre-conditioning, the compact specimens were placed in a pressure chamber. The curing chamber was vacuumed to a pressure of around -0.1 MPa, and maintained for 3 min. After that, CO₂ gas, with a purity of 99%, was injected and maintained at 0.2 MPa at (20 ± 2) ℃ and RH of (60% ±5%) for 3 d.

The carbonation degree of calcium carbonate minerals was determined by a thermal gravimetric analyzer. The phase composition of CO₂ cured calcium silicate minerals was analyzed by XRD analysis, and FTIR spectra were obtained with a Thermo-Scientific IS10 FTIR instrument. Moreover, the microstructure and morphology of paste specimens were examined using Phenom LE SEM. The compressive strength of cylindrical mortar specimens (Φ = 25 mm and h = 25 mm) was tested after CO₂ curing. Meanwhile, the pore structure was tested by a MAG-MED proton nuclear magnetic resonance spectroscopy.

The carbonation degree of γ-C₂S, C₃S₂ and CS exceeded 60%, followed by 50.3% for β-C₂S, while that of PC was only 29.7%. The lowest carbonation degree of PC was because cement particles release more Ca ions than other minerals during early CO₂ curing, rapidly forming a carbonate shell that blocks further carbonation inside the compacts. The carbonation products of calcium silicate minerals were mainly calcium carbonate and silica gel. Calcite was the main crystalline calcium carbonate, along with minor amounts of aragonite and vaterite. The silica gel phases in CO₂ cured γ-C₂S, C₃S₂ and CS showed higher polymerization degree and exhibited a clear demarcation from calcium carbonate, while the silica gel phases in the outer layer of CO₂ cured PC particles were intermixed with calcium carbonate. This was related to the high hydration reactivity of PC, which could react with water and form a certain amount of hydration products (i.e. C-S-H and portlandite) in the pores. The carbonation of porous C-S-H results in the intermixing of calcium carbonate with silica gel. Additionally, the synergy of cement hydration and carbonation also facilitates the leaching of Ca²⁺, thus leading to an evaluated content of amorphous phases in CO₂ cured PC.

After 3 days of CO₂ curing, the compressive strength of β-C₂S and γ-C₂S mortars exceeded 50 MPa, followed by PC and C₃S₂, while the strength of CO₂ cured CS was only 11.6 MPa. The compressive strength of CO₂ cured calcium silicate minerals showed a linear relationship with CO₂ uptake and porosity, increasing with a decrease in porosity and an increase in CO₂ uptake. With the increase of Ca/Si, the CO₂ uptake of CS, C₃S₂ and γ-C₂S increased, the porosity decreased, and thus the compressive strength increased. However, PC and β-C₂S were subjected to both hydration and carbonation, which promote the formation of amorphous phases, leading to lower porosity and higher compressive strength. Moreover, the crystal size of calcite, the content of amorphous calcium carbonate and the interfacial properties of calcium carbonate and silica gel can also impact the compressive strength and microstructure evolution.

The carbonation products of calcium silicate minerals were mainly calcium carbonate and silica gel. The carbonation degree of non-hydraulic γ-C₂S, C₃S₂ and CS was relatively higher, followed by β-C₂S, while PC demonstrated the lowest carbonation degree. The silica gel phases in CO₂ cured γ-C₂S, C₃S₂ and CS showed higher polymerization degree and exhibited a clear demarcation from calcium carbonate, while the silica gel phases in the outer layer of CO₂ cured Portland cement particles were intermixed with calcium carbonate. The compressive strength of CO₂ cured calcium silicate minerals showed a linear relationship with CO₂ uptake and porosity. With the increase of Ca/Si ratio, the CO₂ uptake of CS, C₃S₂ and γ-C₂S increased, the porosity decreased, and thus the compressive strength increased. Portland cement and β-C₂S were subjected to both hydration and carbonation, leading to lower porosity and higher compressive strength. The smaller size of calcite crystals and the higher content of amorphous calcium carbonate also contribute to an increased mechanical property of CO₂ cured Portland cement.

The use of calcium carbonate powder as a mineral admixture in cementitious materials can reduce CO₂ emissions from clinker calcination and fossil fuel combustion, offering a low-carbon solution for sustainable cement production. Calcium carbonate can effectively fill the pores, refine the pore size, and promote the growth and precipitation of early hydration products of cement. On the other hand, calcium carbonate can react with the aluminum phase in cement to generate calcium carbon-aluminate, improving the mechanical properties of cementitious materials. There are two main crystal types of calcium carbonate in nature, calcite and aragonite. Due to the difference in density, stiffness and crystal structure of calcite and aragonite, calcite and aragonite-type calcium carbonate have different effects on the properties of cementitious materials. This paper elucidated the effects of different crystal types of calcium carbonate on the microstructure and mechanical properties of cement pastes and calculated the various effects of calcium carbonate in cement pastes, which would provide a solid and effective theoretical basis for the application of calcite and aragonite in engineering practice.

The raw materials used in this study include Portland cement (P·I 42.5), slag, calcite and aragonite. The calcite and aragonite were prepared by wet carbonation and analyzed for composition and purity by XRD and TG. Calcite and aragonite were separately added to the cement–slag composite system to prepare two groups of ternary systems with a water-cement ratio of 0.4. The simplex-centroid mixture design method was used to optimize the relative composition design of the raw materials in the cement pastes. According to the cement pastes composition design, 20 mm×20 mm×20 mm specimens were moulded for testing compressive strength, ϕ20 mm×40 mm specimens were moulded for testing splitting tensile strength, and ϕ25 mm×25 mm specimens were moulded for pore structure analysis.

The mechanical properties and pore structure of the specimens were tested after curing for 3, 7, 28 d and 90 d in standard curing conditions (temperature (20±1)℃, relative humidity ≥ 96%). The cement paste powders of corresponding age were taken for XRD and TG tests to analyse the type and content of hydration products. Finally, the nucleation effect, dilution effect, filling effect and chemical effect were quantified based on the contribution of calcite and aragonite to the sample compactness.

The addition of approximately 15% calcite or aragonite enhanced the compressive strength of the cement paste during early hydration. This was because calcite or aragonite promoted cement hydration, improved the early hydration degree of cement, and increased the content of hydration products such as Ca(OH)₂ and C-S-H. The compressive strength of the calcite specimen was significantly higher than that of the aragonite specimen at the same dosage. Since the density of calcite was lower than that of aragonite, the filling effect of calcite was higher than that of aragonite, and calcite had a more significant nucleation effect than aragonite, with more hydration products deposited on the surface.

After 28 days of hydration, the compressive strength and splitting tensile strength of specimens mixed with calcium carbonate and slag were significantly increased. The synergistic effect of calcium carbonate and slag significantly improved the chemical reaction degree of calcium carbonate, and increased the content of calcium carboaluminate. Meanwhile, the formation of calcium carboaluminate inhibited the transformation of ettringite into calcium monosulfoaluminate, the solid phase volume of hydrated product increased, and the compactness of cement paste increased. Therefore, the mixture of slag and calcium carbonate was conducive to the continuous improvement of the mechanical properties of cement pastes.

With the increase of hydration age, the toughening effect of aragonite on cement paste was gradually improved. At 90 d of hydration, the content of aragonite was 20%~30% (in mass), the content of slag was 5%~15%, the contribution of chemical effect to the cement paste compactness was 15%~21%, and the splitting tensile strength of the specimen was increased by 39% compared with that of pure cement specimens. The improvement of the tensile strength of aragonite was mainly related to the chemical effect. The calcium carboaluminate formed by the reaction strengthened the bond between aragonite and cement paste, enhancing the toughening effect of aragonite.

The compressive strength of calcite specimens was found to be higher than that of aragonite specimens at the same calcium carbonate content during the early stage of hydration, primarily due to the increased filling and nucleation effects exhibited by calcite compared to aragonite. Due to the toughening effect of fibrous aragonite in cement pastes, the splitting tensile strength of the aragonite system was significantly higher than that of the calcite system. With the increase of hydration age, the reaction degree of calcium carbonate and aluminum phase in the composite system increased, the chemical effect continued to increase, and the compactness of the matrix increased. The addition of calcite or aragonite and slag was beneficial to the continuous improvement of the mechanical properties of the cement pastes. The increase in calcium carboaluminate content enhanced the toughening effect of aragonite and significantly increased the splitting tensile strength of the mixed specimens.