Compressive Strength and Microstructure of CO₂ Cured Calcium Silicate Minerals

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.