Cement industry continues to face significant environmental challenges due to its substantial CO₂ emissions, primarily stemming from carbonate decomposition in raw materials and fossil fuel combustion during production. Carbonatable binders and carbonation curing technologies have a promising potential for CO₂ reduction in cement production, and the existing methods involve the calcination of calcareous and siliceous raw materials to produce low-lime silicates for their emission reduction limits. Natural wollastonite, primarily composed of calcium silicate (CS), has an attractive alternative due to its abundant reserves and ease of extraction. Utilizing natural wollastonite as a carbonatable binder can eliminate both fossil fuel combustion and the need for calcareous raw materials, thereby avoiding the carbonate decomposition entirely. However, its application is currently limited due to its low carbonation reactivity. This study was to investigate the enhancement of carbonation reactivity of natural wollastonite via optimizing calcination activation to develop a high-performance carbonatable binder.

Natural wollastonite was calcinated under various calcination activation condictions (i.e., temperature, time, and cooling regimes). The activated wollastonite powder was mixed with water and compressed at 4 MPa to form the samples with different two dimensions (i.e., 20 mm × 20 mm × 20 mm and 20 mm × 80 mm × 20 mm). These samples underwent carbonation curing for 24 h before characterization. The mechanical properties were evaluated through compressive and flexural strength tests. The phase composition was characterized by X-ray diffraction (XRD) on both the wollastonite powder and carbonated samples before and after calcination activation. The structure of the samples was determined by Fourier-transform infrared spectroscopy (FT-IR) to analyze both the calcination activation effects and the resulting carbonation products. The microstructural evolution was determined by scanning electron microscopy (SEM). CO₂ uptakes were analyzed by thermogravimetric analysis (TGA).

This study reveals that calcination activation significantly enhances the carbonation hardening performance of natural wollastonite via increasing carbonation reactivity. Optimal conditions are identified at a calcination temperature of 1200 ℃, effectively transforming Para-CS and β-CS into metastable α-CS. The effective calcination time for the complete transformation is 60 min, while preserving the material valuable fibrous structure. Water quenching is found to be the most effective cooling method, retaining higher concentrations of crystal defects in the metastable α-CS. The transformation from a chain structure to a ternary ring structure at 1200 ℃ results in an increased structural distortion, thus leading to an enhanced carbonation reactivity of α-CS. The carbonation analysis reveals the extensive formation of carbonation products, primarily calcium carbonate and highly polymerized silica gel, resulting in a dense microstructure. Note that the preservation of the fibrous structure through calcination activation allows unreacted wollastonite to serve as a fibrous reinforcement within the carbonation hardening system, contributing to enhanced mechanical properties.

The results showed that calcination temperature exhibited a dominant effect, followed by calcination time and cooling regime. The optimal calcination temperature of 1200 ℃ led to the improvements in compressive strength, flexural strength, and CO₂ uptakes of the carbonated samples. The calcination time was critical in ensuring a complete phase transformation, while maintaining structural integrity, and a rapid cooling could preserve crystal defects that enhanced carbonation reactivity. The resulting carbonatable binder developed a carbonation hardening system comprising calcium carbonate, highly polymerized silica gel, and unreacted wollastonite fibers that could serve as an effective reinforcement within the carbonation hardening system. These findings could provide a promising pathway for the large-scale utilization of natural wollastonite in sustainable construction materials, having a potential for reducing the environmental impact of cement industry.