Innovative two-step synthesis of ultrafine highly-carbonatable calcium silicate binder from waste carbide slag and silica

This study presents an innovative two-step synthesis method for producing highly-carbonatable calcium silicate (CS) binder using waste carbide slag and silica. The process involves wet chemical synthesis of C-S-H precursors followed by calcination at various temperatures (300–1200°C). The resulting CS materials were characterized and subjected to carbonation curing to evaluate their CO₂ sequestration potential and mechanical properties. X-ray diffraction (XRD) and thermal analyses revealed the formation of β-CS at 800°C and its transformation to α-CS at 1200°C. The synthesized CS exhibited ultrafine crystallite sizes ranging from 20 to 77 nm, contributing to enhanced reactivity. Carbonation experiments demonstrated high CO₂ uptake, with samples calcined at 800°C achieving a remarkable 24 h carbonation strength of 160.41 MPa. Microstructural analysis using SEM and low-field NMR showed that the carbonation products formed a dense, interconnected structure with optimized pore distribution. The synergistic carbonation of residual C-S-H and β-CS, coupled with the small crystallite size, was found to be key in achieving high strength and CO₂ sequestration capacity. This research demonstrates the potential of the two-step synthesis method for producing highly reactive CS binders from industrial waste, offering a promising approach for CO₂ mitigation in the construction sector.