Preparation of new coal gangue-based cementitious material via hydrothermal synthesis–low temperature calcination method

Abstract

Traditional cement-based materials’ economic and environmental concerns urge low-carbon alternatives. This study activates coal gangue via hydrothermal synthesis − low temperature calcination to explore optimal activation conditions for cement substitution. A Box-Behnken response surface experiment was designed with 28d compressive strength as the response index to study the effects of hydrothermal temperature, Ca/Si ratio, NaOH concentration, and calcination temperature on coal gangue-based cementitious materials. Single-factor optimization was conducted based on significant factors identified via response surface analysis. Phase evolution and hydration mechanisms were analyzed using XRD, SEM, TG-DSC, etc. Results indicated that optimal conditions were 90 °C, Ca/Si = 2.0, NaOH = 1.5%, and 800 °C, with the Ca/Si ratio and calcination temperature exerting the most significant impact, achieving a 28d compressive strength of 20.1 MPa. At Ca/Si = 2.0, the highest content of C₁₂A₇ was found in the clinker. The formation of β-C₂S and α’_H-C₂S at 800 °C facilitated the densification of C–S–H with ettringite. Hydrothermal pre-treatment lowered the C₂S synthesis temperature by 200 °C and enhanced the 28d compressive strength to 191% of that obtained using the one-step calcination method. The results provide theoretical support for the utilization of coal gangue as a cementitious material, promoting its sustainable resource utilization.