Deterioration of Activated Coal Gangue Powder Concrete under Combined Sulfate Attack and Freeze-Thaw Cycles

Abstract As a solid waste byproduct generated during coal mining, coal gangue demonstrates significant dual benefits for both environmental sustainability and economic efficiency when applied in concrete production. However, in sulfate-rich low-temperature environments, sulfate attack and freeze-thaw cycles (FTCs) can severely degrade the durability and load-bearing capacity of coal gangue concrete. In this study, coal gangue was subjected to mechanical-microwave composite activation, and the degradation behavior of activated coal gangue powder concrete (ACGPC) with varying replacement ratios was investigated through rapid freeze-thaw tests in a sulfate solution. Experimental parameters, including mass loss, relative dynamic elastic modulus (RDEM), compressive strength, and damage layer thickness (Hf), were evaluated. Scanning electron microscopy revealed the microstructural evolution of ACGPC, while mercury intrusion porosimetry quantitatively characterized its pore size distribution. The results indicate that the deterioration of ACGPC progressively decreases with increasing coal gangue powder (CGP) content. Compared with CGP-free concrete, ACGPC with 20% CGP exhibits superior frost resistance, reducing the RDEM loss and compressive strength loss by 13.68% and 14.51%, respectively, after 275 FTCs. However, the enhancement in frost resistance under sulfate exposure significantly diminishes at a 30% CGP content. With increasing FTC cycles, Hf shows a gradual increase, while the compressive strength of the damaged layer declines more significantly. When CGP content is between 0% and 20%, ACGPC exhibits a gradual reduction in porosity, accompanied by an increased proportion of fine pores and improved pore structure. Notably, incorporating 20% CGP reduces porosity by 24.37%. The micro-filling and secondary hydration properties of CGP enhance the frost resistance of concrete exposed to sulfate environments.