Effect of limestone powder and fly ash on the pH evolution coefficient of concrete in a sulfate-freeze–thaw environment

Limestone powder has been widely used in concrete during fly ash shortages and in the green and sustainable development of concrete materials. Because the effect of limestone powder on concrete alkalinity is not well understood, we research the effect of 10% limestone powder concrete, 10% fly ash concrete, and pure cement concrete at different depths on the pH value of the initial state, microstructure and compressive strength, sulfate immersion, and number of sulfate–freeze–thaw cycles at different pH value coefficients of concrete erosion products formed in a sulfate-freeze–thaw environment. Limestone powder has no obvious effect on the pH value of concrete in the initial state, while fly ash reduces the pH value of concrete in the initial state. In the sulfate-freeze–thaw environment, the protective property of limestone powder concrete is slightly stronger than that of pure cement concrete, and the protective property of fly ash concrete is weaker than that of pure cement concrete. The effect of sulfate immersion and the sulfate-freeze–thaw environment on the pH value of the concrete surface layer is greater, which is confirmed by the results of synchronous thermal analysis. TSA (the thaumasite form of sulfate attack)-type damage to limestone powder concrete occurred in the sulfate-freeze–thaw environment, and the C–S–H gel content in 10% limestone powder concrete was significantly less than that in 10% fly ash concrete and pure cement concrete at depths of 0–5 mm, 5–10 mm and 10–15 mm, which reduced the content of C–S–H gel.