Effect of fly ash content on the leaching resistance of shotcrete

Introduction To investigate the influence of fly ash (FA) content on the leaching resistance of shotcrete, this study adopted FA replacement ratios as the main variables (0%, 15%, 20%, 25%). Methods Shotcrete specimens were prepared using the wet-mix spraying method and subjected to accelerated leaching tests in a 6 mol/L ammonium chloride solution. Systematic analyses of compressive strength, porosity, leaching depth, and calcium ion leaching amount were conducted to examine the effect of FA content on the leaching resistance of shotcrete. Microstructural evolution was also analyzed using scanning electron microscopy (SEM). Results The results indicate that the incorporation of FA significantly enhances the leaching resistance of shotcrete. An appropriate amount of FA promotes the formation of additional C–S–H gel through pozzolanic reactions at later stages, improving concrete density and inhibiting calcium ion migration as well as the advancement of the leaching front. With a 20% FA content, shotcrete exhibited optimal leaching resistance: after 90 days of leaching, the compressive strength loss rate was 34.13%, porosity increased by only 3.09%, leaching depth reached 22.09 mm, and the total calcium ion leaching was 3.7 mol/L. The synergistic effect of the pozzolanic reaction and the micro-aggregate effect of FA optimizes the pore structure and reduces the content of soluble calcium phases, thereby enhancing the chemical erosion resistance of concrete. Discussion The results demonstrate that reactive components in FA can react with calcium hydroxide from cement hydration to form more stable calcium silicate hydrate, significantly delaying the leaching process of shotcrete. At a 20% replacement ratio, leaching damage is effectively suppressed while mechanical performance is maintained. This study provides theoretical and technical support for mix design and engineering application of shotcrete in erosive environments.