Influence of different silica fume contents on the leaching resistance of shotcrete

Shotcrete is a key support material in tunnel engineering, and its durability under underground water decalcification conditions directly affects structural safety and service life. In this study, a 6 mol/L ammonium chloride solution was used to simulate an underground water decalcification environment. The effects of different silica fume (SF) contents (0%, 5%, 7%, 9%) on the leaching resistance of shotcrete were systematically evaluated in terms of compressive strength, porosity, leaching depth, and calcium ion leaching amount. The results show that the physical and mechanical properties of the shotcrete specimens decrease with increasing leaching time. Compared with the specimens without silica fume, the SF-modified specimens exhibit a lower compressive strength loss rate and a smaller increase in porosity after 90 days of leaching. The development of leaching depth is slower, and the calcium ion leaching rate is also significantly reduced. A higher leaching resistance is already observed at early ages. When the SF content is 7%, the shotcrete specimens exhibit the best leaching resistance. Silica fume has high pozzolanic activity. When incorporated into the fresh concrete mixture, it provides an effective micro-filling effect at early ages and increases the density of the concrete. At the same time, the pozzolanic reaction consumes part of the calcium hydroxide, which hinders the further development of leaching and thus improves the leaching resistance.