Optimizing the development and field application of calcium stabilizing agents for preventing calcium leaching in tunnels

Abstract

As a hidden engineering structure, tunnels are inevitably affected by the long-term erosion of groundwater, resulting in the phenomenon of calcium leaching in shotcrete. The effective components of the initial tunnel support are dissolved by groundwater, with a portion remaining in the tunnel drainage system and the rest discharged from the tunnel. This phenomenon can further accelerate the deterioration of the lining and cause blockages in the drainage system, posing new challenges to environmental protection and structural safety. To mitigate the problem of calcium leaching in tunnel shotcrete, new building materials, termed Calcium Stabilizing Agents (CSAs), were fabricated using industrial solid wasters such as silica fume, mineral powder, silicone zirconium, and metakaolin. An orthogonal experiment was implemented to investigate the influence of CSAs on the calcium ion (Ca2+) leaching concentration; the sensitivity analysis was conducted to identify the impact of each ingredient of CSAs on the calcium ion leaching concentration; and microscopic methods such as XRD, SEM, and TG were used to analyze the mechanism of the CSAs on anti-calcium leaching ability of mortar. The results showed that the addition of the CSAs can effectively improve the anti-calcium leaching ability of mortar. For the optimal dosage of the CSAs mortar, the calcium ion concentration of the 28-day CSAs mortar was reduced by 31.55 %, and the compressive strength and flexural strength were enhanced 31.09 % and 37.27 %, respectively. From the sensitivity analysis of calcium ion concentration, compressive strength, and flexural strength of CSAs mortar at 1 day, 7 days, and 28 days, silica fume and zirconium silicone show the greatest influence on the performance of mortar. Consequently, they are deemed as the core components of CSAs. The microscopic analysis showed that the internal cementitious structure of the cement stone grew denser after CSA was added. It also contained long-chain crystalline structures that effectively fill capillary pores to resist erosion. Finally, the CSAs product was applied to a tunnel under construction on site, and the results showed that the shotcrete mixed with CSAs had a good anti-calcium leaching effect and significantly improved compressive strength. The findings of this study can provide a new avenue for preventing and controling the tunnel crystallization diseases.