Groundwater infiltration into tunnels causes water to percolate through the fissure channels in the initial support shotcrete. This results in the dissolution and outflow of calcium hydroxide, a key product of cement hydration. This process significantly incurs the formation of crystallization blockages in the tunnel drainage systems. Optimizing the shotcrete mixing ratio is a feasible way to mitigate these blockages. Therefore, this study conducts calcium dissolution tests to investigate the impact of six admixtures, namely, antialkali agent, nanosilica, nanosilica carbonate, fly ash, sodium methyl silicate waterproofing agents, and silane waterproofing agents, on calcium dissolution resistance. Also, mechanical and microscopic tests are carried out to examine their impact on the strength and pore structure of the shotcrete. The objective of this study is to determine the optimal admixture for enhancing the calcium dissolution resistance of shotcrete. Results indicate that the antialkali agent significantly reduces the calcium leaching content of shotcrete. When the dosage is 14%, the calcium leaching amount is reduced by 68.4% in 28 days. Followed by nanosilica and silane waterproofing agents, with optimal dosages of 12 and 0.4%, respectively, the dissolution amount of calcium ions in shotcrete was reduced by 32.87 and 26.5%, respectively. Fly ash curing for 28 days can also reduce the calcium ion dissolution of shotcrete, while nanocalcium carbonate and sodium methyl silicate have little effect on the calcium dissolution of shotcrete. The antialkali agent with a strong calcium ion dissolution effect can improve the tensile strength of shotcrete under long-term curing conditions, which can be increased by 52%, but it compromises the growth of compressive strength. Nanosilica, fly ash, and silane waterproofing agents can improve both the compressive strength and tensile strength of shotcrete under long-term curing conditions. Specifically, at 28 days of curing, the compressive strength increased by 16.83, 28.8, and 20% and the tensile strength increased by 50.24, 60, and 64.5%. In addition, the microscopy results show that the antialkali agent, nanosilica, and silane waterproofing agents promote the hydration process of cement to form ettringite with a low and stable calcium–silicon ratio and reduce calcium hydroxide crystals. Nanosilica and silane waterproofing agents optimize the pore distribution in shotcrete by increasing beneficial pores, decreasing harmful pores, and reducing total porosity.

中文翻译：

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减少隧道喷射混凝土中的钙溶解：外加剂对喷射混凝土性能的影响

地下水渗入隧道导致水通过初始支撑喷射混凝土中的裂缝通道渗透。这导致水泥水化的关键产物氢氧化钙溶解和流出。该过程显着导致隧道排水系统中结晶堵塞的形成。优化喷射混凝土混合比是缓解这些堵塞的可行方法。因此，本研究通过钙溶解试验，探讨抗碱剂、纳米二氧化硅、纳米碳酸硅、粉煤灰、甲基硅酸钠防水剂、硅烷防水剂6种外加剂对钙溶解性能的影响。此外，还进行了机械和微观测试，以检查它们对喷射混凝土的强度和孔隙结构的影响。本研究的目的是确定增强喷射混凝土抗钙溶解性的最佳外加剂。结果表明，抗碱剂显着降低了喷射混凝土的钙浸出量。当用量为14%时，28天钙浸出量减少68.4%。其次是纳米二氧化硅和硅烷防水剂，最佳掺量分别为12和0.4%，喷射混凝土中钙离子溶出量分别减少32.87和26.5%。粉煤灰养护28天也能减少喷射混凝土的钙离子溶出，而纳米碳酸钙和甲基硅酸钠对喷射混凝土的钙溶出影响不大。钙离子溶解作用较强的抗碱剂可以提高喷射混凝土在长期养护条件下的抗拉强度，可提高52%，但会损害抗压强度的增长。纳米二氧化硅、粉煤灰、硅烷防水剂可以提高喷射混凝土在长期养护条件下的抗压强度和抗拉强度。具体而言，固化28天时，抗压强度分别提高了16.83%、28.8%和20%，抗拉强度分别提高了50.24%、60%和64.5%。此外，显微镜结果表明，抗碱剂、纳米二氧化硅和硅烷防水剂促进水泥的水化过程，形成钙硅比低且稳定的钙矾石，并减少氢氧化钙晶体。纳米二氧化硅和硅烷防水剂通过增加有益孔隙、减少有害孔隙和降低总孔隙率来优化喷射混凝土中的孔隙分布。