Hydration characteristics and mechanical property of superfine cement composite grouting materials modified by nano-SiO₂

In order to explore a new type of high-performance cement based grouting material for mining and improve the bearing effect of the full-length anchoring support system, a high-strength superfine cement composite grouting material was obtained by using ultra-fine Portland cement as cementing material modified with nano-SiO₂, expanding agent, accelerating agent and water reducing agent as admixture. Based on different characterization methods, the effects of nano-SiO₂ content (0, 0.5%, 1% and 2%) on the slurry performance, mechanical properties, hydration products, microstructure and hydration heat were investigated for the composite grouting materials. The coupling mechanism of nano-SiO₂ to superfine cement grouting material was revealed. In addition, it was used as a full-length anchoring agent in laboratory model test, and the mechanical load bearing property of the full-length anchoring system before and after nano-SiO₂ modification were analyzed. The results show that with the increase of nano-SiO₂ content, the fluidity of the superfine cement slurry decreases, the setting time increases first and then decreases, and the gap between the initial and final setting time decreases. Suitable SiO₂ can effectively improve the compressive strength and flexural strength of the materials. Especially, the mechanical properties of the composite grouting material reach the best under the condition of 1% content. The compressive strength and flexural strength of the hardened specimen increase by 49.6% and 91.4% at 1 d age, respectively. At 28 d age, the compressive strength increases by 34.6%, and the early mechanical strength is significantly optimized. Thermodynamic modeling, hydration heat, XRD and SEM confirm that nano-SiO₂ can shorten induction period and accelerate hydration process. Moreover, it promotes Ca(OH)₂ consumption to generate more C-S-H gelling products by volcanic ash effect, resulting in increased hardened specimen densification. The enhancement mechanism can be attributed to the synergistic effect of nano-SiO₂ with micro-aggregate filling effect, volcanic ash effect and crystal nucleus effect on superfine cement composite grouting material. In the mechanical pulling test of the full-length anchoring system, the peak pulling load and corresponding slip of the 1% nano-SiO₂ modified anchoring agent system increases by 59.7% and 177% respectively. The post-peak load curve decreases relatively smoothly, demonstrating enhanced mechanical bearing characteristics and anti-failure ability.