Study on the Applicability of Foam Concrete for Filling Restricted Spaces in Ultralong Distance Tunnels

This study aims to investigate the applicability of foam concrete for filling confined space in ultralong distance tunnels. In this context, foam concrete should achieve high fluidity while maintaining good compressive strength and dry density. By using a comprehensive balance analysis method, and by comparing results with those of a control test group, the effects of various components on the applicability of foam concrete were explored. The exploration included both composite mineral admixtures and single admixtures. The microstructure was characterized by scanning electron microscopy to explore the mechanism of action. The results showed that, in the tests with composite mineral admixtures, the combination of fly ash and tailings sand can significantly improve fluidity while ensuring compressive strength and dry density, with the optimal mixing ratio being 1∶1. In the experiments with single admixture, it was found that redispersible emulsion powder can enhance fluidity and reduce shrinkage while maintaining dry density and compressive strength, with an appropriate dosage of 0.25%–0.4%. Foam stabilizer can improve the compressive strength and softening coefficient of foam concrete, reduce water absorption and drying shrinkage rates, but are not beneficial for improving fluidity, with the optimal dosage being 0.12%. Although sodium gluconate retarder can delay setting and improve fluidity, it causes significant mass loss, and the dosage should not exceed 0.04%. This study provides theoretical references for the practical application of foam concrete in filling confined spaces in ultralong distance tunnels.