Study on the Performance Evolution Law and Microscopic Mechanism of Cement–Sodium Silicate Grout Prepared by Seawater

During the construction of underwater shield tunnels (excavated using a slurry pressure balance shield machine), whether seawater (Sw) can be used to replace freshwater (Fw) in the preparation of cement–sodium silicate grout (CSG) has become a major concern in the engineering community. CSG is formed by mixing components A and B, where component A is a liquid prepared by mixing bentonite, cement, and water, and component B is a sodium silicate solution. In this paper, the CSG was prepared using Sw instead of part of Fw. The properties, including bleeding rate, initial and final setting time, gel time, compressive strength, and microscopic characteristics, were tested to investigate the influence of Sw on the performance of CSG and explore its impact mechanism. The results showed that when expanding bentonite with Sw, the bleeding rate of Component A exceeded 50%, failing to meet the engineering requirement of 10%. However, expanding bentonite with Fw, the seawater replacement ratio has almost no effect on Component A, with all values remaining below 10%. As the seawater replacement ratio increases, the setting time of CSG is significantly shortened. Although the inclusion of seawater results in a marginally lower 1-day strength for CSG, it notably boosts the strength at later ages. Specifically, at a 45% seawater replacement ratio, the 28-day strength showed a marked increase of 52% relative to the CSG without seawater. In the later stage of hydration, the positive effect of Cl− in seawater, promoting the hydrolysis of C3S and C2S on strength, is significantly higher than the negative effect of sulfate ion erosion in seawater on strength. Therefore, seawater significantly increases the 28-day compressive strength of CSG. This study can provide reference and guidance for the application of seawater in the preparation of two-component grout for submarine shield tunnels.