Injectability Analysis of Seawater-Mixed Magnesium Phosphate Cement Slurry Applied to a Sand Layer

• Seawater-mixed magnesium phosphate cement was proposed for subsea tunnel reinforcement. • Properties crucial to grouting were evaluated on seawater-mixed MPC slurry. • Differences between freshwater and seawater as the mixing water were compared. • The injectability of seawater-mixed MPC slurry was evaluated in the case of the sand layer. Magnesium phosphate cement (MPC) is extensively used for construction and quick repairs owing to its high performance and good erosion resistance. This study aims to evaluate the injectability of seawater-mixed MPC slurry into a sand layer for grouting reinforcement. The setting time, fluidity, rheology, particle characteristics of MPC slurries mixed with freshwater and seawater, and the grouting effect are explored. The raw material composition and microstructure are also analyzed. The results revealed that the mass ratio of borax to magnesium (B/M) and magnesium to ammonium dihydrogen phosphate (M/ADP) plays a vital role in the setting time of seawater-mixed MPC slurries. Considering the hydration and rheological behavior of seawater-mixed MPC slurries, an optimal formulation is proposed, which incorporates fixed M/ADP, B/M, and W/C ratios of 2.5, 10%, and 0.2, respectively. Unlike cement-based grouts, MPC has no obvious flocculation effect, and seawater-mixed slurry has a greater viscosity, particle size, and longer setting times during initial hydration. Seawater-mixed MPC slurry has good pumpability as a grouting material; however, its injectability is slightly worse than that of freshwater-mixed slurry.