AbstractA composite ultrafine sulfoaluminate cement–based grouting material (U-SAC) at a high water–cement (W/C) ratio was developed to reinforce broken coal rock for the safety of coal mines. The effects of the W/C ratio on the bleeding, bulk density, viscosity, setting behavior, and mechanical strength of U-SAC pastes were investigated. The phase assemblage, microstructure, and micromechanical properties of hardened U-SAC pastes in the crack of broken coal rock were also revealed by multiple technologies. The results indicated that when the W/C ratio was 1.0, the U-SAC pastes flowed well with a low bleeding rate and low viscosity. More importantly, the mixed pastes could harden within 20 min and reach strengths of 12.1, 17.8, 22.3, and 26.2 MPa at ages of 4 h, 3 days, 28 days, and 60 days, respectively. During the hydration process, a high volume of ettringite (AFt) and alumina gel (AH3) was rapidly generated, consuming water and filling the pores, which was responsible for the rapid setting and high early strength. Scanning electron microscope and nanoindentation results showed that the U-SAC can fill 8-μm microcracks in broken coal rock and the elastic modulus of the hardened U-SAC paste was 2.39 times higher than that of coal rock. These results provide an inexpensive and effective material for the rapid reinforcement of microcracks in structural engineering.