Preparation and performance evaluation of aluminum sulfate-based alkali-free suspension accelerators for shotcrete

An aluminum sulfate-based alkali-free suspension rapid-setting agent (AF-LX-FSA) was formulated, which exhibited optimal performance at a dosage of 8% by mass of cementitious materials. At the optimal dosage, the agent achieved a flowability of 135 mm, remarkably short initial and final setting times of 2.86 min and 4.67 min, and high compressive strengths of 1.06 MPa, 13.2 MPa, and 49 MPa at 6 h, 1 d, and 28 d, respectively, all conforming to the Chinese national standard GB/T 35159-2017. The acceleration mechanism and microstructural evolution were systematically investigated via XRD, TG-DTG, and SEM, revealing that the rapid setting and early strength development were primarily attributed to the rapid formation of a dense network of ettringite (AFt) crystals. The insights into the acceleration mechanism and microstructure provide a valuable theoretical basis for the development of high-performance suspension alkali-free accelerators.