Effects of Prepolymerization and Fly Ash on Exotherm and Flame Retardancy of Polyurethane Mine Grouting Materials

Conventional polyurethane (PU) grouting materials face a severe trade-off between curing exotherm safety, flame retardancy, and mechanical performance in deep coal mining. Herein, we propose a synergistic strategy combining chemical prepolymerization with fly ash (FA) incorporation to develop high-performance prepolymer-based polyurethane/fly ash (PUP/FA) composite grouting materials. Prepolymerization combined with FA addition successfully mitigated the maximum reaction temperature to 98.3 °C while sustaining a rapid curing rate within 3 min. At an optimal FA loading of 20 wt%, the PUP/FA-20% composite sustained a robust compressive strength of 42.5 MPa, satisfying underground reinforcement standards. Crucially, limiting oxygen index (LOI) and cone calorimetry tests demonstrated outstanding flame retardancy and smoke suppression; the LOI reached 28.5%, and the total smoke production plummeted to 21.3 m2. This performance enhancement is governed by a synergistic mechanism where dimethyl methylphosphonate acts via gas-phase radical scavenging, while uniformly dispersed FA particles serve as rigid barrier nodes to construct a dense protective shield in the condensed phase. This work offers a highly effective, waste-valorized, and fire-safe grouting solution for sustainable deep-underground engineering reinforcement.