Investigation on Mechanical Performance and Flame-Retardancy of Polymer Cement-Based Coatings with Ettringite Modified by Amphiphilic Group

Polymer cement-based coatings (PCC) have great potential in protecting underground structures, while their flame -retardancy is limited by the flammability of the polymer components. Developing multifunctional inorganic fillers that can enhance both mechanical properties and flame retardancy is crucial for improving the safety and durability of the coatings. In this study, an ettringite (AFt) modified by amphiphilic group and aluminum hydroxide (AH) was subjected to surface functionalization treatment and then incorporated into the PCC to enhance the overall performance of the material. The tensile strength and bond strength of the coatings modified by amphiphilic group with AFt were increased by 27.7% and 22.4%, respectively. Meanwhile, the flame burn time, flameless burn time, and carbonization length were reduced by 89.1%, 80.6%, and 90.2%, respectively, and the limiting oxygen index (LOI) increased by 28.9%. The amphiphilic groups act as molecular bridges that couple the inorganic modified AFt with the organic VAE phase, thereby strengthening the organic–inorganic interface and promoting a more integrated polymer–cement network. Meanwhile, the well-dispersed inorganic phases provide endothermic dehydration and a protective residue during heating, jointly improving the mechanical reliability and fire safety of the PCC.