Polyurethane (PU)/silicate composite for grouting is prepared using toluene 2,4-diisocyanate (TDI), polyoxypropylene glycol (PPG), and sodium silicate. The reaction temperature change, mechanical properties, flame resistance, thermal performance, chemical structure, and section morphology of PU/silicate composite are investigated by thermometer, mechanical test, LOI and vertical combustion tester, thermogravimetric analysis (TGA), FTIR, and scanning electron microscopy (SEM). The addition of sodium silicate reduced the maximum reaction temperature and improved the mechanical properties and flame resistance of PU composite dramatically. The best comprehensive performance can be achieved when the silicate fraction is 25 wt%, tensile strength is 19.3 MPa, bonding strength is 4.1 MPa, compression strength is 47.2 MPa, the value of LOI is 34.1% and reaches UL 94 V-0 rating. Porous structures with microspheres and spherical crevices are observed in the section of PU/silicate composite, which contributes to the low reaction temperature and higher mechanical performance, the reaction equations and possible formation mechanism of the special microspheres reinforced structure are put forward finally. (C) 2015 Society of Plastics Engineers