A polyurethane-forming reaction mixture characterized by its ability to be adjusted with respect to its viscosity in response to changing temperature and humidity conditions is achieved with a mixture that includes a liquid polyisocyanate having an isocyanate group content of at least 10% by weight, an isocyanate-reactive component which includes at least one high molecular weight polyol, a urethane catalyst and from 0.2 to 5 parts by weight per 100 parts by weight of isocyanate reactive component of a viscosity adjusting component. The viscosity adjusting component is preferably a diamine, a polyamine and/or an amine-terminated polyol. This reaction mixture is particularly useful for producing polyurethane geotextile composite materials useful as liners for ditches and canals.