Liquid isocyanate-terminated polyurethane prepolymers are cured by mixing them with about 5% to 200% by weight, preferably 10% to 100% by weight, based on the weight of prepolymer, of an aqueous colloidal dispersion selected from aqueous alkali metal silicates containing from about 20% to 60% solids and aqueous magnesium oxide containing from about 5% to 40% solids. The water associated with the aqueous dispersion is bound in the cured product and thus the dispersions act as both curing agents and low cost extenders. The cured materials may be non-porous elastomeric products or finely cellular products. Some of the cellular products are rigid and suitable for, e.g., structural applications. Both types of products have good flame retardant properties.