Properties of organo-mineral cements

Alkali silicates react with isocyanates to form hard and brittle materials. The mechanical properties of these organo-mineral cements depend on the types of sodium silicate and isocyanates that are combined, and on the accelerating and modifying agents that may be added. The flexural strength of the pure hardened cements can reach 10 M Pa, and the compressive strength 30 M Pa. The setting times vary from a few seconds to several weeks. The hardened cement seems to be an intimate mixture of at least two phases: an organic phase consisting of a polyisocyanate resin, and an inorganic phase consisting mainly of an amorphous silica gel and various carbonates of alkali and earth alkali metals. Thus, the hardening process takes place simultaneously in the two phases. The isocyanates react to a polymeric material, liberating CO₂. This in turn acts as a catalyst for the setting of the alkali silicates, yielding an SiO₂ gel and various carbonate phases as reaction products. Five different carbonates have so far been identified. The paper summarizes results obtained to date in a current research programme sponsored by the German Research Foundation for pure organo-mineral cements and some mortars made therefrom.