Disclosed is a closed-cell polyisocyanurate foam composition capable of high compressive strength at temperatures up to 200.degree. C. The new composition further exhibits no loss or degradation in conventional mechanical properties--less than that which impacts the intended use. The formulation of the present invention is based on the reaction product of a isocyanate and an epoxide resin catalyzed by a mixture of a tertiary amine and a cyclic amine. Compressive strength is augmented by incorporating a large fraction of a non-reactive bulk filler into the precursor polymer gel.
 This invention was made with Government support under government contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention, including a paid-up license and the right, in limited circumstances, to require the owner of any patent issuing in this invention to license others on reasonable terms.