Abstract: Semisolid polymerizable compositions are obtained by combining unsaturated polyester resin syrup, aqueous alkali-stabilized colloidal silica and finely divided particulate calcium sulfate hemihydrate. The composition rapidly thickens to a semisolid state when the ingredients are mixed and retains its semisolid condition for extended periods of time. The unsaturated polyester resin syrup component of the mixture remains polymerizable until its polymerization is initiated, normally by means of heating in the presence of a high temperature polymerization initiator. Sufficient calcium sulfate hemihydrate is included to react with substantially all of the uncombined water in the other ingredients so that the resulting polymerizable composition is substantially free of uncombined water. The compositions may also contain inert fillers and/or reinforcing fibers. The compositions can be employed to produce coatings, laminates, cast articles, molded articles and other shaped articles.