Abstract: A substantially water-free hardener composition suitable for admixture with a cement to give a product capable of reaction with a silicate to form a hardened mass, comprises: (a) from 5 to 99.9%, preferably 75 to 99.8%, of an organic carbonate hardener for the silicate; (b) from 0.01 to 20%, preferably 0.02 to 10% of an organic retarder, preferably an organic acid retarder such as citric acid for the cement, the % being by weight based on the total weight of (a) and (b). The organic carbonate hardener is preferably a mixture of ethylene and propylene carbonates in proportions of from 3:1 to 1:3 by weight. The hardener may include a cement such as Portland cement. Suitable amounts are (a) organic carbonate hardener from 10 to 60%, (b) organic retarder is from 0.05 to 10% and (c) cement from 20 to 85%.