Abstract: Method for accelerating the hardening and increasing the strength of Portd cement by mixing Portland cement with water and with a mixture containing dialuminum pentahydroxychloride, calcium sulfate and calcium oxide in weight ratios of 1:3.5 to 4.3:1.3 to 1.8, respectively, and up to 2 parts by weight of an alkali metal nitrite at temperatures up to 90.degree. C. and thereafter hardening the mixture at temperatures up to 100.degree. C. The mixing can be carried out either by wet mixing or dry grinding the ingredients.

Abstract: A method for accelerating the hardening and increasing the strength of cets, including cement pastes, cement-sand mortars and concretes, which comprises mixing the cements with water plus an admixture at a temperature up to 60.degree. C., and thereafter allowing the mixture to harden at a temperature up to 100.degree. C. The admixture used comprises bialuminium pentahydroxychloride, gypsum, lime and alkali or alkaline earth nitrite.The method is applicable to different kinds of cement, such as rapid hardening cement, slag-cement, pozzolana-cement and belite-cement.