Abstract: An admixture for cement compositions is prepared by calcining at 630.degree. to 870.degree. C. at least one substance selected from the group consisting of natural kaolin, halloysite and synthetic kaolin, effecting classification so that amorphous portions of alumina/silica having a mixing ratio of 0.5 to 1.3 form main components, with all particles having diameters up to 8 .mu.m and an average diameter of 0.5 to 2 .mu.m, and a specific gravity ranging from 2.45 to 2.55. This admixture is used to produce a cement composition such as mortar or concrete, which realizes excellent fluidity to promote workability, and a high strength structure.

Abstract: A pressure water absorption inhibitor for lightweight aggregates which comprises an aqueous dispersion of a polymeric substance having a viscosity which permits the aqueous dispersion to enter the fine pores in the surface layer of a lightweight aggregate and which dispersion is capable of being thickened to a viscosity of not less than 50 times the viscosity before thickening by elevation of the pH.