The Solidification of Cement

Abstract

This article describes the development of strength resulting from chemical reactions between cement constituents and added water. The main features of the hydration and development of strength of portland cement and high-alumina cement are described in terms of a comparison between the two types. Both cements are comparable with respect to their system of oxides, their hydraulic properties, and their ultimate strength. The products of hydration and their proportions in the hardened paste of portland cement are considered. It is noted that the reaction between cement and water is exothermic and that under typical conditions the temperature of a paste may rise by tens of degrees. The problems caused by this phenonemon are discussed. The study of the microstructure of the hydrates by electron microscopy shows that the gel fibrils consist of fine hollow tubes. The essential features of the hydration of portland cement and in particular the growth of the fibrillar calcium-silicate-hydrate gel material that is an important factor in the development of strength, is discussed. Since cements (and concretes in general) can develop large compressive strength during hydration, it is usually necessary to reinforce concrete beams or to subject them to prestressing or postressing in compression by the inclusion of stretched steel wires or rods. The prospects for useful modification with the well-developed crystaline hydrates of high-alumina cements are uncertain. With the irregular fibers of portland cement some improvement might be expected if, for example, the fibers could be made less regular and more intricately woven.