chapter  6
42 Pages

- Hydration

Portland cement (PC) is a product of intergrinding/blending of cement clinker with gypsum and minor additional constituents/additives. The blended cement is manufactured in the cement plant, partially replacing PC with the mineral admixtures. The mineral admixtures are also added during concrete making, as a partial replacement for cement, along with other constituents, like chemical admixtures, aggregate (coarse and ne), and water. The manufacture of PC, varieties of blended cement, as well as concrete are subject to the provisions of respective national standards. The hydration of cement is the combination of all chemical and physical processes taking place after contact of the anhydrous solid with water. The chemical reactions of clinker minerals and mineral admixtures play a major role but other aspects such as agglomeration, adsorption, evaporation, and release of thermal energy also need to be considered. It begins with the addition of water while making concrete, and nearly 60%–65% of the hydration is complete in 28 days. Although the cement minerals like calcium aluminates and silicates and the calcium sulfate (gypsum) take part in the hydration reactions, the process is affected by all other constituents of concrete. There is evidence to show that, under certain conditions, the hydration reactions may continue up to 5 years. The nature and the progress of cement hydration are responsible for the strength and the durability of concrete. The mineral admixtures added to cement take part in the hydration reactions due to their pozzolanic/ cementitious properties. They affect the early-age properties, like rheology and therefore workability, setting, shrinkage, etc., besides the strength and the durability of concrete. Therefore, in order to understand the difference, the understanding of hydration process of cement is important, both without and with mineral admixtures.