Cement types

Cement is the binder that turns sand and stone into concrete and mortar. Portland cement is made by burning limestone and clay or shale at about 1 450 °C in a kiln to make clinker, then grinding the clinker with a little gypsum to control setting. Its main compounds are tricalcium silicate (C3S), which gives early strength; dicalcium silicate (C2S), which gives later strength; tricalcium aluminate (C3A), which reacts fast and generates heat; and tetracalcium aluminoferrite (C4AF). Changing their proportions or blending in other materials gives cements for different jobs.

Ordinary Portland cement (OPC) is sold in India in strength grades 33, 43 and 53, meaning the minimum 28-day mortar strength in MPa. Portland pozzolana cement (PPC) blends clinker with fly ash or calcined clay; Portland slag cement (PSC) blends it with ground granulated blast furnace slag. Blended cements develop strength more slowly but have lower heat, better long-term strength and better resistance to chlorides, sulphates and alkali-aggregate reaction, and they cut carbon dioxide emissions. Composite cement uses both fly ash and slag.

Special cements include sulphate-resisting cement (low C3A) for sulphate soils and sewage works, rapid-hardening cement for early stripping and repairs, low-heat cement for mass concrete, white cement for finishes, oil-well cement and high-alumina cement for refractory uses. Tests on cement cover fineness, consistency, setting times, soundness (Le Chatelier and autoclave) and compressive strength of mortar cubes. Cement should be stored dry, used first-in first-out, and retested if stored for more than about three months.