Apart from ordinary Portland cement, describe two other cements both in use and manufactured. – NABTEB QUESTION

ANSWER

(i) Rapid Hardening Portland Cement: This is made by mixing together chalk or limestone, with clay or shale, heating them to a clinker and grinding them to finer powder than that for ordinary Portland cement. The setting time is similar to that of O.P.C. But it develops strength more rapidly. It enables form work to be struck earlier.

(ii) Extra-Rapid Hardening P.C: Manufactured by adding an accelerator e.g calcium chloride (CaCl2) to rapid hardening Portland cement. It has quick initial set. Hardening much faster than rapid hardening p.c. It is particularly useful in cold weather. Allows concreting to continue during low (not freezing) temperature. It is also useful for marine work in intertidal conditions.

(iii) High Alumund Cement: Made from chalk and bauxite, black in color. Develops high strength in 24 hours, making it responsibly useful for emergency work and in cold weather. It resists sulphate attack and some weak acids and most organic liquids.

(iv) Hydrophobic Cement: Made by adding substances to Portland cement and during the grinding process, which form a water-repellant film around each grain of cement. The cement can thus be stored under humid and damp conditions without deterioration. During the mixing process, the film is rubbed off and normal hydration takes place.

(v) White Cements: By special manufacturing process and selection of raw materials, the amount of iron in the Portland cement is considerably reduced. It is used for manufacturing precast concrete products and in a production of cement points.