COMPARATIVE STUDY OF SHORT TERM EFFECT OF ALUM (A COAGULANT) ON STRENGTH OF CEMENT MORTAR

摘要

Alum is the most common and universal coagulant used in water treatment plant, however the general concentration of alum of 2% is observed in mixing tank. The present paper highlights upon the aggressive reaction of alum on cement mortar cubes, which are cured in potable water and in 2% concentration of alum in water. The cement mortar used in present investigation was comprised of 43 Grade O.P.C. and natural Kanhan River sand. This investigation was carried out on cement mortar mixes with proportions of 1:3, 1:4 and 1:6 with water cement ratio of 0.46 for a constant value of 20% flow of workable consistency obtained from flow test, which was carried out as per B.I.S.5512-1969. For this purpose each set comprising of 9 cubes for each of the of above mentioned mix proportions were kept immersed in potable water and in 2% alum solution for 3, 7 and 28 days curing from their date of casting. These cubes were tested for compressive strength and density. It was found that cubes immersed in 2% solution of alum had lost their cube strength considerably in respect of 3,7 and 28 days for mixes 1:3, 1:4 and 1:6 as compared with those immersed in potable water at 28 days. The reduction in strength was found to be 25%, 3% and 47% respectively. However the density variation was found to be practically constant. Hence it could be concluded that at early ages also, the detrimental reaction of alum occurs vigorously with cement undergoing hydration during its curing process. Hence early dosing of alum in the tank should be avoided.