Most uses of fly ash arise out of its strength development due to pozzolanic property. For this strength development it is often necessary to add lime to fly ash. However, there is an optimum lime content beyond which strength of fly ash is not enhanced. On the other hand sodium salts can be used to enhance the strength of fly ash lime mixture. The water resistance of this increased strength is not known. This paper reports the effect of sodium additives on the unconfined strength of Muddanur fly ash and resistance to soaking in water. The strength of fly ash, which is at its optimum with 3% of lime, is greatly enhanced with 1% sodium chloride or with 1% sodium hydroxide. The increase in strength is better with sodium chloride than with sodium hydroxide. The strength of fly ash lime mixture further increased with curing. However, the strength loss on soaking in water also increased with curing. It was observed that the presence of sulphate increases the strength of fly ash lime mixture only at shorter curing periods. Curing for longer periods decreased the strength in the presence of sulphate both for soaked as well unsoaked specimens. These changes have been explained based on alteration in fly ash lime reactions in the presence of sodium salts.
Thermal plants in burning of coal for generation of power are producing abundant quantities of ash residue. Fly ash which constitute about 80% of this ash is a major component and poses a major disposal problem due to its fine particle which are blown off by air and due its toxic elements. Efforts are increasingly made to utilize fly ash in many geotechnical applications. Recently, procedures are established to determine the optimum lime content of a fly ash (Sivapullaiah et al, 1995).
