EFFECT OF SOAKING ON THE STRENGTH OF FLY ASH LIME- SODIUM SALTS MIXTURES H. N. Ramesh 1 , M. Sivamohan 2 and P. V. Sivapullaiah 3 ABSTRACT 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. INTRODUCTION 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. Most uses of fly ash depend on its pozzolanic nature to develop sufficient strength. While some fly ashes develop strength without any additive, most fly ashes require addition of lime for development of good pozzolanic strength. However, there is an optimum lime content for enhancing the strength of a fly ash. Recently, procedures are established to determine the optimum lime content of a fly ash (Sivapullaiah et al, 1995). But for most construction materials, as well as fly ash in geotechnical applications enhancement of durability is an important criterion. The strength of fly ash achieved with pozzolanic reactivity is not established. Hence, it is proposed to assess the durability of fly ash. There is no standard test to assess the durability. The water resistance of pozzolanic materials is an important test to assess the durability. To enhance further the strength of fly ash with optimum lime content, sodium salts are often used (Ramesh et al, 1999). The salts are sodium chloride, sodium hydroxide and sodium sulphate. The strength achieved varied considerably depending on the type of the salt used. However, the enhancement of strength is established only for samples cured under ambient conditions and tested without soaking. The effect of sodium salts on the strength fly ash lime mixture cured and soaked in water is not known. Further, the strength loss on soaking can be taken as a measure of durability. Hence, it is also proposed to study the improvement in the water-resistant strength of fly ash with sodium salts and the loss of strength of samples with sodium samples after curing for different curing periods upon soaking. This will help to select better admixture for improving the strength of fly ash. ----------------------------------------------------------------------------------------------------------------------------------- 1. Reader, Department of Civil Engineering, UVCE, Jnana Bharathi, Bangalore-560056, India 2. Former Graduate Student, Department of Civil Engineering, UVCE, Jnana Bharathi, Bangalore-560056, India 3. Assistant Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore-560012, India