Improvement of strength of fly ash with lime and sodium salts

Abstract

It is often necessary to improve the strength of fly ash to enable its use for many geotechnical applications. Lime in some form or other is normally used as an admixture. Methods are established to determine the optimum lime content for fly ash, as the addition of lime beyond the optimum lime content may not be beneficial or economically viable. This paper examines the use of sodium salts to enhance the effect of lime on fly ash. Based on unconfined compression strength tests on a compacted fly-ash–lime–salt specimen, it has been shown that sodium salts improve the strength of a fly-ash–lime mixture. The improvement is salt-specific. This improvement in strength can be attributed to the formation of a greater volume of sodium calcium silicate hydrate gel as opposed to a calcium silicate hydrate. The improvement in strength is greater for sodium hydroxide than for sodium chloride. The increased pH with sodium hydroxide dissolves more silica and can produce more cementitious compounds after reaction with lime. Although sodium sulphate increases the strength initially, its long-term effect is suspect.