The use of recycled bassanite and coal ash to enhance the strength of very soft clay in dry and wet environmental conditions

Abstract

This study investigates the use of recycled bassanite, produced from gypsum waste, in conjunction with coal ash as a stabiliser material to improve the strength of very soft clay soil as well as to improve its durability in a wet environment. Additionally, this study investigated the effect of wet–dry cycles, referred in this study as a wet environment, on durability, strength and mechanical properties of very soft clay soil stabilised with recycled bassanite and coal ash. Four different combinations of the bassanite–soil ratios ranging between 0% and 20% as well as three different combinations of the coal ash–soil ratios ranging between 0% and 20% were used. The results showed that the use of recycled bassanite and coal ash significantly increase strength and improve durability. Both strength and durability improved with an increase of bassanite and coal ash content in the soil mixture. The use of coal ash alone is not recommended because it has a slightly negative effect on soil strength. The increase in the bassanite and coal ash contents is associated with the decrease in moisture content and the increase in dry unit weight. Curing time has a significant effect on the improvement of strength in the early days compared to the later days. The durability of the stabilised soil specimens decreased with an increase in the wet–dry cycles up to the third cycle, and then the effect of the wet–dry cycles decreased. Coal ash has a significant effect on the improvement of durability, especially for samples stabilised with a small amount of bassanite. The influence of the wet–dry cycles on mechanical properties, such as water content, dry unit weight and volume changes, is not significant. Generally, samples stabilised with bassanite and coal ash within the investigated limits in this study are durable against the effects of the wet–dry cycles. It is shown that the incorporation of recycled bassanite and coal ash to improve the strength and durability of very soft clay soil achieves an acceptable performance in both wet and dry environmental conditions. In addition, the effective use of waste gypsum and coal ash contributes to developing a sustainable society by reducing the huge quantities of solid wastes, producing useful material from waste material, and establishing a sound environment.