Swelling and geo-environmental properties of bentonite treated with recycled bassanite

Abstract This paper describes an investigation of the use of recycled bassanite, which is derived from plasterboard waste, as an additive material to mitigate the swelling potential of bentonite considering its environmental impact. Recycled bassanite was mixed with furnace cement and with lime at 2:1 ratios, and then the admixture was mixed with the tested bentonite at four different content ratios. The effects of adding bassanite admixture on the swelling properties, mechanical properties, environmental properties, microstructure, and mineralogical composition of the tested bentonite were investigated. The test results indicated that recycled bassanite has the potential for use as a stabiliser to mitigate swelling in expansive clay. Increasing the admixture content reduced the potential for swelling, the plasticity, the montmorillonite intensity, the percentage of sodium ions and the cation exchange capacity of the bentonite. The compressive strength, unit weight and percentage of calcium ions were increased for all admixture contents used. The different bassanite–cement/lime admixture contents resulted in approximately the same reduction in the swelling potential of the tested bentonite. The bassanite–cement/lime admixture had much more pronounced effects on the swelling and mechanical properties of the bentonite than bassanite alone. The bentonite treated with 6% admixture content exhibited the greatest reduction in swelling potential; hence, this admixture content ratio is considered optimal for stabilising bentonite and mitigating its swelling. The measurements of hydrogen sulphide gas, fluorine solubility and pH of the treated bentonite were found to be within the acceptable limits, indicating that bassanite–cement/lime admixtures used did not have any negative environmental effects. However, bassanite is not recommended for use alone as an amendment for expansive clay because it releases fluorine at amounts that may exceed the allowable limit in some cases.