A novel solidification technique for fluorine-contaminated bassanite using waste materials in ground improvement applications

This study investigates the development of solidification technology, based on the formation of ettringite, for fluorine-contaminated bassanite using waste and recycled materials. Recycled bassanite is mixed with coal ash, blast furnace slag and furnace cement type B in varying proportions to obtain the optimal ettringite production, based on the chemical response. Leaching tests are performed on the different admixtures to measure the solubility of fluorine. Scan electron microscope and X-ray diffraction tests are conducted on the samples to explore the formation and intensity of ettringite. The results show that the suggested admixture has a significant effect on the production of ettringite and increasing the cement content in the admixture increases the formation of ettringite. The increase of the ratio of cement in the admixture has a clear effect on the production of ettringite and the reduction of fluorine solubility compared to the case of cement–bassanite mixture. Both size and length of ettringite crystals increase with an increase in the ratio of cement-admixture/bassanite and intensity of ettringite in the material matrix. The solubility of fluorine decreases with the increase of admixture content because of the concomitant increase in the production of ettringite. The dimensions of the ettringite crystals have a significant effect on the reduction of the solubility of fluorine, i.e., an increase in ettringite prisms size decreases the fluorine solubility. The investigated limits of the suggested admixture do not show any adverse effect on the geo-environmental properties in terms of fluorine release. The suggested admixture is recommended as a stabiliser material for soft clay soil because it is environmentally friendly, economical and meets the environmental standards.