An assessment of geo-environmental properties for utilization of recycled gypsum in earthwork projects

Abstract

This paper presents an assessment of geo-environmental properties for the incorporation of recycled gypsum, produced from gypsum waste, as an additive material in earthwork projects. In this study, the solubility of Fluorine, Boron and hexavalent Chromium and the emission of hydrogen sulfide gas refer to geo-environmental properties. To achieve this objective, the tested soil was mixed with recycled gypsum in conjunction with lime and cement in different proportions to overcome the solubility of gypsum and to reduce the solubility of harmful substances. The results of the study show that the utilization of recycled gypsum alone, as an additive material for earthwork projects, has a negative effect on the emission of hydrogen sulfide, while the addition of a solidification agent, such as cement or lime, to the recycled gypsum reduced the emission of hydrogen sulfide gas. The suggested contents and ratios for the gypsum–cement and gypsum–lime admixtures used in this study are safe against the emission of hydrogen sulfide because their measurements were found to be below the standard limits. Increasing the proportion of cement or lime in a gypsum–soil mixture has a significant effect on the reduction in Fluorine solubility. The content of the admixture has a significant effect on the reduction in Fluorine solubility in the case of gypsum–cement, while there is not much difference in the measured values of Fluorine in the gypsum–lime admixture. The intensity of ettringite increases with the increase in admixture content and the decrease in the admixture ratio, while the intensity of calcite increases with increases in both the content and the ratio of the admixture. The formation of ettringite has a significant effect on the capture of Fluorine, namely, the solubility of Fluorine decreases with the increase in ettringite intensity. The measured values for the solubility of harmful substances, including Fluorine, Boron and hexavalent Chromium and the emission of hydrogen sulfide gas, were found to lie within the standards for the investigated limits of the gypsum–soil mixture treated with cement or lime in the present study. This proves that the utilization of recycled gypsum in conjunction with cement or lime, as a stabilizer material and within the investigated limits, is safe and meets the environmental standards.