Performance assessment of clay soil stabilized with recycled gypsum based on SEM and XRD

Abstract

The use of recycled gypsum, produced from gypsum waste plasterboard, as a stabilizer material for ground improvement projects is initiated in Japan recently. This application is considered one of the appropriate solutions that aims to eliminate the huge quantities of gypsum waste plasterboards and to avoid the cost of their disposal in landfill sites while preserving the environment. Although the incorporation of recycled gypsum in ground improvement projects has many advantages, it poses many challenges since gypsum is a soluble material. Therefore, it is essential to explore the microstructure and mineralogical compositions of clay soil stabilized with recycled gypsum in order to achieve successful results. To achieve this purpose, recycled gypsum was mixed with furnace cement type-B or lime, in dry state, in different ratios to prevent the solubility of gypsum. Subsequently, different contents of these admixtures were mixed with clay soil to mold cylindrical stabilized soil specimens and then subjected to different curing times before testing. SEM and XRD were used to investigate microstructure and mineralogical composition respectively, while unconfined compression test was used to investigate the compressive strength. Test results showed that the addition of recycled gypsum improves the strength of clay soil compared to identical untreated samples. The improvement in strength based on compressive strength results in agreement with the results of SEM images and XRD. The formation of ettringite increases with the increase of recycled gypsum content in soil mixture. XRD results showed that the improvement in strength is not only due to the formation of ettringite but also due to the formation of calcium carbonate and hydrate calcium sulphate in soil mixture. Curing time has a significant effect on the formation of ettringite and the improvement of strength especially in the early curing times. The initial improvement in strength is related to the potential of gypsum for water absorption while the permanent improvement is related to the formation of ettringite and calcium carbonate in soil mixture.