# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
###### Abstract
Moreover, soil stabilization improves the California bearing ratio, tensile strength, and friction angle of the subgrade soil used in road and highway construction [15]. Reducing volumetric changes in expansive soil requires the addition of admixtures that aim to modify the swelling behavior of clay [2]. Cement, lime, and fly ash are common additives in stabilizing weak soil types by improving their mechanical properties [16]. The cementitious properties of these agents introduce a considerable increase in the strength of the soil. Like concrete, tricalcium silicate and dicalcium silicate were the compounds responsible for this in cement [17], while calcium oxide corresponded to lime and fly ash. Due to the inclination of the existing literature to these materials, little definitive information can be obtained from other alternative additives [18]. The extensive use of cement and lime made these materials expensive, opening new avenues for cheaper additives such as waste materials [16]. Among these waste materials, those with promising stabilizing potential include gypsum and rice husk ash. Gypsum is a mineral that is primarily associated with compounds containing calcium sulfate (CaSO\({}_{4}\)) [19], containing 32.6% calcium oxide, 46.5% sulfur trioxide, and 20.6% water [13]. This material is commonly used in industrial cement and plasterboard production. Recycling plaster boards has been popularized to improve the ground in Japan [15; 20; 21]. Past findings [19; 22] indicate that the disposal of gypsum boards as a waste material harms the environment. Gypsum boards constitute almost 1% of global construction and demolition waste [23]. When present in landfills, sulfates from the gypsum may react with organic material in the paper sheets that hold the material together. This anaerobic reaction produces hydrogen sulfide gas that can harm the health of humans and animals and the well-being of the surrounding environment. While regulations are currently implemented to limit the presence of these materials in disposal sites, more is needed to combat such an issue. Therefore, recycling plasterboards as an additive for soil stabilization should be made known as a viable option for many to consider. Recent research has shown promising results on adding specific percentages of gypsum to various soil types. Studies have attempted to use gypsum as a stand-alone additive. The investigations by [PERSON] et al. [16] involved the addition of 20% gypsum to reduce the liquid limit of the tropical peat sample from 144% to 123%. [PERSON], [PERSON], and [PERSON] [21] evaluated high-plasticity clay from Gunma Prefecture in Japan treated with gypsum, which resulted in greater unit weights, with an observed rise from 17.75 kN/m\({}^{3}\) to 18.75 kN/m\({}^{3}\). This was associated with the particle flocculation exhibited by expansive soil in the presence of calcium components. Positively charged calcium ions are attracted to negatively charged soil particles. The same study determined a significant increase in compressive strength in clayey soil from 50 kPa to 125 kPa. They introduced gypsum into the clay soil-induced particle hardening, which enhanced the cohesive strength between soil grains. Water absorption following the change in the composition of gypsum from its hemihydrate to hydrate forms reduced voids between particles. In naturally occurring gypsum, gypsum, gypsum, and sulfate-bearing soils, high concentrations of gypsum reduce the capacity of the soil to become an effective subgrade material for roads and highways, particularly when faced with prolonged soaking [24]. Moisture absorption is increased in these soils due to gypsum's soluble properties, which promote the presence of caverns and pores and continuous settlements. [PERSON] and [PERSON] [25] also reported problems associated with highly gypsum soils with properties such as strength losses and a sharp increase in compressibility when saturated.