# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
###### Abstract
[PERSON] and [PERSON] [20] stated that the solubility of gypsum negatively impacts soil stability due to bonding within the soil being undermined. The inclusion of gypsum in the soil introduced sulfates in the sample, which can induce swelling, especially in reactions with water. This may prove that a second additive is required to address concerns about gypsum solubility. Other research tried different proportions of gypsum together with other additives. [PERSON] and [PERSON] [26] examined the role of gypsum on the physical and strength behavior of soil stabilized with fly ash-line. Accelerated strength increase after 14 days for soil with fly ash, 6% lime, and gypsum was observed due to the development of ettrinigite within the voids and cementitious binders that enhance particle interlock. [PERSON], [PERSON], and [PERSON] [27] investigated using recycled bassanite, produced from gypsum waste, in conjunction with coal ash as a stabilizer material to improve the strength of very soft clay soil. The moisture absorption of bassanite formed calcium hydroxide, which changed to calcium carbonate after a reaction with atmospheric carbon dioxide. Calcium carbonate initiated the hardening process between particles. Despite these positive outcomes, complementary additives have been primarily limited to lime, cement, and fly ash. Although extensive research attests to their effectiveness, there are concerns about the sustainable use of these materials. Rice husk is defined as the external layer of the rice grain obtained as a by-product in rice milling [28]. This layer constitutes about 23% of the initial weight of a rice grain [29]. From the estimated 545 million metric tons of global rice production [30], one-fifth of the worldwide rice paddy cultivation was converted to rice husk [31]. Most global production is attributable to several Southeast Asian countries with favorable soil and climate conditions. While rice production is significant, issues in disposal arise due to the sheer abundance of harvest, the lack of nutritional value, and the long decomposition time [32]. Rice husk is mainly composed of several organic materials such as hemicellulose, cellulose, and lignin. These components comprise 75-90% of the material, while approximately 17-20% includes ash primarily composed of silica and other metallic impurities such as iron, manganese, calcium, sodium, potassium, and magnesium [28]. [PERSON] et al. [33] discussed that the abundance of silicates in plant cuticles like rice husks is widespread among crops grown regularly. Rice husk is converted to ash form through thermal decomposition or burning. Depending on the component, temperature, and burning time, this process produces non-crystalline silica ash, which contains pozzolanic properties [29]. This feature makes the material a possible admixture for expansive soils [33]. [PERSON] [12] added 7.5%, 10%, and 12.5% rice husk ash (RHA) combined with 2, 4, 6, and 10 percent lime by dry weight on Indonesian soils, decreasing the plasticity index. [PERSON] et al. [34] determined that 20% and 25% RHA effectively reduced the expansion index to a very low potential according to ASTM D4829-11 [35] criteria. However, the same RHA contents resulted in a decrease in maximum dry density and unconfined compression strength and an increase in optimum moisture content. These are indicators that RHArequires a cementitious binder to produce an increase in strength. [PERSON] et al. [36] added 8% lime to 16% RHA, which saw a significant improvement in the optimal moisture content (10%), a friction angle of 38.5 degrees (770% increase), and an improved maximum shear strength (46% increase). This study treated Kauswagan clay with varying amounts of recycled gypsum and rice husk ash to assess its shear strength and durability characteristics. Previous work by [PERSON] and [PERSON] [37] and [PERSON] and [PERSON] [6] highlighted the efficacy of rice husk ask and recycled gypsum in treating the shrink-swell potential of expansive soils.