# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
###### Abstract
However, previous studies only focused on the short-term performance of several index properties. The present research subjected expansive soils treated with varying proportions of recycled gypsum mixture to direct drained shear loading to identify shear strength parameters. Although gypsum and RHA effectively improve several properties of expansive soils, each has limitations. Gypsum has cementitious properties, yet also manifests plasticity at high water content. RHA has no cementitious properties, but its lack of cohesion effectively reduces expansion. Therefore, a second round of direct shear tests was performed with soils treated with recycled gypsum and 10% RHA, whose content was adopted from [PERSON] et al. [34]. Direct shear specimens' conditions characterized expansive soil's behavior under short-term loading conditions. The resulting stress-strain behavior was assessed versus the calculated prediction from a modified hyperbolic stress-strain model [38] derived from direct shear test results. There is also a disparity in the literature comparing the performance of soils treated with a gypsum-RHA combination in short-term shear strength and long-term conditions under fluctuating moisture conditions due to wetting, drying, and flooding. Similar studies can be found from [PERSON] [39], who examined the influence of cyclic drying and wetting on the swelling behavior of lime-treated black cotton and ash-modified soils, and [PERSON], [PERSON], and [PERSON] [27], who explored the durability and strength of very soft clay treated with bassanite and coal ash in dry and wet environmental conditions. [PERSON] and [PERSON] [40] also assessed the performance of gypsum and RHA in a soaked CBR test. However, these studies have yet to consider variations in the curing period of additive treatment, and the expansion index test is another option to assess the shrink-swell potential of soils after multiple dry-wet cycles. The results of this study can help identify new approaches to improving the strength and durability of expansive soils under varied curing and moisture conditions. Investigating low-cost and waste materials such as recycled gypsum and rice husk ash to enhance the engineering properties of weak expansive soils may show potential to reduce the number of harmful substances deposited in waste disposals that contribute to the cumulative impact of pollution on the environment. Utilizing natural and recycled gypsum as a primary additive and a complement to established soil stabilization supplements creates new opportunities to openly discuss and analyze its short- and long-term effects on clay soil characteristics. This research may also contribute to the current pool of knowledge built on using gypsum to stabilize the soil. Moreover, new alternatives can be introduced in the construction industry, particularly in the Philippine setting, to propose methods that economize the cost and time of certain construction activities while reducing unattended debris in disposal sites. This investigation may also inspire further explorative work in the local setting, which could likely perform analyses in other critical areas with a significant presence of weak clayey and expansive soils.