# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
## 4 Conclusions  
Gypsum was introduced into fine-grained soils considered expansive to evaluate the effectiveness of the admixture in stabilizing expansive soils under short- and long-term conditions. A fixed amount of 10% RHA was added as a complementary additive to gypsum to address their respective weakness in soil improvement. The test soil samples and additives were proportioned in the treated specimen by volume. The outcome of various laboratory tests based on ASTM standards converge to the following conclusions. Drained direct shear test samples exhibited ductile and compressive behavior at different normal effective stresses. A rise in shear strength resulted from increased additive content for gypsum-only specimens. Samples with 15% gypsum achieved the highest shear strength. A decrease in strength occurred with the addition of RHA. Similar behavior was recorded with the friction angle of gypsum-only samples, which yielded high-angle values. Peak friction angle was achieved at 5% gypsum + 10% RHA, but was followed by a significant drop with further addition of gypsum. The ductile behavior shown by the stress-strain response of the test specimen exhibited a hyperbolic form. A modified hyperbolic model was generated to predict the treated specimen's shear-stress-shear-strain and volumetric-strain-shear-strain relationship. The resulting model yielded a reasonable projection of the stress-strain and volumetric-strain-shear-strain response due to the ductile and compressive behavior of the test specimen. The durability of specimens with 15% recycled gypsum and 10% was assessed by comparing the unconfined compression strengths of trials not subjected to moisture fluctuations and trials subjected to wet-dry cycles and water immersion. Control specimens manifested a linear increase in compressive strength with longer curing days. Treated samples subjected to durability tests showed significant strength loss compared to control samples, regardless of the number of cycles. Samples with shorter curing days withstood the cyclic wetting and drying better as new water was introduced during the early curing stage. Rapid exothermic hydration occurred during the first ten days of curing. The results indicate that the curing period influences the resistance of soil to cyclic wetting and drying, as opposed to the number of cycles of capillary soaking. Subsequent expansion index tests determined that treated samples subjected to durability tests had reached a very low expansion potential. All samples also experienced hydro-compression settlement due to water intrusion into the pore spaces. However, the collapse strain of the soil sample shows that the treated soil has a negligible degree of collapsibility. A response surface methodology generated a predictive model for the UCS of soil as a function of curing days and number of cycles. It was identified that eight wetting and drying cycles would result in the complete loss of soil strength. With this model, the  
Figure 10: Variation of UCS of soil cured for 30 days as a function of cycles. unconfined compressive strength of the treated soil can be determined at any curing day and any value of the wetting and drying cycle. Without available data, the model can evaluate the critical number of wetting and drying cycles for soil treated with 15% gypsum and 10% RHA free of sulphatic contamination under in situ conditions. Based on test results, the RHA-gypsum admixture is an effective treatment to control the expansiveness of the soil; however, the treated soil has some concerns about its durability when exposed to moisture fluctuation and extreme wetting conditions. Under these circumstances, the soil treated with RHA and gypsum must include additional preventive measures to maintain the soil's mechanical properties. Further research on other gypsum-RHA proportions or any other materials with cementitious and pozzolanic properties can be conducted to explore approaches to improving soil durability.