# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
## 3 Results and Discussion
### Modified Durability Test  
The results of the unconfined compression tests conducted on the modified durability samples are summarized in Table 12. Samples cured for prolonged periods had weaker UCS than the 7- and 14-day samples. A sharp decrease in the unconfined compressive strength was observed after the first cycle of the modified durability test, which was kept almost constant across all three cycles. Test results in Table 13 indicate that earlier curing days show better resistance to cyclic wetting and drying, as lower strength losses were recorded for shorter curing periods. This coincided with higher strength values in Table 12 for the same curing periods. The better resistance of earlier curing days may be attributed to rapid exothermic hydration occurring during the first ten days of the early curing stage. The rapid hydration process contributed to the durability of early curing day samples, such as 7-day and 14-day curing samples, which led to better management of newly introduced water into the hydration process, lowering potential strength loss. However, a significant strength loss was observed when samples were exposed to prolonged wetting and drying. Strength losses from 75% to 95% were incurred after 1, 2, and 3 wet-dry cycles. Similar studies on lime-treated soils revealed significant particle rearrangement after wetting and drying cycles [59]. The first drying cycle induced cracks due to water loss in saturated soil samples, as suction in the soil formed tensile stresses within the specimen. The succeeding wetting cycle exacerbated damage as moisture filled the newly formed cracks, reducing soil strength and stiffness. The contamination of these lime-treated soils containing sulfates increases the risk of induced heave and distress of structures [60]. Strengths of samples after cycles 1, 2, and 3 did not exhibit significant differences due to the initial degradation that occurred in cycle 1. This meant a minimum compression strength was reached before the succeeding cycles. The ingress of water within the sample pores alters the microstructure of the soil, where microcracks develop within the sample, resulting in significant deterioration [61], as observed in cycle one samples. The modified durability test conducts capillary soaking to allow fluctuation of moisture levels to simulate cyclic wetting and drying [45]. Seasonal water content changes caused by periods of precipitation and evaporation result in volume and void ratio changes in the soil [62]. Water absorption during cyclic wetting led to a decrease in unconfined compressive strength due to a reduction in cohesive forces that ensure particle interlock. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cycles** & **1** & **2** & **3** \\ \hline
**Curing Period (Days)** & & & \\ \hline
7 & 83.22 & 74.89 & 81.71 \\
14 & 86.54 & 89.67 & 84.50 \\
21 & 94.22 & 94.07 & 93.70 \\
28 & 94.73 & 94.95 & 95.00 \\
35 & 96.56 & 96.40 & 96.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Average strength loss of treated soil samples subjected to modified durability tests. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Cycles** & \multicolumn{2}{c}{**1**} & \multicolumn{2}{c}{**2**} & \multicolumn{2}{c}{**3**} \\ \hline
**Curing Period** & **UCS (kPa)** & **w (\%)** & **UCS (kPa)** & **w (\%)** & **UCS (kPa)** & **w (\%)** \\ \hline
7 & 22.12 & 59.28 & 33.10 & 54.47 & 24.12 & 56.11 \\
14 & 21.96 & 56.74 & 16.85 & 54.93 & 25.30 & 59.47 \\
21 & 13.97 & 57.86 & 14.31 & 53.02 & 15.21 & 58.51 \\
28 & 13.66 & 51.14 & 13.09 & 57.09 & 12.96 & 59.38 \\
35 & 12 & 59.28 & 13.80 & 63.17 & 15.35 & 66.77 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Average unconfined compressive strength (UCS) and moisture content (w%) of treated samples subjected to modified durability tests. [PERSON] [49] suggested that cyclic wetting and drying of soil caused soil particles in residual soils to flocculate. The drying phase initiated the cementation of particles, resulting in relatively large void spaces between aggregated soil lumps.