## 4 Strength Properties  
### Unconfined Compressive Strength of Lime-Cement Stabilized Clay Soil  
The unconfined compressive strength tests on clay soil stabilized with lime (0-30%) and cement (0-8%) over 7, 14, and 28 days revealed significant strength enhancement patterns driven by complex physicochemical mechanisms (Figure 12(a)-c). The unstabilized soil exhibited 779 kPa at 7 days, while optimal performance occurred at 15% lime and 6% cement, achieving 1584.62 kPa, 2250.42 kPa, and 2670 kPa at 7, 14, and 28 days, respectively, representing a remarkable 222.5% increase over unstabilized soil at 28 days. These improvements result from calcium-induced pozzolanic reactions that form calcium silicate  
Figure 12: Principal component analysis of compressibility modulus in lime-cement stabilized soil. hydrate and calcium aluminate hydrate gels, creating water-resistant inter-particle bonds through cation exchange processes and progressive crystalline structure development. The most pronounced strength development occurred between 7 and 14 days, with the optimal mixture gaining 40.4% strength compared to only 18.5% from 14 to 28 days, indicating accelerated pozzolanic reactions and gel formation during the initial two weeks when calcium ion availability and clay mineral reactivity are highest. Lime content beyond 15% consistently decreased strength across all curing periods and cement combinations, establishing an optimal threshold where excess calcium disrupts the delicate balance of crystalline structure formation. Similarly, cement effectiveness diminished at higher percentages, with strength increases of 23.9% when adding 0-2% cement but only 8.3% when increasing from 6 to 8% cement at 28 days with 15% lime, suggesting saturation of available reaction sites and potential interference with optimal hydration processes. These results demonstrate that while prolonged curing enhances strength development through continued pozzolanic activity, the critical improvements occur within the first two weeks when primary cementitious reactions establish the fundamental microstructural network, and optimal additive proportions exist beyond which additional materials provide diminishing returns due to disrupted chemical equilibrium. Figure 13: Line plot showing unconfined compressive strength (UCS) values after days of curing, with varying cement (0–8%) and lime (0–30%) contents. Lines show strength development patterns: (**a**). UCS values for 7-day curing time. (**b**). UCS values for 14-day curing time. (**c**). UCS values for 28-day curing time. The failure modes provide crucial insights into stabilization mechanisms. Unstabilized clay specimens exhibited typical ductile failure with gradual deformation and barrel-shaped bulging, behavior that persisted with low lime content (0-5%) where insufficient pozzolanic reactions failed to establish rigid bonds. At optimal lime content (10-15%), specimens transitioned to brittle failure characterized by well-defined shear planes and sudden load drops, reflecting calcium silicate hydrate and calcium aluminate hydrate gel formation that creates rigid crystalline structures. This brittle behavior indicates successful cementitious bonding with failure planes oriented at 45-60 degrees to the loading axis. Cement incorporation at optimal percentages (4-6%) further enhanced brittle characteristics with sharp, clean fracture surfaces and minimal post-peak ductility, demonstrating effective reinforcement of the lime-induced matrix. However, excessive lime (>20%) or cement (>6%) produced erratic failure patterns with multiple micro-fractures, suggesting disrupted microstructural integrity where excess additives interfere with optimal gel formation. Temporal evolution showed progressive embrittlement as reactions advanced, with 7-day specimens displaying transitional ductile-brittle behavior while 28-day specimens demonstrated fully brittle failure consistent with mature cementitious microstructures.