# Prediction of Unconfined Compressive Strength in Cement-Treated Soils: A Machine Learning Approach
## 3 Results and Discussion
### Experimental Test Results
#### 3.1.3 Effect of Cement Content on Strength Development  
The influence of cement content on UCS was further examined through detailed analysis of strength measurements across different cement percentages. Figure 6 presents a consolidated boxplot analysis of UCS values grouped by cement content and curing period.  
The results demonstrate a clear monotonic increase in UCS with increasing cement content across all curing periods. Specimens prepared with 2.5% cement exhibited median UCS values of approximately 1000 kPa after 28 days of curing, while those with 10% cement achieved median strengths of approximately 4500 kPa over the same curing period--more than a fourfold increase. This substantial strength enhancement can be attributed to the increased formation of cementitious hydration products that bind soil particles and create a more rigid soil matrix.  
A noteworthy observation is the accelerating rate of strength gain with increasing cement content. The experimental data reveals that the relationship between cement content and UCS is not strictly linear but exhibits a positive second-order component. The incremental strength increase when moving from 2.5% to 5% cement was less pronounced than the increase observed when transitioning from 7.5% to 10% cement. This non-linearity suggests that beyond a certain threshold, additional cement provides disproportionately greater strength benefits, likely due to the formation of a more continuous cementitious network throughout the soil matrix.  
The physical mechanism behind this behavior can be understood through the fundamental principles of soil-cement interaction. At low cement contents (2.5%), the cementitious products primarily strengthen individual contact points between soil particles. As cement content increases to intermediate levels (5-7.5%), these localized cementation zones begin to overlap, creating stronger particle clusters. At higher cement contents (10%), a more continuous cementitious matrix forms, dramatically enhancing the soil's resistance to deformation and failure.  
Additionally, the interquartile range (IQR) of UCS values progressively expanded at higher cement percentages, particularly at 7.5% and 10%. This increased variability indicates that at higher cement contents, the stabilized soil becomes more sensitive to other factors such as mixing homogeneity, moisture distribution, and microstructural development during curing. From a practical perspective, this finding suggests that while higher cement contents yield greater strengths, they may also require more stringent quality control measures during mixing and placement to ensure consistent performance.  
For geotechnical applications with moderate strength requirements (UCS \(\leq\) 2000 kPa), cement contents in the range of 5-7.5% appear to offer an optimal balance between strength enhancement and economic considerations. For applications demanding higher strength levels, the 10% cement content consistently delivered UCS values exceeding 3000 kPa after 14 days of curing, making it suitable for more demanding applications. These recommendations are consistent with typical values reported in the literature for geotechnical applications, as well as with international practice [23; 39; 45; 46].  
Figure 6: Consolidated UCS analysis: effect of cement content and curing time. The red circles indicate statistical outliers, defined as values falling outside 1.5 times the interquartile range from the box edges.