## 3 Results and Discussions  
### Compressive Strength of PU-Cement Mortar  
Figure 7 shows the compressive strength of the PU-cement mortar containing nano silica. The average compressive strength of the control mortar (PUCM0-0) at 28 d curing age was 60.28 MPa. All PU-cement-modified specimens revealed a decrease in compressive strength due to adding the PU binder. As depicted in Figure 7, the addition of 10% PU binder without NS led to a decrease in strength of 22.81%. However, the rate of strength reduction was mitigated due to the reinforcing effect of the NS particles at various contents, as noted in PUCM10-1, PUCM10-2, and PUCM10-3, which reduced by 19.81%, 15.56%, and 17.44%, respectively, compared to the reference sample. The result indicated that the addition of the NS material mitigated some of strength lost due to the PU binder. This finding agreed with previous studies [43; 44], which showed that NS improves the compressive strength of mortar, and the improvement mechanism is due to the filling of  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Matrix** & **Equation** & **Description** \\ \hline R\({}^{2}\) & \(\left[\frac{\sum_{i=1}^{n}\left(m_{i}-\overline{m}\right)\left(p_{i}-\overline{p }\right)}{\sum_{i=1}^{n}\left(m_{i}-\overline{m}\right)\sum_{i=1}^{n}\left(p_{i }-\overline{p}\right)^{2}}\right]^{2}\) & R\({}^{2}\) is a commonly used performance metric to describe how well a model predicts a given variable. Its value ranges from 0 to 1. When R\({}^{2}\) is near to 1, high prediction accuracy is attained [54; 55]. \\ MSE & \(\frac{1}{n}\sum_{i=1}^{n}\left(m_{i}-\overline{m}_{i}\right)^{2}\) & The statistical error demonstrating the model’s performance. The MSE value was very near to zero, which indicates excellent prediction accuracy. The difference between the predicted value and the observed value is indicated by RMSE. When the RMSE value approaches 0, better performance is achieved. \\ MAE & \(\frac{1}{2}\sum\limits_{i=1}^{n}\left|m_{i}-p_{i}\right|\) & MAE revealed the mean absolute error value between the predicted and observed value. It has a range between 0 \(<\) MAE \(<\)\(\infty\). \\ \hline \hline \end{tabular}
\end{table}
Table 4: Evaluation matrices.  
Figure 6: Hyperparameter tuning process.  
pore structure in the cement mortar and promotes the hydration process of the matrix. Additionally, a further decrease in the compressive strength was noted in the PU-cement mortar containing 15%PU binder. The PUCM15-0 revealed a compressive strength of 41.52 MPa, which was 31.51% lower than the control specimen (PUCM0-0). Similarly, a remarkable reduction in the compressive strength of the PU-cement mortar was noted in the specimens involving the highest PU binder content (25% PU binder), as revealed in the PUCM25-0 specimen with 31.34 MPa, which appeared to be the lowest strength recorded among all mixes. This strength was improved due to the addition of NS, as demonstrated in PUCM25-1, PUCM25-2, and PUCM25-3. This indicates the reinforcing effect of NS. In spite of the negative effect of PU binder content when added in lower and higher content, NS particles were able to mitigate some of the strength loss due to the introduction of the polymer materials.  
Previous research has indicated that the polyurethane binder significantly decreases the mechanical properties, particularly the compressive strength of cement-based composites, as reported in the literature [6; 26; 42]; the strength decreases with the increase in PU binder content. The reduced strength is attributed to the cement particles' adsorption to reduce the mixtures' hydration process and the formation of more void in the cement mortar. However, the incorporation of PU binder in the cement-based composite improved the desirable performance of durability-related properties such as water absorption, early and long-term shrinkage, etc. Due to its viscoelastic behavior, the influence of PU binder prevents crack propagation in cement-based materials [56].