## 3 Experimental Test Results and Analysis
### Compressive Strength (CS)  
Figure 5 provides valuable insights into how the incorporation of OWA affects the compressive strength of the cement paste. As illustrated, the compressive strength increases with time in all samples. For instance, the compressive strength values for cement paste without OWA rise from 28 to 47 MPa between 7 and 28 days, followed by a further increase, reaching a measurement of 50 MPa at 90 days. Furthermore, the samples containing OWA show values lower than those for the reference cement paste. In the case of the addition of 5% OWA, the compressive strength value is 32 MPa at 90 days of curing, which is 36% lower than the control mixture. However, the rate of the drop is significantly reduced for the inclusion of 10% OWA, reaching about an 8% decline, corresponding to a compressive strength magnitude of 46 MPa. It appears from the chemical composition that OWA possesses two important properties: First, OWA exhibits some cementitious characteristics due to the presence of good amounts of CaO (36.13%) [13]. Second, OWA can be regarded as a pozzolanic material in light of the presence of SiO\({}_{2}\) (24.7%) [36]. When OWA reacts with water, there is a formation of additional C-S-H gel, which is the main contributor to strength development [13; 36]. Therefore, the incorporation of 10% OWA could be appropriate to generate the optimal compressive strength. On the other side, as the OWA substitution level exceeds 10%, especially at 15 and 20%, a significant reduction in compressive strength is noted. For instance, the 90-day compressive strength is recorded at 31 and 20 MPa, corresponding to a reduction of 38% and 60%, respectively, in comparison to the reference mixture. Generally, this decline is mostly due to two reasons. Firstly, the loss of the amount of cement contributes to a decrease in the amount of hydration products, such as dicalcium silicate (C\({}_{2}\)S) and tricalcium silicate (C\({}_{3}\)S), which are primarily responsible for strengthening the matrix [20]. Another possible reason is that as the proportions of OWA go up, the porosity of the body structure becomes greater, which is evidenced by the decrease in the density as witnessed earlier [20; 28]. \begin{table}
\begin{tabular}{c c c c c c} \hline  & & & **Density (kg/m\({}^{3}\))** & \\ \hline
**Days** & **0\% OWA** & **5\% OWA** & **10\% OWA** & **15\% OWA** & **20\% OWA** \\ \hline
1 & 1888 & 1868 & 1852 & 1826 & 1820 \\ \hline
7 & 1908 & 1910 & 1904 & 1900 & 1838 \\ \hline
28 & 1920 & 1916 & 1912 & 1904 & 1860 \\ \hline
90 & 1938 & 1920 & 1916 & 1908 & 1892 \\ \hline \end{tabular}
\end{table} 
Table 3: Density for all pastes. Determining the pozzolanic activity index (PAI) is imperative to emphasize that OWA undergoes pozzolanic reactivity. The PAI represents the ratio of compressive strength between OWA-based paste at diverse levels and the reference paste measured after 28 days of curing. At a substitution level of 10%, the PAI stands at 90%, satisfying the ASTM C618-00 guidelines [53]. This demonstrates that 10% OWA can achieve 90% of the compressive strength of the control mixture, validating that OWA is a pozzolanic material that generates additional hydration products responsible for strength gain. The set of chemical reactions between various mixture components is detailed in Figure 6.  
The hyperbolic model yields satisfactory results, demonstrating a strong agreement with the experimental data. The coefficient of determination (R\({}^{2}\)) is high, with values of 0.97, 0.99, 0.99, 0.99, and 0.98 for cement replacement of 0, 5, 10, 15, and 20% OWA, respectively. This mathematical model confirms its viability for practical applications in OWA-based matrices. Figure 7 displays the results of the IRCS and UCS values for each mixture. As seen, the IRCS values witness a consistent trend with that of the compressive  
Figure 5: Compressive strength of pastes (experimental and estimated). Figure 6: Set for chemical reactions between various mixture components. strength analysis.