## 3 Results
### Fire Resistance  
The results of fire resistance tests reveal a worrying trend in the incorporation of washing fines in mortars. All formulations containing washing fines (L10, L20, L30) showed a significant reduction in flexural (Table 9) and compressive strength (Table 10) compared with the reference formulation (Ref) at all temperatures tested (200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 900 \({}^{\circ}\)C). This drop in performance became more pronounced as the rate of substitution of cement by washing fines increased. At 900 \({}^{\circ}\)C, all formulations, including the reference, show a drastic loss of strength, which is consistent with the typical behavior of cementitious materials at high temperatures. However, the incorporation of wash fines seems to exacerbate this loss of strength.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Formulation**} & \multicolumn{3}{c}{**Compressive Strength [MPa]**} \\ \cline{2-4}  & **200 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **900 \({}^{\circ}\)C** \\ \hline Ref & 27.7 & 25.9 & 4.4 \\ L10 & 18.7 & 16.9 & 3.6 \\ L20 & 15.6 & 10.6 & 2.5 \\ L30 & 6.2 & 4.9 & 1.3 \\ \hline \hline \end{tabular}
\end{table}
Table 10: Compressive strength of mortar after exposure to fire.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Formulation**} & \multicolumn{3}{c}{**Compressive Strength**} \\ \cline{2-3}  & **14 Days [MPa]** & **28 Days [MPa]** \\ \hline Ref & 47 \(\pm\) 1.8 & 51.1 \(\pm\) 1.2 \\ L10 & 33.3 \(\pm\) 1.1 & 36.1 \(\pm\) 1.2 \\ L20 & 26.9 \(\pm\) 1.3 & 29.3 \(\pm\) 1.0 \\ L30 & 17.9 \(\pm\) 1.2 & 21.5 \(\pm\) 1.3 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Compressive strength of mortar incorporating washing fines.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Formulation**} & \multicolumn{3}{c}{**Flexural Strength [MPa]**} \\ \cline{2-3}  & **200 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **900 \({}^{\circ}\)C** \\ \hline Ref & 5.8 & 4.8 & 0.6 \\ L10 & 5.1 & 3.3 & 0.4 \\ L20 & 3.7 & 2.7 & 0.3 \\ L30 & 1.1 & 0.9 & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Flexural strength of mortar incorporating washing fines after exposure to fire.  
This heat-induced degradation of mechanical properties could be explained by several factors. Firstly, the increased porosity of mortars containing washing fines, as observed in the porosity results, could facilitate heat penetration and accelerate the degradation of the material's internal structure. Secondly, the partial substitution of cement by washing fines could reduce the amount of cement hydrates formed, which are primarily responsible for the mortar's mechanical strength. Finally, potential differences in thermal behavior between washing fines and Portland cement could induce internal stresses and microcracks when exposed to high temperatures, further weakening the mortar's structure.  
These results underline the importance of considering fire resistance when using washing fines as a partial cement substitute, particularly in applications where fire safety is paramount. Although the incorporation of washing fines offers environmental and economic benefits, as demonstrated in the previous sections, their negative impact on fire resistance could limit their use in certain contexts. However, it is important to note that the results obtained are preliminary and require more detailed analysis and research to determine the feasibility and effectiveness of these mortars in practical applications. Further studies will be required to comprehensively assess the fire performance of these materials, taking into account various parameters such as processing conditions and variations in composition.