# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 3 Results and Discussion
In order to further contribute to scientific literature, the prepared mortars were tested to freezing and thawing resistance. Mechanical properties were measured after 300 freeze-thaw (F-T) cycles, using the properties of mortars after 90 days of curing, corresponding with Figures 11 and 12. The freezing and thawing resistance of RSPX is similar to that of the control mortar. The greater porosity that arises from the incorporation of the recycled aggregate favors the dissipation of hydraulic pressures. Only when the voids system is so developed that a significant decline in the initial mechanical properties can be determined, as for RSRX mortar, a slightly reduced resistance to F-T cycles is observed. The present investigation outcome confirms some previously reported findings [92; 93; 94; 95; 96; 97]. A sulphate attack is also a major concern for the durability of construction. Sulphate-rich environments expose mortar and concrete to expansion and degradation due to the formation of ettrinigite. It causes high internal stresses, leading to an extended cracking pattern [98; 99]. Figure 11: The mortars’ compressive strength after 300 F-T cycles. Figure 12: The mortars’ flexural strength after 300 F-T cycles. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixes**} & \multicolumn{3}{c}{**Sulphate Attack (6 Months)**} & \multicolumn{3}{c}{**Freeze–Thaw Cycles (300 Cycles)**} \\ \cline{2-9}  & \(\Delta\) **Weight** & **\(\Delta\) **Flexural** & **\(\Delta\) **Compressive** & **Penetration** & **\(\Delta\) **Weight** & **\(\Delta\) **Flexural** & **\(\Delta\) **Compressive** \\  & **(\%)** & **Strength (\%)** & **Strength (\%)** & **[cm]** & **(\%)** & **Strength (\%)** & **Strength (\%)** \\ \hline NSPX & \(-16.9\%\) & \(-63.0\%\) & \(-80.6\%\) & \(1.5\) & \(-1.0\%\) & \(-9.6\%\) & \(-10.0\%\) \\ RSPX & \(-15.8\%\) & \(-55.1\%\) & \(-79.3\%\) & \(1.7\) & \(-1.0\%\) & \(-11.6\%\) & \(-10.4\%\) \\ NSRX & \(-12.5\%\) & \(-69.4\%\) & \(-86.1\%\) & \(1.1\) & \(-1.7\%\) & \(-9.7\%\) & \(-9.8\%\) \\ RSRX & \(-11.7\%\) & \(-61.8\%\) & \(-77.1\%\) & \(1.2\) & \(-1.2\%\) & \(-14.0\%\) & \(-18.1\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The weight and mechanical properties variation after freeze–thaw cycles and sulphate attack. The literature regarding recycled aggregate mortar and concrete performance against external sulphate attack (ESA) is not conclusive. This is mainly due to the several parameters involved in ESA. [PERSON] et al. [30] replaced the coarse fraction of concrete with recycled aggregate and tested specimens after 60 days of immersion in sulfate solution. They reported a lower resistance of recycled aggregate concrete (RSC) compared to the control one in light of the sulphate attack, causing strength losses of around 26% and 20%, respectively. They attributed the findings to the higher porosity and water absorption of RSC, leading to higher sulphate ion penetration. However, some studies present a similar or even better performance for RSC than for natural aggregate mortar and concrete (NSC). [PERSON] et al. [100] reported that the exposure to a sulphate solution has a negative effect only for RSC with replacement ratios higher than 30%. [PERSON] et al. [101] reported that RSC with 100% replacement shows worse performance than NSC against ESA only when a high water-cement ratio (w/c = 0.55) and a non-sulphate-resistant cement were used. Since the scientific community has reached no consensus by now, the prepared mortars have been tested for sulphate attack.