# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 3 Results and Discussion
Therefore, the thermal degradation characteristics of the prepared recycled aggregate mortars and the control ones were investigated by exposing the prismatic specimens, after a curing period of 90 days, up to 700 \({}^{\circ}\)C in steps of 100 \({}^{\circ}\)C in a muffle furnace. Each thermal cycle consisted of a soaking period of 12 h followed by a cooling period of 24 h. The degradation of the physical and mechanical properties was evaluated by the non-destructive ultrasonic reflection technique (Figure 16) and weight loss (Figure 17). Ultrasonic non-destructive testing (NDT-UT) techniques estimate the quality of mortar and concrete by measuring the propagation velocity of the ultrasonic wave [110]. Its magnitude depends on the material's density or the pore volume fraction; thus, it is closely related to their mechanical properties and modulus of elasticity [2]. The propagation velocities of the ultrasonic wave measured on specimens at room temperature and reported in Figure 16 are consistent with porosity and mechanical properties of the prepared mortars. In particular, a 5% difference between NSPX and RSPX and a 14% difference between NSRX and RSRX propagation pulse velocities (UPV) have been found. As the temperature increases (Figure 16), all the mortar experience a decrease in the propagation velocity as a consequence of the thermally induced damage. Four temperature intervals can be distinguished. Figure 16: Propagation velocities of ultrasonic pulse vs. temperature for the tested mortars. Interval 25-100 \({}^{\circ}\)C: the integrity of the specimen structure is close to that at room temperature. At these temperatures, the free water or capillary water inside the concrete are heated to evaporate and form water vapors. NSPX and NSRX mortars show a decrease in weight of 0.2% and 2%, respectively, which is lower than that of the corresponding mortars produced with recycled aggregate equal to 1.2% (RSPX) and 4.4% (RSRX) (Figure 17). This can be ascribed to the greater difficulty for water vapors to escape in more densely structured mortars [107]. The steeper decrease in UPV measured for RSPX (around 13%) and RSRX (9.3%) mortars can be explained by the greater ease of water vapor to escape in mortars that are more porous and of the fine-cracks to develop, under vapor pressure, from original defect of the recycled aggregate at pore boundaries. Interval 100-400 \({}^{\circ}\)C: the capillary water continues to evaporate. Resins used as additive are thermally degraded. Furthermore, several reactions occur in the cement matrix. Ettringite (AFt) is thermally decomposed [110]; the dehydration-dehydroxylation of calcium silicate hydrate (C-S-H gel) cause the loss of crystal water and an increase in structural porosity. The loss of bonded water results in an increased weight loss for all the tested mortars, larger for mortars produced with recycled aggregate (5.6% for RSPX vs. 3.5% for NRPX and 10.4% for RSRX vs. 9.8% for NRSX). This finding can be explained by the following processes:  
* The dehydration-dehydroxylation of calcium silicate hydrate (C-S-H gel) in RSPX and RSRX mortars not only involves the cement matrix, but also the recycled aggregate, mainly composed by concrete rubble. * The higher amount of water vapor produced can easily escape in the more porous structures of mortars produced with recycled aggregate. In this temperature range, a significant decrease in UPV is observed for all the mortars. However, it is steeper for mortars produced with natural sand (28.9% for RSPX vs. 36.9% for NRPX and 30.8% for RSRX vs. 37.1% for NRSX). This can be ascribed to their denser structure which hinders the migration of water vapors that are blocked and accumulated in the pores. The increased vapor pressure on pore boundaries favors the formation of fine cracks. Interval 400-500 \({}^{\circ}\)C: the reactions of dehydration-dehydroxylation of calcium silicate hydrate (C-S-H gel) continue, and most of the calcium hydroxide (CH) decomposes into calcium oxide (CaO) [111, 112].