# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 3 Results and Discussion  
In Figure 7, the workability of the prepared mortars is reported. Both self-leveling and thixotropic mortars produced with RS show only a moderate decline in initial workability with respect to control mortars. It can be also observed that the decline in spreading value is lower in the case of the thixotropic mortar. As reported by many authors, several factors affect the workability of these types of mortars. A significant role is played by the RS content, the degree of saturation, the kinetics of water absorption, and the surface texture [53; 54]. The chemical kit also has a pivotal role in defining their fresh state properties. When dry surface saturated aggregate is used, as in the present investigation, the more irregular and rougher surface of RS offers a higher resistance to the inter-particle movements, promotes particles' interlocking, and requires more water to be wetted [75]. The presence of adherent cement paste on aggregate, due to its porosity, also affects the workability [67]. The resin and surfactant included in the chemical kit of both the types of mortars limited the loss of workability by a ball bearing action and a dispersing effect, respectively. The higher amount of cellulose ethers (CE) in the chemical compound of the thixotropic mortars, used as viscosity enhancing and water retention additive to prevent the rapid loss of water by suction, hinders the wettability of the surface of recycled aggregate, thus lowering the gap in the initial mortar workability between NSRX and RSRX  
Figure 7: Spreading of thixotropic and self-leveling mortars produced with natural and recycled aggregate. mortars. The chemical kit used for self-leveling mortars, characterized by a higher amount of surfactant and a lower amount of cellulose, enhances the wettability of recycled sand surface, thus decreasing the initial workability in the RSPX mortar. In Figures 8 and 9, the evolution of compressive and flexural strengths of the prepared mortars with curing time is reported. The compressive and flexural strengths of thixotropic (RSRX) and self-leveling (RSPX) mortars are lower than those of the corresponding control mortars made with siliceous sand. However, RSRX and RSPX show different behaviors, in terms of the extent of compressive strength decrease. The thixotropic RSRX mortar shows a compressive strength 15% lower than the reference mortar after two days of curing. The gap increases to around 40% after seven curing days and then remains stable for a longer curing time. The self-leveling RSPX mortar shows a limited compressive strength gap compared to the control mortar, varying from 11% to 6% with curing time. As reported by several investigations [76, 77, 78, 79, 80],  
Figure 8: Compressive strength vs. curing days. Figure 9: Flexural strength vs. curing days. the reduction in mechanical properties in mortars produced using recycled aggregate can be caused by:  
* the poorer mechanical properties of recycled sand in comparison to the natural sand used;
* the higher porosity of recycled sand;
* the presence of adherent cement paste on aggregate;
* the moisture conditions of aggregate;
* a weaker interfacial transition zone (ITZ). As evidenced in Figures 1A and 2, the recycled sand, mainly composed by particles of mortar and concrete, shows micro-cracks occurring at the weaker interfacial transition zone (ITZ) because of the crushing process. They modestly participate, through the so-called \"ceiling effect\" [56], to lower the strength of the RSRX and RSPX mortars, in comparison to those with natural aggregates [81, 82]. The chemical kit has an important role in explaining the different behavior of the two type of mortars with recycled sand. It contains sulphoaluminate cement in the case of the thixotropic mortars, which has been partially replaced by metakaolin in the mix used to produce the self-leveling mortars.