# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 3 Results and Discussion
After an immersion period of 6 months, all the mortars experience a significant loss of mechanical properties (see Figures 13 and 14) that is explained by following reactions [102; 103]:  
* the formation of gypsum (calcium sulphate dihydrate), leading to softening and loss of concrete strength: \[\text{Ca(OH)}_{2}+\text{Na}_{2}\text{SO}_{4}\cdot 10\text{H}_{2}\text{O}= \text{CaSO}_{4}\cdot 2\text{H}_{2}\text{O}+2\text{NaOH}+8\text{H}_{2}\text{O}\] (2)
* The higher amount of gypsum converts tricalcium aluminate (C3A) in the mortars to ettrinigite (calcium aluminate trisulphate). The expansive reaction leads to high internal stresses and cracking: \[\text{3 CaO}\cdot 1\text{Al}_{2}\text{O}_{3}+26\text{H}_{2}\text{O}+3(\text{ CaSO}_{4}\cdot 2\text{H}_{2}\text{O})=\text{3 CaO}\cdot 1\text{Al}_{2}\text{O}_{3}\cdot 3 \text{CaSO}_{4}\cdot 32\text{H}_{2}\text{O}\] (3)  
Figure 13: The mortars’ compressive strength after 6 months of immersion in sodium sulphate solution. Notably, RSPX and NSPX showed a similar loss of compressive strength (Table 5), whereas RSRX mortar showed a better performance than the control mortar (NSRX). The recycled aggregate used in the present campaign is mainly composed by cementitious compounds (concretes and mortars), as well as materials of ceramic origin (brick), as evidenced by EXD and X-Ray analyses. Therefore, mortars produced with the recycled aggregate contain more Portland cement hydration products which can react with external sulfate. Furthermore, as reported in Table 4, RSPX and RSRX mortars show increased overall porosities that favor and accelerate the ESA kinetics. The obtained results, thus, cannot be explained based on only these considerations and further contributions participate to define the observed behavior. According to authors, because ESA damage is mainly due to the formation of expansive products in confined pores, greater porosity can help to reduce the internal restriction of mortar to deform and decrease the generated internal stresses. The capacity of mortar and concrete produced with recycled aggregate to relief stresses due to their high porosity has been also reported by [PERSON] et al. [104] and [PERSON] et al. [105]. Figure 15 shows the cross-section of specimens exposed to sulphate attack post bending test and after using BaCl\({}_{2}\)+KMnO\({}_{4}\) solutions [106] to measure the sulphate penetration depth (Table 5). Figure 14: The mortars’ flexural strength after 6 months of immersion in in sodium sulphate solution. Figure 15: Sulphate penetration depth in tested mortars, using BaCl\({}_{2}\) + KMnO\({}_{4}\) solutions and after 6 months of immersion in a sodium sulphate solution. Recycled aggregate mortars show greater ion penetration depth than the control ones (Table 5). This can be explained by their higher open porosity. Interestingly, self-leveling mortars show higher sulphate ion penetration depths than the more porous thixotropic mortars. An important role in the observed behaviour can be attributed to a different efficiency in the redispersible powder dispersion. During mixing, polymer particles firstly disperse in the cement paste. As cement hydration proceeds and capillary water is consumed, the polymer particles flocculate to form a close-packed layer on the surfaces of capillary pores that can also effectively block the transportation of invasion ions based on the continuity polymer network [27]. The continuity polymer network depends on the amount and dispersion of the redispersible powder. The exposure of cementitious materials to high temperatures, as it occurs during a fire, deteriorates their mechanical properties and affects their durability. The main causes of this decline can be ascribed to the decomposition of hydrates, the coarsening of porous structure, thermal cracking induced by water vapor pressure, and thermal mismatches [107; 108]. The last two mechanisms considerably depend on the type of aggregate used in mortars [109].