# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 3 Results and Discussion
Sulphoaluminate cement (CSA cement), used to provide a shorter setting time [83, 84] and to lower the overall shrinkage, reacts according the following reaction:  
\[3\text{CaO}\cdot 1\text{Al}_{2}\text{O}_{3}\cdot 6\text{H}_{2}\text{O}+3 \text{(CaSO}_{4}\cdot 2\text{H}_{2}\text{O})+19\text{H}_{2}\text{O}=3\text{CaO}\cdot 1 \text{Al}_{2}\text{O}_{3}\cdot 3\text{CaSO}_{4}\cdot 31\text{H}_{2}\text{O} \tag{1}\]  
The main hydration product of CSA cement is AFt (cf. Equation (1)). Its generation process requires a high amount of water. Metakaolin reacts with the calcium hydroxide produced by the cement hydration reaction, which already exists in both the adherent cement paste on recycled aggregate and the recycled aggregate itself, as highlighted by X-Ray diffraction analysis (Figure 4). The pozzolanic reaction produces calcium silicate hydrate, thus increasing the density of the matrix especially the transition zone area between recycled aggregates and cement mortar and filling the porosity and the cracks of recycled aggregates, as revealed by [PERSON] and [PERSON] [85] and [PERSON] et al. [66]. The use of cellulose ethers (CE) and sulphoaluminate cement in the mix design of RSRX leads to an initial water retention, followed by faster consumption which produces expansive Aft. The addition of calcium formate reduces set times and improves early strengths by accelerating C-S-H formation. It is useful in repair systems, but further contributes to the fast water consumption. As an overall consequence, the wettability of adherent cement paste on recycled aggregate is partially hindered, the micro-cracks occurring in recycled aggregate can widen, the porosity increases, and mechanical performances decline. In the chemical compound used for RSPX, the calcium formate is replaced by lithium carbonate, commonly used in the production of the self-leveling system, which only promotes fast setting. The partial replacement of sulphoaluminate cement with metakaolin leads to the reduced formation of expansive Aft and the densification of the transition zone in the recycled aggregates and between the cement paste and the recycled aggregate [85]. Therefore, the pore structure is improved, enhancing the mechanical performances, as reported in Figures 8 and 9. The porosity of the produced mortars was measured to validate the proposed explanation. Porosity is defined as the fraction of the total volume that is occupied by pores. It can be further classified into open porosity and closed porosity [86]. The difference between them is the interconnection of pores. The measure porosities are reported in Table 4.  
The higher porosity of the NSRX than NSPX can be attributed to the entrapped air due to the lower workability of the thixotropic mortar that can promote air entrapping. The RSRX and RSPX mortars show higher open and total porosities than NSRX and NRPX mortars, used as the control. Furthermore, the RSRX mortar has higher porosity than RSPX. The obtained results are consistent with the measured mortars' mechanical performances because, as expected, greater porosity is associated with lower strengths. The drying shrinkage of the prepared mortars over early curing time is reported in Figure 10. All specimens experienced a continuous increase in shrinkage during the test period, with a fairly rapid growth during the first curing days, followed by a slower increase. Compared to the control mortars NSPX and NSRX, the RSPX and RSRX mortars show a shrinkage increase of 13.5% and 23.1%, respectively, after 28 days of curing. As expected, greater porosity leads total larger drying shrinkage. Similar results are reported in scientific literature by [PERSON] et al. [87]. The frost resistance of mortars produced using recycled aggregate has been investigated in recent papers with divergent results that can be addressed to both the variability of the quality of the recycled aggregate and the different employed methods [88; 89; 90; 91; 92].