**4. Conclusions**

This study analysed the effects of a sulphuric acid exposure on the physical and mechanical properties of mortars manufactured with a crystalline admixture in the short term (up to 90 days). The acid attack had significant consequences on the microstructure and the physical and mechanical properties of mortars. According to the results obtained in this study, the following can be concluded:


This study has verified that the incorporation of crystalline admixtures improves the durability of mortars exposed to aggressive environments (sulphuric acid attack). Compared to normal mortars, crystalline admixtures added to mortars increase compressive strength and reduce mass loss. Consequently, a lower reduction of ultrasonic velocity is observed. Furthermore, incorporating crystalline admixtures in the mixing of the cementitious materials does not have the drawbacks of superficial treatments, such as the difficulty of adhesion with the substrate, the uneven thickness layer or the cost of applying the coating. Although, in the short term, the use of durable materials implies a higher initial cost without apparent benefit, in the long term, the decrease of maintenance and rehabilitation actions as well as the increase in the construction's lifecycle is more cost efficient and, importantly, reduces the negative environmental impact.

**Author Contributions:** Conceptualization, V.E.G.-V. and M.L.; Data curation, V.E.G.-V. and A.J.T.-A.; Formal analysis, V.E.G.-V. and M.L.; Funding acquisition, A.J.T.-A. and J.M.S.; Investigation, V.E.G.-V., A.J.T.-A. and J.M.S.; Methodology, V.E.G.-V. and A.J.T.-A.; Project administration, J.M.S.; Resources, A.J.T.-A. and J.M.S.; Supervision,

A.J.T.-A., J.M.S. and M.L.; Validation, M.L.; Writin—original draft, V.E.G.-V.; Writing—review & editing, V.E.G.-V., A.J.T.-A., J.M.S. and M.L.

**Funding:** This research was funded by the University of Alicante (GRE13-03) and (VIGROB-256).

**Acknowledgments:** The authors would like to thank Imrepol S.L. and Xypex España for providing the admixture used in this study. The authors also would like to thank Lidia Adeva Gil and Wanda Cecilia Candia Santos for their technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
