**4. Conclusions**

The presented research aimed to evaluate the nano-silica (NS) influence on the strength recovery of the previously heated cement mortars. The tests have shown that NS fosters compressive strength recovery by the mortars heated up to 600 ◦C. The composites regain more than 40% of the compressive strength after heating at 400 ◦C and about 25% after heating at 600 ◦C. The main reason for the strength recovery is the rehydration of the C-S-H phase and the hydration of other, previously not hydrated, components. Additionally, the hydration of yet unhydrated nano-silica contributes to regaining the compressive strength when the volume of delivered water is sufficient. The strength recovery phenomenon fades away at 800 ◦C when the damages in the material's internal structure are too big for significant strength recovery.

**Author Contributions:** Conceptualization, E.H. and P.Ł.; methodology, E.H. and P.Ł.; formal analysis, E.H.; investigation, E.H.; writing—original draft preparation, E.H.; writing—review and editing, P.Ł. All authors have read and agreed to the published version of the manuscript.

**Funding:** The publication cost of this paper was covered with funds from the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**


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