*3.2. Mechanical Property Recovery by Mortars Damaged at High Temperature*

Some of the heated mortar specimens were stored for 24 months in the moisture conditions (see Point 2.2) and then tested. The results of compressive strength determination for these specimens are presented in Figure 3. The highest growth of the compressive strength was observed for M3 mortar specimens within the entire range of temperatures. However, the significant recovery of the strength after 24 months of storage was noted for all tested mortars.

**Figure 3.** Compressive strength of the mortars after heating and curing for the next 24 months.

The recovery of the mechanical properties of the heated cement mortars after 24 months of storage in the moisture conditions is presented in Figure 4. The compressive strength determined after heating was accepted as the reference strength, and the strength after 24 months of storage was compared to it.

**Figure 4.** Relative compressive strength of the heated mortars after 24 months of further curing.

After 24 months of storage, all mortar specimens achieved a compressive strength higher than that obtained immediately after heating, except for the M0 mortar, which demonstrated a slight decrease in the average strength (by 1%) after heating at 800 ◦C. The positive effect of the NS's presence on the mortars' structure recovery is visible. The increase in the mortars' strength after heating at 200 ◦C was proportional to the NS content. The moisture delivered during storage in the climate chamber after heating made it possible for the NS to hydrate fully, which led to the growth of strength. The highest increase in the strength was observed for the specimens heated at 400 ◦C. The compressive strength of the mortars containing NS increased by 38% to 45%, while the compressive strength of the mortar M0 without NS increased by only 22%. The strength of the mortars with NS, heated at 600 ◦C, also rose after 24 months of storage. However, the increase was smaller (23% to 28%), except for in mortar M5, where a strength increase of 43% was noted. The phenomenon of a strength increase fades away after heating at 800 ◦C. Some compressive strength growth was observed only for the mortars with the higher NS content; it was 9% for the mortar M4 and 8% for the mortar M5. For the other mortars, the strength increased by about 5%, which is on the border of the measurement error.

The compressive strength increase in the heated cement mortars is caused by the rehydration of the C-S-H phase and hydration of other, previously not hydrated, components, such as cement grains, NS, and CaO [16,17]. The partial regaining of the compressive strength by the mortars containing the NS is possible when the volume of delivered water is sufficient for the hydration of yet unhydrated nano-silica. As shown in the study described in [18], the NS agglomerates forming in the cement matrix at the NS content above 3% are the reason for decreasing strength after heating the mortars compared to the mortars without nano-silica. The downfall of the strength is caused, among other reasons, by the air voids formed in the area of NS agglomerates. The tests presented in this paper

have shown that sufficient water delivery leads to the hydration of yet unhydrated NS and, consequently, the filling of the created air pores and voids. The filling of the empty spaces causes, in turn, the higher compressive strength increase for the mortars M4 and M5. At the temperature of 800 ◦C, the damages in the material's internal structure are too big for significant strength recovery. The authors plan to continue the investigation using more sophisticated testing methods, including scanning electron microscopy, X-ray diffractometry, and X-ray computer tomography.
