*2.1. Materials*

The mortars to be tested were prepared with the use of Portland cement CEM I 42.5N, a magnetite aggregate with a grain size up to 2 mm and apparent density of 4.77 g/cm3, and the water dispersion of nano-silica. The water dispersion of NS with the density of 1.4 g/cm3 contained 50% of the colloidal NS by volume. The diameters of the NS particles were from 40 to 140 nm. The TEM images of NS particles are presented in Figure 1.

**Figure 1.** TEM images of nano-silica (NS) [14].

### *2.2. Samples and Their Storage*

The tests were carried out on the cement mortars whose compositions are presented in Table 1. The water content was reduced in mortars containing nano-silica in the form of dispersion. Specimens of the size 40 mm × 40 mm × 160 mm were prepared according to the European Standard PN-EN 196-1 [15].


**Table 1.** Composition of the tested mortars.

The demoulded specimens were stored in a climate chamber on the grate above water at 20 ± 2 ◦C. After 28 days of curing, the specimens were heated. Then, part of the samples were tested, and the others were stored in the chamber for 24 months until further testing. In total, sixty specimens of the tested mortars were conditioned for two years, from which two specimens of each mortar were initially heated at the given temperature.

#### *2.3. Test Methods*

After 28 days of curing, the specimens were heated in the medium-temperature oven. In the first stage, the temperature in the oven rose at the rate of 1 ◦C/min until reaching the specified value. The specimens were heated at 200, 400, 600, and 800 ◦C. The final heating temperature was maintained for 60 min so that the entire volume of the specimens

could achieve the proper uniform temperature. Then, the specimens were cooled down at the rate of 1 ◦C/min. The cooled samples were removed from the oven and weighed. The compressive strength of the remaining specimens was determined after 24 months of storage in the climate chamber. The compressive strength was determined according to the European Standard PN-EN 196-1 [15].

#### **3. Results and Discussion**

#### *3.1. Compressive Strength after Heating*

The results of the compressive strength testing after heating are presented in Figure 2. The positive effect of the NS admixture becomes visible with heating at an increasing temperature. The highest compressive strength within the tested temperature range was determined for M3 mortar specimens.

**Figure 2.** Compressive strength of the heated mortars.
