*4.2. Physical Properties*

The initial (20 ◦C) apparent densities were 1825 kg/m3 for M0-K and 2070 kg/m<sup>3</sup> for M50-K, and their true (specific) densities evaluated with a helium pycnometer were, respectively, 2490 kg/m<sup>3</sup> and 2475 kg/m3. Samples containing 50% (M50-K) of FA and Metakaolin had a higher apparent density compared to those containing 100% of FA (M0-K), as shown in Figure 4. The evaluated total porosities reflected this observation. The porosity of the M0-K was 40% and that of M50-K was 27% (Figure 5). The lower apparent density of M0-K was attributed to a more significant number of pores in the material and their larger size. This also affects the differences in the initial mechanical properties.

**Figure 4.** (**a**) Specific density ρ<sup>T</sup> and (**b**) apparent density ρoT in kg/m3 of heated samples.

**Figure 5.** (**a**) Ultrasound Pulse Velocity (UPV—VT) and (**b**) porosity (P) with temperature.

After high temperature exposure, the apparent density of the samples slightly decreased due to the drying process and changes in the matrix. Once exposed to high temperatures, the components of the samples, such as aggregate, became dry. The moisture contained in the material was gradually removed through heating. Furthermore, heating did not significantly affect the change in density. The physically bound water was entirely evaporated, and the hydroxyl groups were removed at high temperatures. Some sources showed that the dihydroxylation process begins at 250 ◦C and continues up to 600 ◦C [10], which causes the geopolymer binder to shrink.

For the M0-K, an increase in density after exposure to 200 ◦C was observed, while for the M50-K, density increased to 400 ◦C. This is related to the continuous geopolymerization process under elevated temperature; the mortar behaves similarly to an autoclaving mortar, with binder connections developing and strengthening. This was confirmed by the increase in tensile strength and applies more to a matrix without GGBSF. Another insignificant drop in density over 400 ◦C was related to the cracking of the matrix.

The density (*ρ*) and the apparent density (*ρ*<sup>0</sup> ) values enabled the total porosity (P) to be determined. The variation in total porosity P with increasing temperature is presented in Figure 5. The M0-K presents an increase in porosity between 20 ◦C and 200 ◦C. In this temperature range, the porosity remained stable (47%) from 200 ◦C up to 800 ◦C. On the other hand, for the M50-K, a slight increase in porosity from 20 ◦C (27%) to 800 ◦C (45%) was observed. The stable porosity level in the entire range of tested temperatures ensured constant thermal insulation parameters with increasing temperature, which is extremely important in cases of fire protection or fire-resistant material. Therefore, the M0-K, independently from its lower initial mechanical properties, can be developed in the direction of fire protection applications.

The UPV values decreased in the entire temperature range (20–800 ◦C), Figure 5a. However, in M0-K, a stable velocity was measured between 20 ◦C and 200 ◦C. The stable UPV up to 200 ◦C confirms the development of the M0-K matrix at elevated temperatures, as the UPC reflects the amount of air pores in the material. Moreover, the final UPV after exposure to 800 ◦C reached similar values for both the M0-K and M50-K. The same quality was confirmed for both the mechanical and physical properties of the tested materials when exposed to 800 ◦C.
