*4.1. Mechanical Properties*

In general, a decrease in mechanical properties was observed (except for an increase observed at 200 ◦C for M0-K due to the geopolymerisation progress). For compressive strength, the reference value (at 20 ◦C) for M0-K equaled 11.16 MPa, and after exposure to 800 ◦C was 6.77 MPa, while for M50-K, the reference value was 74.4 MPa and at 800 ◦C was 11.50 MPa. A decrease of 39.33% and 84.54%, respectively, in compressive strength for the M0-K and M50-K was noted at 800 ◦C.

Above 400 ◦C, it is reported that the mechanical properties of mortar significantly decrease compared to undamaged samples. This is ascribed to the movement of physically bonded water and OH groups, phase transformation of geopolymers, the development of anhydrous products, and the sintering process [6,7]. At elevated temperatures, nonuniform recrystallization occurred, which resulted in the formation of large cracks that consequently reduced the strength [8,9].

Considering the mechanical properties of the two tested mortars, it can be highlighted that replacing 50% of FA with GGBSF resulted in an increase of over 700% in compressive strength and over 350% in tensile strength. Nevertheless, the drop in compressive strength in the M50-K was more significant at 800 ◦C, reaching the same level as the M0-K. Similarly, the tensile strength of the M50-K dropped suddenly after being exposed to 200 ◦C and continued to decrease, reaching the same value as M0-K at 800 ◦C. It seems that the geopolymer bond created by GGBSF is very sensitive to high temperature exposure and is susceptible to damage at temperatures over 200 ◦C.

The changes in compressive strength and tensile strength for the M0-K and M50-K are presented in Figure 3.

**Figure 3.** The changes of compressive and tensile strength with temperature (T) for M0-K and M50-K: (**a**) compressive strength fcT, (**b**) tensile strength, ftT.
