*3.1. Heating Procedures*

After 21 days of curing, the samples were heated in an electrical furnace with a heating rate of 1 ◦C/min at a set temperature of 200, 400, 600, 800 and 1000 ◦C, Figure 1b. The heating rate followed RILEM recommendations [4] to minimize stress due to the temperature differences between hotter surfaces and the cooler inner parts of the sample. Concrete and mineral materials such as geopolymers are suitable for these heating conditions for evaluating the impact of temperature. Maintaining the temperature for 1 h until homogenization in the entire cross-section of the sample is required when the set temperature in the range of 200–1000 ◦C has been reached. After temperature homogenization, the samples were cooled down and their physical and mechanical properties were measured.

#### *3.2. Physical and Mechanical Properties*

The following properties of mortars were evaluated after temperature exposure.

### 3.2.1. Mechanical Properties

The compressive strength and tensile strength, tested in bending, were measured for the undamaged materials and for the materials after high-temperature exposure.

Firstly, the mortar samples were tested in a three-point bending test using the control testing machine. The loading rate applied was 50 N/s, as this is recommended in PN-EN 196-1 for testing cement mortars. Apart from the bending test, the compressive strength was evaluated according to the remaining prisms (40 × 40 × approx. 80 mm) produced by bending or three-point tests. The load was applied at a loading rate of 2400 N/s for the compressive strength, according to PN-EN 196-1.

The specimens after compressive and bending tests are presented in Figure 2.

**Figure 2.** Samples after mechanical tests: (**a**) after bending tests, (**b**) after compressive tests.
