*4.2. Differential Thermal Analysis*

The thermal curve differential thermal analysis/thermal gravimetric analysis (DTA/TG), differential scanning calorimetry (DSC) analysis, and total ion current (TIC) gas emission analysis are presented in Figures 6–8. Materials containing organic fibers were selected for these analyses (Table 1). Based on the performed tests, it can be concluded that the emission of the detected gases is related to the gases supplied to the sample during the sample's heating, i.e., O2(32) and N2(28), as well as Ar(39), introduced as an inert gas to protect the microbalance. The emission of water was observed in all of the samples, and the amount of water changed successively during the heating of the samples. The highest amount of water was observed at around 140 ◦C, and the lowest amount, related to the dehydroxylation of the cement products, was observed at 480 ◦C. In the tested samples, weight loss in the temperature range of 30–600 ◦C was also observed. The complex endothermic effect observed in the temperature range from about 50 to about 350 ◦C is related to the dehydration of silicates of type C–S–H, hydrated calcium aluminates and aluminosulphates, and the decomposition of gypsum. Another endothermic effect in the temperature range from about 400 to about 410 ◦C is attributed to C2SH decomposition (2CaO·SiO2·H2O). The endothermic effect in the temperature range from about 490 to about 510 ◦C is attributed to Ca(OH)2dehydroxylation. The total weight loss of the samples varied and was 10.2% for sample 2, almost 7% for sample 3, and almost 9% for sample 5. This is due to differences in the composition of the individual materials.

**Figure 6.** Thermal analysis differential thermal analysis/differential scanning calorimetry (DTA/DSC) of sample 2.

The observed mass loss is different in particular types of concretes. These differences most likely result from different aging times, i.e., the time from concrete preparation to measurement (different humidity values), and different amounts and types of aggregates. In general, it can be concluded that the addition of modifiers causes an increase in the temperature of the cycle in which the DTA/DSC analysis takes place and causes an increase in the weight loss. Organic additives likely facilitate the emission of water at about 140 ◦C.

**Figure 7.** Thermal analysis DTA/DSC of sample 3.

**Figure 8.** Thermal analysis DTA/DSC of sample 5.

The tests showed that the specific heat of the concrete material strongly depends on the amount of free water that is released in the temperature range of 50–90 ◦C. Values of the specific heat measured by this method significantly drop at higher temperatures, which is likely related to the absorption of heat during the dehydration reactions of concrete materials.

#### *4.3. Mechanical Properties*

Table 9 shows the maximum values of the mechanical strength under bending for the tested fireplace materials after the different aging periods.

Samples 1 and 3 showed the highest mechanical strength at more than twice that of the other tested materials. Their bending strength after 28 days of aging was 12.3 and 13.2 MPa, respectively. The main additives in both samples were cement and aggregate. As a reinforcing additive, sample 1 included glass fibers, while sample 3 used glass and polypropylene fibers. Samples 2, 4, and 5 included a minor addition of steel fibers, which are also used as a strengthening additive to maintain the concrete's structure during operations at high temperatures, causing significant stress in the system. Sample 5, despite the addition of reinforcing fibers, was characterized by low mechanical strength (4.8 MPa) due to the addition of a mafic aggregate containing more than 2% free biotite. Increased mica content causes a significant and constant decrease in compressive strength [40], which is especially important in the case of the fine aggregates (e.g., mafic sand) [41].


**Table 9.** Values of the mechanical strength under bending of the tested fireplace materials.
