**4. Results**

#### *4.1. Thermal Properties*

To standardize the data needed to calculate, among others, thermal parameters, the mass, volume, and density of the samples were determined (Table 2).


**Table 2.** Physical parameters of the tested samples.

Table 3 outlines the temperatures obtained depending on the cooling time. At the beginning of the measurement, the temperature between measurements was measured every few seconds, every 30 s, and then every 1 min. Using the thermal imaging camera, it was possible to implement a program for reading the temperature values on the surfaces of the samples in different places (Figure 4). To calculate the thermal capacity, the value a is used, i.e., the temperature measured in the middle of the sample.

**Figure 4.** Example thermal image of the temperature distribution on the surface of the tested sample.


**Table 3.** Temperature of the tested samples depending on the cooling time.

Figure 5 shows a diagram of temperature changes (value a) on the surfaces of the cooling samples.

**Figure 5.** Combined temperature distribution diagram on the surface of cooling samples.

Calculation of Heat Capacity

The specific heat is an additive quantity, i.e., each degree of freedom in a given system contributes to the total specific heat of the system, and, as a result, the total specific heat is the sum of the different contributions [35]. On the basis of the percentage content of individual raw materials (Table 1) and their table values of *c*<sup>v</sup> specific heat (Table 4) [36], the total specific heat of the examined materials was calculated:

$$
\mathfrak{c}\_{\mathbf{V}} = \Sigma^{\mathbf{0}} \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\/ \!\!\!\!\!\!\!\!\!\/ \!\!\!\!\!\!\!\!\!\!\!\/) \tag{5}
$$

where %*x* is the percentage of raw material in the sample [-], and *c*vx is the table-specific heat value (J/kgK).

The calculations are presented in Table 5.

**Table 4.** Specific heat of raw materials used to prepare samples.


**Table 5.** The total specific heat of the tested samples.


On the basis of the value of total specific heat Σ *c*<sup>v</sup> (Table 5), the mass of the tested samples *m* (Table 2) and Δ*T* (the difference in the temperature of a given body before and after thermal transformation; Table 6) and Δ*Q*, i.e., the change in the thermal energy of the tested materials, were calculated. The results of the calculations are presented in Table 6.


**Table 6.** The Δ*T* and Δ*Q* values of the materials tested.

Another value that characterizes the thermal properties of materials is volumetric thermal capacity b, which describes the ability of amaterial to accumulate heat. The calculated values of the volumetric thermal capacity are shown in Table 7.



When describing the thermal properties of fireplace materials, it is also important to know the thermal power of the tested materials *P*, i.e., the parameter determining the transfer of heat from the body to the system. The relevant calculations are presented in Table 8.

In the initial cooling phase (up to 10 min), all analyzed samples lost heat at a similar rate; then, the trend changed. The curves in Figure 5 and the temperature values in Table 3 indicate that sample no. 4 (with 60.477% weight of sand content) lost heat the fastest, while sample no. 5 (with 32.027% weight of sodium–calcium feldspar content) lost heat the slowest.

**Table 8.** Thermal power of the tested materials.


Beyond the different compositions of the samples, this phenomenon could also be influenced by the fact that the samples differed significantly in their density. The density of sample no. 4 was 1.50 g/cm3, while that of sample no. 5 was 2.16 g/cm3. The values of their thermal parameters (thermal power, thermal volume capacity, and thermal energy), however, were found to be similar. The highest values were recorded for sample no. 5 consisting of magnesium silicates and sodium–calcium feldspar. The weakest thermal parameters were found for sample no. 4, as well as sample no. 2, which consisted mainly of cement, sand, and refractory aggregates. This is consistent with the literature data.

For example, materials with the highest specific density always have the highest thermal capacity [37]. The thermal capacity of metals with a density of 7000–9000 kg/m3 is 1,500,000–3,500,000 J/(m3K). Casting rocks used as aggregates in concretes have an even smaller thermal capacity than metals (granite is about 1,800,000 J/(m3K)). Brick and sand have an even lower volumetric heat capacity at about 1,200,000 J/(m3K). For normal and refractory concrete with a density of about 2400 kg/m3, the volume heat capacity is about 2,770,000 J/(m3K) [38,39].
