*4.4. Microstructure of Fireplace Concrete*

Scanning electron microscope(SEM) microphotographs with the energy dispersive spectroscopy (EDS)system of the tested samples are shown in Figures 9–13. Based on the conducted research, it is possible to determine the compact microstructure of all of the concrete materials, except for sample 4, which is characterized by the presence of large spherical pores (Figure 12). A compact structure was also present in the concrete materials containing glass fibers (Figure 9) or polypropylene fibers (Figures 11–13). Generally, a homogeneous microstructure was found in the analyzed concrete materials; however, in the case of sample 5, the uneven distribution of fibers was noted (Figure 13). There were also visible pores (except for sample 4), which may have resulted from inappropriate selection of the aggregate graining (filler) of the concrete material or deaeration. This is an important factor because the presence of pores lowers the density and thermal properties of the concrete material. EDS measurements confirmed the presence of cementderived aluminates and aluminosilicates, as well as the presence of fibers, aggregate grains, and pigments.

Quartz grains and zirconium were present in the microstructure of sample 1 (Figure 9). Sample 3 (Figure 11) contained increased amounts of calcium aluminosilicate and strengthening organic fibers. Sample 4 (Figure 12) is a foamed concrete featuring the addition of calcium carbonate. In the composition of sample 2 (Figure 10), increased amounts of magnesium, zirconium, and calcium can be seen. In particular, in micro-area b, dolomite crystals formed with the addition of titanium white reinforced with organic fibers were found. Concrete no. 5 (Figure 13) is a typical concrete reinforced with organic fibers and aggregate, where the presence of iron comes from the minerals of the aggregate.

**Figure 9.** SEM microphotograph and energy dispersive spectroscopy (EDS) analysis of sample 1.

**Figure 10.** SEM microphotograph and EDS analysis of sample 2.

**Figure 11.** SEM microphotograph and EDS analysis of sample 3.

**Figure 12.** SEM microphotograph and EDS analysis of sample 4.

**Figure 13.** SEM microphotograph and EDS analysis of sample 5.

From an application perspective, concrete systems can be reduced to a mixture of three components: cement, fine aggregate, and water. Cement is a composite system in which aggregate grains are surrounded by a hardened cement slurry. Concrete formation is determined by the cement's hydration reactions. Cement is a mixture of solid phases and mainly contains various types of aluminates and calcium aluminosilicates: allite C3S-(3CaO·SiO2), bellite C2S-(2CaO·SiO2), tricalcium aluminateC3A-(3CaO·Al2O3), brownmilleriteC4AF- (4CaO·Al2O3·Fe2O3), calcium oxide (free)-CaO, and gypsum-(CaSO4·0.5H2O).

Cement hydration is a multi-stage process. However, it can be assumed that the main phases of hydration are as follows:


The hydration of tricalcium aluminate, however, is more complicated. There are three basic stages in this process. In stage I, under the initially high concentration of gypsum, ettringiteis formed according to the following reaction:

$$\text{\textbullet\text{CaO}\cdot\text{Al}\_2\text{O}\_3 + \text{\textbulletCaSO}\_4\cdot2\text{H}\_2\text{O} + 2\text{6H}\_2\text{O} \rightarrow \text{\textbulletCaO}\cdot\text{Al}\_2\text{O}\_3 \cdot \text{\textbulletCaSO}\_4\cdot32\text{H}\_2\text{O}.\tag{7}$$

In stage II, when gypsum is lacking, a transitional phase is formed:

$$\text{\textbullet\text{CaO}\cdot\text{Al}\_2\text{O}\_3\cdot\text{Ca(OH)}\_2\cdot12\text{H}\_2\text{O}.\tag{8}$$

In stage III, the second stage phase reacts with ettringite, giving the low-sulfate form of aluminum sulphate according to the following reaction:

3CaO·Al2O3·3CaSO4·32H2O + 3CaO·Al2O3·Ca(OH)2·12H2O → 3CaO·Al2O3·CaSO4·12H2O + 2Ca(OH)2 + 20H2O. (9)

The hydration of braunmillerite is similar to that of tricalcium aluminate. In the presence of gypsum and water, the following structures are formed:

4CaO·Al2O3·Fe2O3 + CaSO4·2H2O + Ca(OH)2 + aq → 3CaO·(Al2O3, Fe2O3)·3CaSO4·32H2O. (10)

After braunmillerite is exhausted, the structure becomes 3CaO·(Al2O3,Fe2O3)·CaSO4·12H2O [42,43].

The hardened cement slurry is largely dominated by two phases: the CSH gel phase and the portlandite (Ca(OH)2) phase. There are also additional phases resulting from the hydration of tricalcium aluminate and braunmillerite. It should be noted that these phases contain large amounts of crystalline water, and this water is the main destructive factor for cement mortars at high temperatures, which accompanies the use of fireplaces and heating stoves. However, the content of elements such as magnesium, aluminum, calcium, and silicon significantly affects the thermal properties of the concrete [44].

#### **5. Discussion**

The addition of fibers to concrete improves the mechanical properties of mortars and may improve heat flow in concrete materials. At elevated temperatures, polypropylene melts and is absorbed by the surrounding cement matrix, creating a network of channels through which moisture and the resulting water vapor can flow [45]. Hardened cement mortar can also be treated as a fibrous composite in which the matrix is the hardened cement mortar. In the case of such a composite, the addition of a small amount of fibers, even with an elasticity modulus lower than that of the matrix, significantly improves the mechanical properties of the mortar. This is because during the hydration process of the paste components, a number of reactionsoccur, leading to the formation of mechanical stresses. The resulting stresses, which are related to internal contractions, are leveled until the structure stiffens. However, in the initial stage of hardening, the hardening paste has too low a mechanical strength to compensate for the stresses accompanying the hydration process (as confirmed by the results presented in Table 9).

The results of the mechanical strength after 28 days confirm this explanation. The highest bending strength was found for the samples with the highest amount of glass and polypropylene fibers, i.e., sample 1 (0.266 wt % of glass fibers) and sample 3 (0.209 wt % of glass fibers and 0.02 wt % of polypropylene fibers). The addition of fewer of these fibers or the addition of steel fibers (even in combination with glass fibers) caused more than a twofold decrease in mechanical strength in the described case, from 13.2 MPa for sample 3 to 4.7 MPa for sample 4.

When the proportion of organic fibers is so large that the fibers form a self-connected network of channels, it is known as a parallel transport model. That is, the transport of liquids and gases at temperatures above 400 ◦C takes place both through the network formed after melting the fibers and through the pores and cracks existing in the material. When there are fewer fibers, transport can be described by the so-called series-parallel model, where the incompletely connected network of channels is supplemented by the existing defects in the material [46,47].

The introduction of short polypropylene fibers, in addition to preventing detonation splashes during rapid heating or contact with fire, also has a very positive effect on the mechanical properties of mortars. Hardened cement mortar can be treated as a fibrous composite in which the matrix is hardened cement paste. In the case of such a composite, the addition of a small amount of fibers, even with a modulus of elasticity lower than that of the matrix, results in a significant improvement in the composite's mechanical properties. This is because several stress reactions take place during the hydration of the cement slurry components. In the subsequent period, the stresses arising from internal contraction are leveled until the structure is stiffened. However, in the initial stage of hardening, the hardening cement slurry still has too low a mechanical strength to compensate for the stresses accompanying the hydration. This yields the formation of a micro-fracture network, leading to material defects and a loss of continuity at the microscale. The addition of fibers causes bridging of the resulting cracks, which reduces their size and improves the material's resistance to brittle fractures [48].

However, the addition of short fibers to the cement matrix may regulate more than the matrix's hydration and hardening properties. Through the addition of fibers with an elasticity modulus higher than the matrix modulus, the strength properties of the material can be directly modulated. The addition of such fibers ensures that the material does not disintegrate when the cement matrix breaks, allowing the matrix to still transfer some loads.

The reactions taking place in the cement during heating can be divided into five stages:


The effect of high temperature, i.e., that above 100 ◦C, on hardened cement material is very specific and causes the rapid desorption of moisture from the outer layers. The resulting water vapor flows towards the colder inner layers, where it is reabsorbed. As the temperature increases, the thickness of the heated layer increases. When the moisturesaturated layer does not move quickly enough, it is overtaken by the wandering temperature front, which leads to the evaporation of water at the front border and an increase in internal pressure. This creates tensile forces perpendicular to the temperature front, which in turn leads to rapid removal of the surface layers in the form of microcracks and spatters. This phenomenon mayoccur cyclically, destroying increasingly deeper layers of the material.

To counteract this phenomenon, it is necessary to improve the heat flow, which will entail a reduction in the pressure inside the heated element. The best way to do this is to create an interconnected network of channels in the material through which moisture and the resulting water vapor can flow, or toimprove the material's thermal conductivity.

This can be achieved via the introduction of a fibrous material that melts at an elevated temperature and is easily absorbed by the surrounding cement matrix. Polypropylene is perfect for this purpose. This material melts at 170 ◦C, which is below the temperature at which water vapor is rapidly released and does not damage the concrete matrix. At low temperatures (<100 ◦C), all forms of water in concrete (capillary, adsorptive, and crystalline) become very good heat accumulators (the heat of water evaporation is approximately 80 kcal/mol). Exterior cladding concrete shapes used in furnaces are a good choice for this purpose.

Another type of thermal accumulation involves using the excess heat energy generated inside the fireplace or the stove to heat rooms over a longer timeframe. Such furnaces are made of natural soapstone (a greenish-gray rock containing mainly magnesium aluminosilicates), which has a very good heat accumulation capacity. In these furnaces, due to the accumulation capacity of soapstone and various technical solutions (e.g., the multiple circulation and swirling of exhaust gases), the temperature in the furnace can reach about 1200 ◦C (normally about 600 ◦C), which allows the so-called afterburning of exhaust gases (soot combustion). This results in high efficiency of the furnace and, above all, has a positive impact on the environment.

The temperatures in storage furnaces can reach 400 ◦C and higher. Ordinary concrete is rarely used as a material for furnaces due to its low resistance to such temperatures (the safe limit temperature is 450 ◦C). This is why, as a rule, ceramics that are much more resistant to high temperatures are used for this application, most often fireclay. In our case, sample 5 showed the highest thermal power (Table 8) (19.70 W), whereas sample 2 showed the lowestthermal power (8.55 W). In addition to the significant effect of density (sample 5—2.16 g/cm3; sample 2—1.76 g/m3), composition itself plays an important role. Sample 2 consisted mainly of cement, sand, and refractory aggregate, whereas sample 5 consisted mainly of magnesium silicates and sodium–calcium feldspar. Therefore, it is worth analyzing, in each case, the physicochemical processes thatoccur during the formation of hydration materials and their destruction under the influence of high temperatures.

#### **6. Conclusions**

Concrete elements can be successfully used in the construction of domestic fireplaces, thus taking advantage of the good heat accumulation properties of these elements. Both thermal and strength properties can be modified significantly via the addition of various aggregates as well as propylene and steel fibers. This modification consists mainly in changing the mechanisms of the heat flow and the decomposition products of concrete elements during heating—mainly water, although the density of the materials also has a significant impact.

From the perspective of thermal energy use, our study showed the clear advantage of concrete materials containing magnesium and sodium–calcium aggregates (feldspars). The highest values were recorded for sample 5 (which contained over 70% of the listed components). In turn, from the perspective of mechanical properties, samples 1 and 3 showed the highest mechanical strength and were reinforced with glass and polypropylene fibers, respectively. The results, however, depend on the applied temperature range. Therefore, in the case of fireplaces, it is suggested to use different multi-layer systems of concrete materials to enable the long-term heating of rooms via heat accumulation.

**Author Contributions:** Conceptualization, A.S., J.M.-P. and P.I.; methodology, J.M.-P., Ł.W. and T.G.; software, A.S.; validation, A.S., P.I. and J.M.-P.; formal analysis, J.M.-P., Ł.W. and T.G.; investigation, A.S. and J.M.-P.; writing—original draft preparation, A.S.; writing—review and editing, J.M.-P.; visualization, A.S.; supervision, M.K. and P.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the agreement with Northstar Poland.

**Acknowledgments:** We would like to thank Northstar Poland for its cooperation and for providing research materials.

**Conflicts of Interest:** The authors declare no conflict of interest.
