Mineral composition
Roofing slate samples from Moravian Formation are relatively uniform in macroscopic terms showing a grey colour, a greasy touch and slaty cleavage.
Samples of slates coming from Flysch Carpathians region are more varied. Sample VEG1, representing Vendrynĕ Formation, is grey calcareous clayey siltstone. Sample TRG1, also from Vendrynĕ Formation, is grey calcareous silty claystone. Sample OSTR1 of Hradiště Formation represents black non-calcareous claystone. Sample KUN2 from Veřovice Formation is dark grey non-calcareous shale with trace fossils, and iron oxide stains. All of these flysch samples are of very good shale cleavage.
As it can be noted in Table 2, rocks from Moravian formation have relatively high quartz content (26.42 to 49.11%). The proportion between rigid and elastic minerals determines the slate’s hardness, with respect to mining, production and finishing of the roof [2]. When taking into account content of rigid minerals, such as quartz and feldspar, and that of elastic minerals—micas, it should be noted that the Moravian formation rocks belong to medium-hard and hard slates. It should be noted also, that slates belonging to this group usually contain some carbonaceous matter (in form of graphite) which gives dark grey colour to these rocks. This matter, however, could not be detected by XRD measurement.
Table 2 Mineral composition (XRD [mass%]) of the analysed samples The rocks coming from Flysch Carpathians region are more differentiated in terms of mineral composition. In case of samples VEG1 and TRG1 the dominant minerals are carbonates (calcite and dolomite—altogether 69.79–72.82%), whereas in case of samples OSTR1 and KUN2 the most important component is quartz (57.87–70.82%). In both cases rigid minerals prevail; hence, these rocks are expected to be hard.
Thermal properties
The used device for thermal conductivity measurement (TCi) is very convenient measuring tool, which was also documented by Cha et al. [15], who used a range of thermal conductivity meters for building materials.
The obtained results of the thermal properties are presented in Table 3 and Fig. 4. Thermal conductivity of the Moravian slates in the direction perpendicular to the bedding (k┴) ranges from 1.43 to 1.79 W m−1 K−1. This parameter measured in the direction parallel to the rock bedding (kII) is in range of 3.66 to 3.92 W m−1 K−1. In case of samples from Carpathian region k┴ ranges from 1.99 to 3.15 W m−1 K−1, whereas kII is in range of 2.69 to 3.40 W m−1 K−1.
Table 3 Thermal parameters of examined slates measured at room temperature Roofing slates are described in the literature as rocks of very low thermal and electrical conductivity, and of relatively high resistance to temperature changes. The obtained values of thermal conductivity for the examined rocks are, however, not within the range given for shale rock by Blackwell and Steele [18], i.e. 1.05–1.45 W m−1 K−1. The reason for this discrepancy is probably the relatively high quartz content in the samples tested. When analysing the obtained results, it is visible that the higher values of thermal conductivity correlate with the higher content of quartz in the sample. Unfortunately, the XRD analysis does not provide information on the content of organic matter in rocks, which would additionally facilitate interpretation. The presence of organic matter usually contributes to lowering the thermal conductivity of rocks [19].
The difference between values of kII and k┴ is expressed by anisotropy coefficient (kII/k┴). In most cases of Moravian slates, the anisotropy value is over 2, which means that kII is more than twice the value of k┴. In case of Carpathian shales, this coefficient is significantly lower (mostly 1.1), which means that the slate separation in the second group of rocks is much smaller and has practically no effect on the thermal parameters. The anisotropy coefficient of thermal conductivity for the Moravian slate samples ranges from 2.1 to 2.6 (Table 2, Fig. 5). This is a relatively high value, resulting from the presence of minerals with a foliated (lamellar) habit (muscovite, chlorites) in the studied rocks. However, due to the use of roofing slates, the most important parameter is thermal conductivity (and derivative thermal parameters) measured in a plane perpendicular to the surface of separation.
Effusivity measured along the bedding is between 2319.8 and 2877.5 W s0.5 m−2 K−1, while in the range of 1674.3–2535.1 W s0.5 m−2 K−1 in perpendicular direction. Thermal diffusivity values are differentiated for the two regarded rock groups; they are generally higher for Moravian slates. In this group in the direction parallel to the bedding thermal diffusivity values fall between 16.5 × 10−7 and 18.6 × 10−7 m2 s−1 while in perpendicular direction range between 7.3 × 10−7 and 9.2 × 10−7 m2 s−1. In case of Carpathian flysh rocks thermal diffusivity values in direction parallel to the bedding fall between 13.4 × 10−7 and 16.4 × 10−7 m2 s−1, while in perpendicular direction range between 1.0 × 10−7 and 1.5 × 10−7 m2 s−1. The above comparison shows that heat moves more rapidly in direction parallel to bedding, in both groups, which is not surprising phenomenon. Moreover, it is visible that the rock material which reacts quicker to a change in temperature is Moravian slate.
The thermal conductivity values measured for directions perpendicular to the separation surface under variable temperature conditions are presented in Fig. 6. The lowest value was obtained for sample CER5 under temperature 17 °C (1.03 W m−1 K−1); and the highest for MOK1 and MOK2 under temperature 86 °C (3.42 and 3.39 W m−1 K−1, respectively), and for sample VEG1 under temperature 100 °C (3.69 W m−1 K−1).
Generally, the measurements of thermal conductivity at temperatures different from room temperature indicate a clear increase in this parameter with the temperature of the sample (Fig. 6). The calculated growth gradient of thermal conductivity ranges from 0.014 to 0.526 W m−1 K−1 per 10 °C.
The strongest growth was recorded for sample LHO4. The stronger increase in the conductivity value along with the temperature is recorded for the Moravian slates, while weaker for the Carpathian flysch rocks. The reason is the texture of these rocks, showing weaker shale separation, due to the genesis of these rocks, as it was already explained in the introductory part of this paper. As it is also noted in the literature, the upward tendency is characteristic for metamorphic rocks, in contrast to sedimentary and igneous rocks [20]. In practice, this means that roofs made of slate have better insulating properties in conditions of lower temperatures, while they are less insulated in conditions of strong sunlight.