**3. Results**

Table 2 shows the different rate of calcium diffusion through the samples. The intensity of calcium diffusion was higher for the Vysoké Mýto sample and the diffusion of calcium through this sample began during the first 24 h of the experiment. Calcium diffusion through the Holešov-Žopy sample began one day later and the diffusion was less intensive. The diffusion took place faster on the surface (0–1 cm) than in the core of the bricks (2–3 cm).

**Table 2.** Diffusion experiment with the Vysoké Mýto and Holešov-Žopy bricks. The results are expressed as a cumulation of calcium in distilled water over the time period.


The photographs from the biological experiment (Figures 5–8) confirmed that both types of bricks and all their investigated layers were very intensively covered by microorganisms The composition of biofilm was observed under a microscope (Figures 6 and 8). Both samples were covered by a mix of algae and cyanobacterial species. The density of the biofilm was expressed as an absorbance (see Table 3). The absorbance values were the highest for the surface layer of bricks (0–1 cm). The highest difference was found for the 2–3 cm layer where the two kinds of brick should be considered.

**Figure 5.** Samples of the Vysoké Mýto bricks covered by a biofilm layer of 0–1 cm, 1–2 cm and 2–3 cm (from left to right).

**Figure 6.** Biofilm on the surface of the Vysoké Mýto bricks. A photo was taken under the light microscope Olympus BX43 with a CMOS camera (magnification 400×).

**Figure 7.** Samples of the Holešov-Žopy bricks covered by a biofilm layer of 0–1 cm, 1–2 cm and 2–3 cm (from left to right).

**Figure 8.** Biofilm on the surface of the Holešov-Žopy bricks. The photo was taken under the light microscope Olympus BX43 with a CMOS camera (magnification 400×).

**Table 3.** Biological experiment with the Vysoké Mýto and Holešov-Žopy bricks. Biofilm density on the surface of samples expressed by absorbance values.


### **4. Discussion**

Calcium diffusion through bricks has never been studied, with the exception of a study where calcium was used to detect the age of bricks [19]. For this reason, we are not able to compare our results with the data from the literature. However, calcium diffusion was studied for sandstone [6,20]. Calcium diffusion for the Holešov-Žopy sample was relatively similar to calcium diffusion through the Mšené sandstone, but the Vysoké Mýto sample caused higher diffusion than the sandstone by about one-fifth during the same time period. In the case of sandstone, in the sample from the Hoˇrice locality, calcium diffusion was about one order of magnitude lower. These discrepancies are affected by the composition of the studied building materials and their physical–chemical properties. Moreover, the sandstones by themselves are a highly variable group of rocks with a wide range of physical properties.

The microstructure of brick samples may be described by several metrices, which are obviously interrelated. The highest and positive correlation between the increasing intensity of diffusion and an increasing specific surface area was observed (Table 1). It may indicate that the diffusion of chlorides is taking place by a surface diffusion mechanism where chloride ions are adsorbed on the brick surface. The Vysoké Mýto brick has lower differences in percentage composition between individual layers than the Holešov-Žopy brick; the total porosity of the Vysoké Mýto sample was from 40 to 43% of all the studied layers while the Holešov-Žopy sample had a total porosity of 42% (0–1 cm), 49% (1–2 cm) and 37% (2–3 cm). This fact could have significance not only for diffusion but also for biofilm density. The Holešov-Žopy brick also has a greater amount of the highest pores (10–100 μm).

Bio-colonization of buildings is affected by moisture and nutrient availability, favorable pH, essential and trace metal availability and suitable solar radiation, e.g., Ref. [20]. Biodegradation of bricks was studied many times [21] but species diversity and abundance

have never been studied in depth among the individual layers in the bricks. In the present study, the bricks' layers were completely immersed in water to accelerate the growth of the biofilm and it was clearly visible both on the surface of the bricks and in the surrounding water. The biofilm coatings were photographed, and their analysis shows the presence of cyanobacteria and green algae. At least two species were observed microscopically in both types of bricks and their layers (Figures 6 and 8). These conclusions seem to be logical, and it likely cannot be assumed that more species of these organisms would be present in the depths.

The samples contained unicellular and multicellular organisms and it is therefore not possible to clearly determine the number of cells in the solution. The density of the organisms was indirectly expressed spectrometrically, but we did not recalculate the absorbance values on cell density for any volume unit. The density of the biofilm was once again higher in the samples with a higher visual porosity (0–1 cm layers) and for the Vysoké Mýto sample than for the Holešov-Žopy sample. Such broken surfaces and crevices probably allow algae to colonize the surface of building materials more intensively. The intensity of bio-colonization is more- or less-increasing with the rate of ions diffusion in the material as well as with the specific surface area. It again indicates that colonies are partially controlled by the available surface of the material.
