Microorganisms in Red Ceramic Building Materials—A Review

Abstract

Ceramic materials have a very long tradition of use in construction. Their durability is related to the surface of the material and the action of the corrosive environment. One of the corrosive factors acting on ceramic materials is microorganisms. They can contribute to the deterioration of the technical and performance properties of building materials. Aesthetic, physical, and chemical deterioration are considered to be the main destructive processes in ceramic materials. This work shows how the different types of the most commonly used ceramic materials, i.e., brick and tiles, are damaged. Each of these types is susceptible to microbial growth. Most microorganisms that occur on ceramic materials produce staining substances and thus form coloured biofilms. The direct action of metabolic products secreted by organisms on inorganic substrates is the main cause of chemical biodeterioration. Therefore, this work presents the impact of microorganisms on ceramic building materials.

1. Introduction

Building ceramic materials, especially brick, have been used for more than 8000 years. Due to their long history of use in construction, they are proven materials that are still used in many countries in Europe and worldwide [1]. There are many types of ceramic building materials that differ in structure (porosity), composition, application, properties, shape, and dimensions [2]. With regard to application, ceramic products can be divided into three categories [3], which are shown in Figure 1. They are used both externally and internally. They are used both outside and inside the building. They are then exposed to different environments. Considering their long history, these are also objects, buildings, and finishing elements that are part of the so-called national heritage.

Analytical procedures to analyse the biodiversity and to assess the degree of infestation of ceramic materials are not included in this review. Such procedures will be discussed in another article. Normally, the coating material should be protective for other building elements and is therefore presented as a solution to the problem of corrosion damage. However, in the case of microorganisms, it is their activity that produces a coating, a biofilm, which causes the material to deteriorate. Therefore, the aim of this article is to review the microbial corrosion of structural ceramic building materials based on their composition and taking into account the types of microorganisms and the factors influencing the development of corrosion.

The durability of ceramic objects and elements, whether in historic or modern buildings, depends largely on the quality of the ceramic materials used and the mortars that bind them. Environmental conditions and aggressive factors also add to this [4]. Particularly important are, among others, snow and rainfall; temperature and humidity differences; wind; and solar radiation [5]. Depending on the intensity of the factors acting and the quality of the materials used, different material or construction reactions may occur. The most important causes behind damage to ceramic objects are shown in Figure 2. The most common impurities in ceramic materials are sulphates, i.e., dissolved salts and carbonate compounds. Dissolved sulphates migrate towards the surface of the partition and crystallise. The consequence is the appearance of deposits and efflorescence [6]. Calcium and magnesium carbonate, on the other hand, increase in volume when exposed to moisture and cause cracking, peeling, or spalling of materials.

For ceramic masonry in contact with corroded timber structures, both the ceramic material and the mortar in the joint may corrode. The external masonry in the lower parts of the walls is directly exposed to the weather. This is particularly true of low temperatures. Corroded mortar causes settlement of the masonry and leads to a change in the stress distribution in the masonry [7]. In the case of masonry or individually applied ceramic elements, contamination of ceramic materials by microorganisms, i.e., fungi, bacteria, or algae, can occur as a result of favourable conditions such as substrate and air humidity as well as temperature. These organisms are small in size and so bacteria are single-celled microorganisms between 0.1 and a few micrometres in size, which take the food they need to grow. They reproduce much faster than other living organisms [5]. Fungi are a group of eukaryotic organisms that are unable to assimilate CO₂ from the air. Consequently, they are forced to feed on ready nutrients in the form of cellulose, lignin, sugars, and proteins, i.e., they are able to derive nutrients from all plant and animal organisms, both living and dead [7]. The fungal organism consists of mycelium, consisting of thin, single, filaments, colourless or coloured. They are arranged in more or less loose or tangled elements. They are usually 3–10 μm in diameter [8]. In the construction industry, the most common are domestic and filamentous fungi [9]. Basidiomycetes are most often found in buildings. The second group is made up of moulds belonging to the classes Zygomycetes, Ascomycetes, and Deuteromycetes [10]. Algae are a separate, large phylogenetically diverse kingdom. It is a morphologically and ecologically distinct group that is not a systematic unit. It forms groups of self-living, eukaryotic and prokaryotic plant-like organisms, sometimes unrelated to each other. Algae are green, autotrophic organisms that show the ability to synthesise organic compounds due to their chlorophyll content [11].

Lichens are organisms that are formed from a combination of (Figure 3):

(a)

a resident fungus;, i.e., a mycobiont;

(b)

an extracellular arrangement of one or more photosynthetic partners, i.e., green algae and/or photobionts;

(c)

an unspecified number of other microscopic organisms [12], i.e., the lichen microbiome, comprising mainly bacteria and additional fungi.

Figure 3. Mechanisms of biodeterioration of natural rocks.

Figure 3. Mechanisms of biodeterioration of natural rocks.

Genotypes of all these components may be present in a single symbiotic mould. However, it appears as a visually distinct structured individual, which is the symbiotic phenotype of the original fungal member. It gives its name to the whole conglomerate [13]. The body of lichens is abundant. In most species, the fungus is responsible for the shape of the lichen “body”. The alga, as a self-fertilising organism, produces food and nourishes the entire mould. The fungi provide water and mineral salts, as well as some organic compounds, and they protect the algae and attach them to the substrate. In return, the fungi feed on the products of photosynthesis, i.e., organic substances produced by the algae.

Microorganisms do not occur as pure cultures of individual cells but accumulate to form “aggregates” called biofilms. In most biofilms, microorganisms make up less than 10% of the dry weight, while the matrix can make up more than 90%. The matrix is an extracellular material, produced primarily by the organisms themselves, in which the biofilm cells are embedded [14]. The biofilm is formed by complex, multicellular structures in which numerous microbial cells are surrounded by a layer of mucus [15]. Each biofilm is unique, although some structural elements are reproducible. It is formed by monolayer or multilayer growth of microorganisms. They are arranged in ordered, complex structures that consist of microcolonies [16]. A biofilm is a complex ecosystem that allows the organisms living in it to thrive. It protects against changing environmental conditions. It allows easier access to nutrients and expansion to further surfaces. Biofilm adheres to the surface of solids in aqueous environments, as well as in moist air, on the surface of tissues of other organisms and at the liquid–gas interface [17].

Colonization by microorganisms and the formation of biofilm as well as the release of metabolic products result in the biodegradation of materials [18], including ceramic materials. All of these groups of organisms can contribute to the deterioration of the technical and functional properties of building materials and also have a negative impact on human health. Two unfavourable processes are combined here: environmental contamination with mycotoxins and biodeterioration of building materials [19]. Material destruction processes stimulated by the activity of organisms are referred to as biodeterioration. Microbial colonisation plays an important role in the destruction of ceramics [20]. Fungi play a key role in biodeterioration and, according to many publications, are the most important factor in the destruction of cultural heritage materials [21,22]. They are heterotrophic microorganisms with enormous potential. They can grow on any substrate and produce a wide range of metabolites, such as enzymes, organic acids, and pigments, and cause physical degradation through mycelial growth [23]. Fungi, due to their pigment production, have a negative impact on the aesthetics of ceramic building materials [23]. Some researchers have noted the presence of fungi on glazed tiles [19]. The growth of microorganisms is also influenced by factors such as mineral composition or the presence of nutrients [4]. In addition to fungi, bacteria are also responsible for destroying ceramic materials, as they excrete by-products into the environment in the form of organic acids (e.g., citric acid, oxalic acid), as well as compounds that can potentially (after oxidation or reduction) become harmful to these materials, such as sulphuric acid (VI) or nitric acid (V).

2. Factors Determining Microbial Growth

The detection of microorganisms on cultural objects and elements does not always imply a modification of the chemical composition and/or physical properties of the materials [24]. Certain conditions must occur that, together with other factors, can trigger the onset of biological destruction of materials. The damage is not always visible immediately, and very often the growth of microorganisms is slow. Consequently, the damage may not be visible until many years later.

Therefore, biodeterioration occurs in close connection with chemical and physical factors that affect microorganisms. The growth and development of microorganisms is stimulated by external stimuli, i.e., environmental factors. Microorganisms are sensitive to changes in environmental parameters such as, for example, light, humidity, pH, and the physicochemical properties of the material [25]. It is difficult to determine unequivocally which factors play the most important role. The resulting effect of overlapping factors determines the growth potential of a specific population [26].

For microorganisms on materials, the humidity of the air and substrate is an important factor. The range of water activity (aw) at which microbial growth occurs is wide. They grow best at a hygroscopic equilibrium index aw close to a value of 1.0. Lowering it below the optimum value is associated with a reduction in growth rate or generation time. Below the minimum value, microbial growth is inhibited. Of all microorganisms, fungi are the least demanding and, depending on the strain, can already grow in an environment with a relative humidity of 60% to 99% or a water activity value of 0.6 to 0.9. However, for most of these fungi, growth inhibition occurs at a value of 0.80 [27].

Temperature is another important factor for the growth and development of microorganisms. Microorganisms are able to grow over a very wide temperature range: from −5 °C to 100 °C. Most organisms thrive in the temperature range found in buildings, where the temperature range is usually 18–20 °C. Fungi have the ability to survive at temperatures much lower or higher than these. Although they do not have the ability to grow, they can survive by inhibiting their growth [28]. Most can thrive at 3 °C. However, the optimum range is 22–30 °C, and the limit is around 40 °C. Spores can survive in environments with much lower or higher air temperatures.

An important parameter for microbial growth is the pH value, which indicates the reaction of the environment. Microorganisms can grow at different pH values. Most bacteria thrive in a neutral or slightly alkaline pH (pH about 7–7.5), while most fungi thrive in an acidic environment (pH about 5.2–5.6). Their metabolic products contain a certain amount of acid, which lowers the pH value of the substrate [29]. Moulds grow best at a pH in the range 4–4.5 and are not very sensitive to changes in environmental pH.

The value of the oxidoreductive potential is directly proportional to oxygen availability. If it is high, the environmental conditions are characterised by a high oxygen concentration. Fungal growth is dependent on oxygen availability [30]. Under anaerobic conditions, in the presence of carbon dioxide and hydrogen, the mycelium dies after a few days. Fungal growth is inhibited by excess carbon dioxide. The mycelium stops growing when the carbon dioxide content exceeds 80%.

All microorganisms require carbon, hydrogen, and oxygen for growth, as well as energy sources and electrons. Division of microorganisms according to their use of energy and carbon sources [28,31]:

• Photolithotrophic autotrophy (photolithoautotrophy):
  • Sources of energy: Sources of energy;
  • Hydrogen/electrons: Inorganic hydrogen/electron (H/e⁻) donor;
  • Sources of carbon: CO₂;
  • Microorganisms: Algae, purple and green sulphur bacteria, Cyanobacteria.
• Photoorganotrophic heterotrophy (photoorganoheterotrophy):
  • Sources of energy: Sources of energy;
  • Hydrogen/electrons: Organic H/e⁻ donor;
  • Sources of carbon: Organic carbon or CO₂;
  • Microorganisms: Purple nonsulphur bacteria, green nonsulphur bacteria.
• Chemolithotrophic autotrophy (chemolithoautotrophy):
  • Sources of energy: Chemical energy source (inorganic);
  • Hydrogen/electrons: Inorganic H/e⁻ donor;
  • Sources of carbon: CO₂;
  • Microorganisms: Sulphur-oxidizing bacteria, hydrogen bacteria, nitrifying bacteria, iron-oxidizing bacteria.
• Chemoorganotrophic heterotrophy (chemoorganoheterotrophy):
  • Sources of energy: Chemical energy source (inorganic);
  • Hydrogen/electrons: Inorganic H/e⁻ donor;
  • Sources of carbon: Organic carbon;
  • Microorganisms: Protozoa, Fungi, most nonphotosynthetic bacteria (including most pathogens).

Light radiation can activate, inhibit, or have no effect on the growth of microorganisms. Microorganisms require a certain amount of light radiation to produce healthy, fully reproductive fruiting bodies. Cocci that have been produced without light do not produce spores [30]. On the other hand, the sensitivity of microorganisms to UV radiation is variable and depends on the structure of the cell wall and its thickness, as well as on the composition and structure of the nucleic acid, the physiological state of the microorganism, and finally on the ability of the cells to repair UV-induced damage [32].

Altitude is a factor in the biodiversity of microorganisms present. High-altitude environments generally experience low temperature, variable precipitation, reduced atmospheric pressure, and soil nutrient stress. Cold environments at high altitudes have been successfully colonised by cold-adapted microorganisms. These microorganisms are able to thrive and maintain metabolic activity at sub-zero temperatures. Aspergillus spp., Fusarium spp., and Aspergillus flavus have been identified at the following altitudes: 1500, 1800, and 2200 m a.s.l., respectively. However, their level of growth depends on storage time and initial moisture content [33]. In contrast, cyanobacteria produce toxins in polar and hot deserts, hypersaline and alkaline waters, and hot springs.

Microbial activity increases in countries with rainy seasons. There, almost ideal conditions for their development are created, namely, temperatures exceeding 25 °C and relative humidity ranging from 80 to 100% [34]. Fungi of the genera Aureobasidium, Cladosporium, Dothideomycetes, and Pestalotiopsis, which are the main components of the biofilm that develops on the surface of glass materials, may be present [35]. Microorganisms, like any living organism, need a source of energy, carbon, nitrogen, and many other nutrients to develop, grow, and reproduce [30]. Those needed in the greatest quantities are those that constitute the basic building blocks, namely, carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulphur. In addition to these components, the presence of iron, magnesium, and the so-called trace elements (Co, Mn, Zn, Cu, Mo, Ca), which are required in very low concentrations, is essential [8].

The fungi most commonly found in ambient air are Alternaria, Cladosporium, Penicillium, and Aspergillus [36]. Indoor air additionally contains Mucor, Ulocladium, Trichoderma, Chaetomium [27], Fusarium, and Stachybotrys [37]. The spore concentrations of mould fungi, such as Cladosporium and Alternaria, which are found in indoor air, differ significantly from those found outside the building [38]. This means that the number and spectrum of moulds and bacteria present in buildings are strongly influenced by people, pets, plants, ventilation systems, and the time of year.

The way building materials are manufactured and stored [39] also influences the growth of microorganisms. Failure to maintain proper conditions during their production and improper storage means that they can be a source of mould spore emissions, mycotoxins, and volatile compounds [40]. When contaminated material is incorporated into the building envelope, it will constitute a biological agent. Under favourable conditions, microbial growth will occur and spread to subsequent materials.

The colonisation of the surface of solids depends on the surface properties of the two interacting bodies. Such properties include surface parameters, i.e., surface charge, surface tension, wettability and roughness, and structural parameters such as composition and porosity [41]. Surface charge and surface tension are responsible for short- and long-range interactions [42].

Materials that are similar in terms of their mineralogical composition can be attacked by the same type of microorganisms and corrosion processes then occur according to the same mechanisms. The ability of microorganisms to weather rocks and minerals contributes significantly to the degradation of clay-based ceramic building materials. There are two main mechanisms involved, i.e., processes of physical nature (biomechanical action) and biochemical processes. Figure 3 shows the characteristics of these mechanisms.

The biogeochemical mechanism is associated with processes and reactions that include the following:

• acid hydrolysis, which is coupled with chemolithotrophic processes that release sulphuric and nitric acids into the environment;
• oxidation-reduction reactions involving ions, e.g., oxidation of iron and manganese;
• phototrophic processes, where organic matter is accumulated and oxygen is supplied [43,44].

The above processes are triggered by metabolic products. These compounds dissolve the mineral matrix, resulting in subsequent weakening of the binding system [45]. The end result of the occurrence of biological corrosion is localised pitting and a flaking or peeling of the stone surface. In turn, this damage exposes the stone to erosion and damage from freezing and thawing processes [46]

As a result of acidic gases from the air and precipitation and deposition of particles from the atmosphere, structural changes and chemical transformations of the top layers of stones occur. Crystalline crusts or microcrystalline or amorphous incrustations form on their surface [47]. The formation of grey- to black-coloured crusts occurs due to the release of gases, e.g., SO₂, NO_x, and CO₂; organic compounds; iron and water oxides; and particles from dust, soot, and metal. One reason for the formation of crusts is that microorganisms catalyse various reactions [48].

Biogeophysical mechanisms of biodeterioration play a major role in their formation, which include the following:

• a change in stone porosity or pore size, caused by the formation of a biological membrane on the stone surface;
• alteration of gas diffusion within the material, caused by extracellular polymeric substances of EPS or compounds that reduce surface tension;
• formation of discolouration due to the release of biogenic pigments, i.e., melamine, chlorophyll, and changes in thermal and moisture properties;
• use of biofilm as an absorbent to trap atmospheric pollutants and as a precursor to crust formation;
• facilitating the migration of salts deep into the stone;
• alternation of aerobic and anaerobic conditions [49].

The weathering of silicate minerals which include wolastonite, olivine, pyroxene, serpentine, and brucite causes the release of Mg²⁺ and Ca²⁺ ions to capture CO₂ through the formation of carbonates [50]. Microorganisms such as archaeons, bacteria, and fungi accelerate the formation of carbonates as they participate in the weathering of minerals. Microorganisms also accelerate the dissolution of silicates. This occurs through the production of excess protons, organic acids, siderophores, and EPS [51]. Microorganisms consume the primary elements potassium and phosphorus, which lowers their concentrations in water. This results in the dissolution of scale minerals due to the equilibrium effect. Filamentous fungi effectively promote mineral dissolution by means of filamentous fungi, through which there is strong adhesion to the mineral, and this lowers the pH in the vicinity of the cells and in effect disrupts the mineral network [52].

The biodeterioration process of natural stones can occur in an alkaline environment. Microorganisms have the ability to degrade nitrogen compounds and sodium salts of organic acids. The resulting products are ammonium and sodium salts. Through their formation, the pH reaches a value of approximately 9. In addition, carbon dioxide is also formed during photosynthesis, which also leads to an increase in pH to a value of approximately 8.3. On the other hand, when the pH value decreases, silica dissolution occurs. This effect is achieved by certain bacterial species [43].

3. Biodeterioration of Ceramics

Biodeterioration is a common process that causes damage to ceramic materials. Physical and/or chemical biodeterioration of unglazed ceramics can occur, which is strongly dependent on the porosity and structure of the substrate.

The susceptibility of a material to colonisation by microorganisms is based on its intrinsic properties. These have been defined as bioreceptivity, or susceptibility to biological colonisation [53]. Three groups can be distinguished: primary bioreceptivity, which is due to the internal structure of the material; secondary (secondary) bioreceptivity, which is due to the ability of the material to be altered over time by physical and chemical factors; and tertiary bioreceptivity, which is the factor of biological colonisation of a material altered by the human hand (e.g., after protective measures or preservation) [54]. Each of these groups can, together or separately, induce biodegradation of mineral building materials. The last group can be further divided into two groups [55], i.e., tertiary bioreceptivity, which is used when human actions cause physical changes to materials, such as the use of laser methods, and quaternary bioreceptivity, which is used when additives are introduced to materials or coatings are applied to the surface of materials. Mineral materials are initially difficult to colonise by microorganisms due to their surface reaction, but over time they become colonised by biologicals [56].

3.1. Brick

The traditional ceramic material is bricks made of polymeric red clays such as illite, kaolinite, chlorite, vermiculite, or smectite [57]. Typically, bricks have porous and uneven surfaces that can provide anchorage and shelter for microorganisms. The bricks may harbour microorganisms such as bacteria, e.g., Bacillus sp. and Streptomyces sp. [58].

-

cyanobacteria, e.g., Aphanothece castagnei, Chroococcus minor, Lyngbya corticicola [59,60];

-

filamentous fungi, e.g., Alternaria sp., Fusarium sp., Trichoderma, Penicillium sp. [61];

-

lichens, bryophytes, and plants [62].

Here, Bacillus sp., Methylobacterium sp., Paenibacillus sp., and Pseudomonas sp. are the most commonly identified bacterial genera from the early stages of colonisation [58]. Biological corrosion affects brick masonry. The appearance of microorganisms depends on the environmental conditions and will be caused mainly by capillary moisture and also by environmental pollution. Clay bricks can be contaminated by compounds containing chloride or sulphate ions (acids and mostly salts). Typically, these harmful compounds dissolve in water and distribute in the masonry. The presence of soluble salts eventually leads to the destruction of the brick [63]. When such a brick is exposed to a biological agent, contamination by microorganisms may occur, which will use the water and crystallised salts for their metabolic processes. This will result in further deterioration of the material [64]. Here, we can speak of secondary contamination, where chemical agents first act on the material and, as a result of their action, new products are formed. These compounds then become substrates for the next contamination, which is biological. As a result, further products are formed that will affect the integrity of the entire material. Figure 4 shows a photograph of an ordinary brick originally contaminated with MgSO₄ salt and then with the fungus Cladosporium herbarum. The photograph (b) shows hyphals of the fungus.

3.2. Clay Roof Tiles

The second type of ceramic material is clay roof tiles. These are fired clay slabs with a composition similar to brick, consisting of clay with quartz and carbonates [65]. The primary function of roof tiles is to protect the interior of a building from rain, wind, and solar radiation. They are exposed to weathering, including biological agents that can induce biodeterioration phenomena [66]. Figure 5 shows the contamination associated with the presence of microorganisms. They are usually laid on a roof with a specific pitch angle, which makes them easily accessible to microorganisms that are difficult to flush from the surface.

Noteworthy is the lower biodiversity of photoautotrophs, i.e., algae and cyanobacteria, found on ceramic tiles compared to bricks. The lower biodiversity of microorganisms is due to the fact that they tend to be exposed to extreme weather conditions such as varying temperatures, exposure to radiation, and rapid surface drainage [67]. All types of fungi found on clay tiles have also been identified on stone building materials [68]. Clay tiles can also be colonised by lichens, bryophytes, and vascular plants.

4. Nature of Corrosion

4.1. Physical

A common problem in ceramic materials is biological colonisation by pioneer microorganisms such as cyanobacteria, bacteria, algae, and fungi [69,70]. These may be followed by secondary colonisers, namely, lichens and mosses [70]. At the final stage, vascular plants may develop [70]. Among the final colonisers, Bryum argentum or Tortula muralis were found as bryophytes, Bromus hordeaceus or Sedum sediforme as plants, and Endocarpon pusillum or Verrucaria nigrescens as lichens [71]. However, the occurrence of the individual stages depends on the climate and regions of corrosion [72].

Physical destruction also occurs through the mere presence of microorganisms. This is especially true of fungi and lichens [49].

Most of the microorganisms found on ceramic materials [70] produce staining substances, thus forming coloured biofilms. Due to the use of ceramic materials as a finishing element, aesthetic properties such as colour and texture are very important. The appearance of “aesthetic” biodeterioration is the first clear indication of a biocorrosive effect on the ceramic substrate [60,73]. Properties of ceramic surfaces, such as roughness, surface defects, and porosity, make these materials easily discolourable [74]. The formation of stains on wall tiles and bricks leads to aesthetic changes [60]. Colour changes from red to dark can also occur on ceramic tiles. Many microbial organic pigments, which are strongly bound to inorganic substrates, cause colour changes [75]. Certain types of cyanobacteria, such as Lyngbya, Gloeocapsa, and Scytonema, may be responsible for colour changes [60]. In addition, the activity of metabolic products can also induce substrate staining. Biofilm development causes visual changes on the surface of glazed ceramics, wall tiles, and bricks [76].

The main factors affecting the primary biosensitivity of ceramic building materials are porosity and roughness. For this reason, unglazed materials showed significantly higher sensitivity compared to glazed ceramic substrates. The influence of surface chemistry and pH on the biological corrosion of ceramic materials has been little studied [77].

In addition to their aesthetic function (Figure 6), ceramic building materials also have other properties, such as structural, heat and moisture insulation, and solar reflectivity. These properties can be altered due to biological growth. The development of biofilm and the accumulation of biomass on roofs is associated with a loss of properties such as solar reflectance and insulation of clay roof tiles [78]. Such changes are associated with the development of biofilms composed of phototrophs and fungi. Lichens, such as Verrucaria nigrescens [79], are also responsible for substrate changes. Biophysical and physical damage occurs due to loosening of the intergranular bond. This is due to volume changes, such as expansion and contraction of microorganisms, which weaken the ceramic material [80]. Volume fluctuations of microorganisms can have a strong impact on the physical state of ceramic materials [81]. These changes can result from changes in the volume of microorganism cells during growth or the penetration of biological structures into the material, such as fungal hyphae or lichens [82].

Fungal colonisation is favoured by spatial pore structure, internal structure, porosity, permeability and capillarity, and surface roughness [83,84]. However, the action of the filamentous fungi Aspergillus niger and Cladosporium sp. action on clay tiles causes minor changes in porosity, especially in the size of small pores (<1 μm) [85]. Lichens can also cause changes in the pore structure of clay tiles as a result of lichen penetration into pores, cracks (fissures), and crevices [65]. Volume changes in ceramic materials can also induce extracellular polymeric substances (EPS), which is related to the ability to absorb and drain water. Consequently, biofilms allow large amounts of water to be retained within their structure, while protecting cells from drying out [86]. Depending on the climate, changes in daily humidity levels can lead to extreme fluctuations in EPS volume. As a result, they exert a strong influence on the substrate while causing decomposition [87]. Some microorganisms can produce a gelatinous sheath to protect them from these changes. Such organisms include cyanobacteria (e.g., Gloeocapsa sp.), bacteria, and filamentous fungi. When these microorganisms enter pores or surface imperfections, their sheath can undergo dimensional changes and ultimately cause damage to the ceramic architectural elements due to internal stresses [88].

4.2. Chemical

The direct action of metabolic products secreted by organisms on inorganic substrates is the main cause of chemical biodeterioration. This phenomenon promotes an increase in capillary porosity of the ceramic body and the formation of efflorescence, which can weaken the ceramic material and increase susceptibility to other types of corrosion [80]. Some organic acids excreted by fungi and lichens act as chelating agents, reacting with cations from the substrate, such as Ca²⁺, Mg²⁺, Fe³⁺, Mn²⁺, and Mn³⁺, precipitating as salts [81]. In filamentous fungi, acids are produced by the mycelium [89]. The mycelium can produce acids such as citric, tartaric, itaconic, gluconic, malic, succinic, fumaric, and oxalic acids [42]. The mechanism of destruction of inorganic materials is as follows [90]: processes occur through chemical reactions between the aggressive agent, i.e., organic acids, and the compounds that are components of the building material. The reactions produce salts that are soluble or insoluble in water [44]. For example, they produce oxalic acid, which can chelate calcium and form calcium oxalates, according to reaction Equation (1):

Ca(OH)₂ + (COOH)₂ → (COO)₂Ca + 2H₂O,

(1)

The deposition of such a product in the ceramic matrix causes chemical and mechanical damage, associated with changes in porosity and leading to material deterioration [85]. In addition, organic acids can still react with cations that have been washed out of the material during the corrosion process [55]. Depending on the type of products formed, corrosion will or will not occur. In the case of the formation of readily soluble salts in water, the corrosion effect will be a lack of cohesion, surface flaking, and crumbling. However, the degree of damage depends on the type of acid and the duration of its action. In addition, different species of fungi secrete different organic acids in unequal amounts, so the degree of their destructive effect varies [91]. Most organic acids induce a corrosive process in mineral materials [92].

The formation of calcium oxalate, as a result of the secretion of oxalic acid, is thought to have a protective effect [93].

Building materials that have carbonate compounds in their composition are exposed to organic acids.

Another metabolic product is carbon dioxide. Its action on the components of ceramic materials results in weakening of the material due to leaching of corrosive products such as calcium bicarbonate. The effect of biogenic carbon dioxide, secreted by domestic fungi, moulds, or bacteria, is similar for all mineral materials, including lime mortar, clay brick, and cement concrete [5,94]. The presence of carbon dioxide promotes carbonate corrosion in the presence of moisture. In building materials containing calcium hydroxide and/or calcium carbonate, CO₂ dissolves in water and converts to calcium carbonate (2) and later to calcium bicarbonate (3). The product of the last reaction is easily soluble and is therefore sometimes washed out of the material. As a result, the material loses its cohesiveness and binding force. The process follows the reactions:

Ca(OH)₂ + CO₂ = CaCO₃ + H₂O,

(2)

CaCO₃ + H₂O + CO₂ = Ca(HCO₃)₂,

(3)

In addition to leaching, conversion of calcium oxide to calcium hydroxide may also occur, accompanied by an increase in volume. Carbon dioxide does not damage clay bricks that have been well fired and are dominated by mullite. It may have a slight effect on sillimanite found in poorly fired bricks. Marl may be present in the brick and will then react strongly with carbon dioxide, but due to the small amount of carbon dioxide, the destruction will be localised.

Leaching of components from ceramic materials, intensified by the presence of bacteria, has also been observed on bricks [95]. More than two dozen taxa, belonging to the Firmicutes and Proteobacteria, enhance silica dissolution. Halophilic bacteria are also capable of biological dissolution of components [96]. Salt crystallisation severely damages porous materials [71]. Biofilm growth [49] significantly increases the decomposition of various silica derivatives, such as quartz sand, crystalline quartz, and commercial glass. Many of the minerals required for microbial growth are present in ceramic and vitreous compositions. On the other hand, microorganisms rapidly take up elements for nutritional purposes, as is the case with stone [70].

Micro-organisms, especially bacteria, can produce sulphur or nitrogen compounds. This action can result in the formation of acidic compounds, such as inorganic acids such as hydrogen sulphide acid, nitric acid (V), and sulphuric acid (VI). The products that are formed depend on the food of the bacteria and the availability of oxygen. The reduced sulphur and nitrogen compounds are used as energy sources, and then strong inorganic acids (sulphur (VI) and nitric (III) acid, hydrogen sulphide) are released as end-metabolic products of these reactions [97]. Autotrophic bacteria, which are unlikely to be found in air distribution systems, function in this way. Heterotrophic bacteria, on the other hand, use organic carbon compounds as their main source of energy and then secrete a wide range of organic acids and metabolites. The heterotrophic feeding mode is the most common mode of energy acquisition among bacteria. It is also the only mode used by fungi and higher organisms. This group of bacteria is often found on wall surfaces in canals [98].

Nitrifying bacteria (Nitrosomonas, Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus) [99] produce biogenic nitric acid (V) by oxidation of ammonium ions followed by nitrogen (III) ions. The process takes place in two steps (4) and (5):

2NH₄Cl + 3O₂ → 2HNO₂ + 2HCl + 2H₂O Nitrosomonas,

(4)

2HNO₂ + O₂ →2HNO₃ Nitrobacter,

(5)

Nitric acid (V) production can also be triggered by the action of Nitrobacter and Nitrococcus bacteria. The action of nitric (III) and (V) acids with all the reactive components of concrete/mortar (calcium hydroxide and calcium carbonate) produces soluble salts.

Figure 7 shows the biochemical processes involved in the destruction of ceramics by various bacteria.

The term “biodeterioration” had its origins in processes different from those used today. It denoted the phenomenon of beneficial microbial activity. At the time, they removed pollutants and waste from the environment [100]. Over the years, the term came to mean the negative action of all living organisms (micro- and macro-organisms) on all technical materials, whether inorganic or organic. Therefore, it should come as no surprise that there are reports in the literature of positive effects of micro-organisms on materials, as well as the simultaneous occurrence of biodeterioration and bio-protection. In [101], it was shown that species such as Protoparmeliopsis muralis and Lecanora campestris that are part of the lichen group can have a protective effect on ceramics characterised by high porosity and very small pores. This is due to the weak penetration of the lichens into the ceramic substrate and, consequently, they do not have a negative effect on the surface. At the same time, they limit the penetration of water into the structure. Biodeterioration functions as a negative factor and emerges as a positive factor—bio-protection. Is there also a simultaneous effect of these two states? It appears that such a situation can occur. They can occur on the same substrate (here a material) and are induced by specific microorganisms [102]. In the end, resilience/sustainability results from the equilibrium occurring between these processes. Obviously occurring under specific environmental conditions. However, information on this is scarce and requires much research. However, microorganisms are also used to create materials, so-called Biomaterials such as biocement in which different species of bacteria or fungi are used to create biomaterials that are self-repairing with little damage as a result of the production of biomineralisation [103].

5. Conclusions

The presence of micro-organisms, especially bacteria and fungi, in the environment is enormous. They have the ability to adapt to virtually any environmental condition. Microorganisms are able to colonize all technical materials, including ceramic materials, due to their ability to produce metabolites. This paper describes the microbial degradation of red and glazed ceramics, which were found to be resistant to microorganisms. However, the right temperature and humidity, as well as material composition or pH, allow growth on such raw material. All processes occurring on the material, i.e., microbial decomposition and corrosion, are related to metabolic processes and the very presence of microorganisms on or in the material. The mechanism of destruction depends on the composition of the ceramic material on which the microorganisms develop. Biofilm develops on any material and occurs in almost any environment, as long as there is water in it.

The subject of microbial corrosion of clay ceramics has been known and studied for decades. However, its mechanism is not yet fully understood. The same is true for prevention and monitoring methods. Biodeterioration is such a broad topic and so much of the spectrum is affected by it that a joint effort of different scientific and technical disciplines such as microbiology, materials engineering, corrosion engineering, and integrity management is needed to fathom its mystery and help materials fight the deterioration process. The joint combination of knowledge is of paramount importance in order to identify the causes, mechanisms, and strategies for further action for ceramic clay materials of the present day, but above all the one that constitutes Our History, so that future generations can learn history from it in person and not only through photographs and electronic 3D images.