XRD analysis

The XRD results indicate that Muttalip green clay is composed of SiO₂, albite (Ab), plombièrite (Plm) and muscovite (Ms) 2M ₁ (Fig. 2). SiO₂ is present in ~95% of the materials used in the construction industry. Nayak & Singh (Reference Nayak and Singh2007) characterized the clay in their study and determined that quartz (Qz) was the main dense component.

Pure Ab (NaAlSi₃O₈) can be white, greenish or reddish, and it is often combined with minerals such as Ms, biotite (Bt), hornblende (Hbl) and Qz. As a precious stone, Ab is used in industries such as glass and ceramic production. Ms is a hydrated phyllosilicate Al- and K-based mineral with the formula KF₂(Al₂O₃)₃(SiO₂)₆H₂O or KAl₂AlSi₃O₁₀(F,OH)₂. It can be colourless, greenish, yellowish or reddish. Ms possesses significant Al contents and is employed as a lubricant in the manufacture of non-combustible and insulating materials. In addition to being lightweight, Ms exhibits good flexibility, reflectivity, inertness, dielectric properties, elasticity, hydrophilicity and refractivity (Yuan et al., Reference Yuan, Yang, Ma, Su, Chang and Komarneni2018). It is stable when exposed to electricity, light, humidity and extreme temperatures. Because of all of these features, Ms has been applied widely in various industries. There are also studies in which Ms has been synthesized from an ash produced from biotite-rich coal wastes (e.g. Khoshdast et al., Reference Khoshdast, Shojaei, Hassanzadeh, Niedoba and Surowiak2021), increasing the availability of this important industrial material. Plm has the formula (Ca₄Si₆O₁₆(OH)₂⋅2H₂O)⋅(Ca⋅5H₂O).

XRF measurements

XRF spectroscopy was performed on the Muttalip green clay (Table 2) and it was shown to contain SiO₂, Al₂O₃, Fe₂O₃, TiO₂, CaO, MgO, Na₂O, K₂O and SO₃. SiO₂ and Al₂O₃ as the main components. The large SiO₂ content of the sample confirmed the presence of Qz, as indicated by the XRD results (Fig. 2).

Table 2. Chemical composition (wt.%) of Muttalip green clay.

1LOI at 1050°C.

The CaO in the clay indicates the presence of calcite (Cal), as reported by Coletti et al. (Reference Coletti, Maritan, Cultrone and Mazzoli2016). Cal (CaCO₃) often contains small amounts of Fe, small levels of contamination by clay particles and Qz (Milošević et al., Reference Milošević, Logar and Djordjević2020). Residual Cal is converted to waste during roof-tile firing, which produces more open pores in these tiles (Montero et al., Reference Montero, Jordán, Hernández-Crespo and Sanfeliu2009), leading to increased water absorption (Jordán et al., Reference Jordán, Almendro-Candel, Romero and Rincón2005). The organic matter in the clay may be eliminated using thermal treatment, which also increases the open porosity of the tile (Jordán et al., Reference Jordán, Almendro-Candel, Romero and Rincón2005; De Silva & Mallwattha, Reference De Silva and Mallwattha2018). During firing, CaO expands in the ceramic material, leading to the formation of cracks in the tiles. Depending on the reactions that occur at the firing temperature, these cracks can reduce the flexural strength of the material (Assunção et al., Reference Assunção, Correia, Vasconcelos, Cabral, Angélica and da Costa2021).

XRF analysis of the Muttalip green clay revealed a significant amount of CaO in the clay structure. CaO expanded in the tile during firing and decreased its flexural strength; however, when the firing temperature was increased from 900 to 950°C, CaO reacted with amorphous SiO₂ and increased the flexural strength of the tile. When the firing temperature was further increased, CaO decreased the flexural strength of the tile by promoting stress concentration.

LOI enables volatiles to escape from the tiles at high temperatures until their mass stabilizes. The high value of LOI obtained in this study may be attributed to the loss of volatiles, including water (hydrates and unstable hydroxy compounds), organic materials and carbonates (Milheiro et al., Reference Milheiro, Freire, Silva and Holanda2005; Abdelmalek et al., Reference Abdelmalek, Rekia, Youcef, Lakhdar and Nathalie2017; Milošević et al., Reference Milošević, Logar and Djordjević2020).

The chemical analysis results of the Muttalip green clay were compared with those of other clays used in tile production (Bilgin et al., Reference Bilgin, Bilgin and Yeprem2008; Rivera et al., Reference Rivera, Cuarán-Cuarán, Vanegas-Bonilla and de Gutiérrez2018; Moreno-Maroto et al., Reference Moreno-Maroto, Uceda-Rodríguez, Cobo-Ceacero, Cotes-Palomino, Martínez-García and Alonso-Azcarate2020; Akçin et al., Reference Akçin, Bulut, Ekinci Şans and Esenli2022). Among the clays investigated, the Muttalip green clay exhibited the smallest SiO₂ and largest LOI values.

FE-SEM measurements

FE-SEM images of the Muttalip green clay were obtained at various magnifications to analyse its composition and surface morphology. The FE-SEM image obtained at 150× magnification (Fig. 3a) shows particles ranging in size from 30 to 240 μm with an irregular morphology. In a study of tile production using all of the waste from glass bottles, fluorescent lamps and soda-lime window glass, Rivera et al. (Reference Rivera, Cuarán-Cuarán, Vanegas-Bonilla and de Gutiérrez2018) found smooth, flat-faced and elongated particles; the authors did not observe spherical particles. At 600× magnification (Fig. 3b), smaller clay particles were observed. Small structural cracks were clearly visible at 100,000× magnification (Fig. 3c). Pitarch et al. (Reference Pitarch, Reig, Tomás, Forcada, Soriano and Borrachero2021) found that the FE-SEM morphologies of various ground ceramic particles depended on their water absorption. These small variations in morphology are important because tile ceramic particles exhibit low porosity and a smooth surface. A small number of small pores could increase the flexural strength of the resulting tile. The FE-SEM images of ceramic materials obtained after sintering at high temperatures (Ahmed et al., Reference Ahmed, Sultana, Zaman and Rahman2021) indicated that glassy phase formation and pores could decrease the flexural strength of tile samples.

Fig. 3. FE-SEM images of Muttalip green clay at various magnifications.

Moisture rate

Clays present as powders in the absence of water. Thus, a suitable amount of water should be added to a clay to achieve the desired plasticity because water facilitates the weak electrostatic forces necessary for clay–particle attraction (Barnes, Reference Barnes2013). The moisture rate of the Muttalip green clay was 18.25%.

Drying shrinkage

The Muttalip green clay and water were mixed at a humidity level of 15–20% and then poured into a vacuum press to prepare 0.05 × 0.10 m² galette tiles containing no air bubbles. The galette tiles were removed from the vacuum press and dried in an oven at 80°C for ~6 h. When the drying temperature was >80°C, the tiles developed cracks, leading to product loss. When the temperature was <80°C, the drying process lasted significantly longer. The width, length and weight of wet (W) and dry (W _d) galette tiles were measured (Table 3) and surface images of the tiles after drying were obtained (Fig. 4). No cracks or breaks were observed in the galette tiles. The surfaces of tiles must be defect-free to achieve a product with good flexural strength. Cracks were observed on the surfaces of galette tiles obtained from Muttalip red clays (Kuru Mutlu, Reference Kuru Mutlu2022), which exhibited a high SiO₂ content and were dried at 80°C. However, cracks were absent in galette tiles obtained from schist materials (Kuru Mutlu & Mutlu, Reference Kuru Mutlu and Mutlu2022), which also exhibited a high SiO₂ content and were dried at 80°C. Thus, cracking is not related strongly to the SiO₂ content of the sample.

Fig. 4. Images of the galette tiles after drying and heating at 80°C: (a) galette sample 1, (b) galette sample 2 and (c) galette sample 3.

Table 3. Physical measurements of the galette tiles after drying and firing.

The masses of the galette tiles before and after drying were compared, and their drying shrinkage values were calculated according to Equation 1 (Table 4; Huggett, Reference Huggett2015). In this study, the drying and weight shrinkage values of the tiles were obtained and referred to collectively as the ‘drying shrinkage’. The tiles exhibited a drying shrinkage value that was only 0.45% greater than that of similar tile materials (Kuru Mutlu & Mutlu, Reference Kuru Mutlu and Mutlu2022); this difference is not significant.

(1)$${\rm Drying\ shrinkage} = \displaystyle{{W\;-\;W}_{\rm d} \over W}\;\times \;100\% $$

Table 4. Average drying shrinkage of the galette tiles.

Firing shrinkage

The surfaces of the galettes were defect-free after drying at 80°C. Heat treatment was performed at 900°C (Sample 1), 950°C (Sample 2) and 1000°C (Sample 3) to examine the effects of various firing temperatures on the flexural strength of the tiles. The galette tiles were not removed immediately after firing; instead, the following process was used: the galettes were first heated to 80°C and then rested. Next, they were heated to 473°C and then rested for 1 h. Each sample was then heated to 900, 950 or 1000°C and maintained at this temperature for 2 h. Finally, the galettes were left to air-cool to room temperature.

The width, length and weight (W _f; Table 3) and surface images (Fig. 5) of the fired galette tiles were obtained. None of the tiles showed cracks or fractures because they all contained a small amount of Qz, which serves as a stress concentrator and forms microcracks due to its allotropic transformation (Barnes, Reference Barnes2013). Furthermore, CaO reacts with amorphous SiO₂ to form CaSiO₃ during firing and so prevents crack formation as a stress intensifier (Assunção et al., Reference Assunção, Correia, Vasconcelos, Cabral, Angélica and da Costa2021).

Fig. 5. Images of the galette tiles fired at (a) 900°C, (b) 950°C and (c) 1000°C.

All of the ceramics exhibited the reddish colour (Babisk et al., Reference Babisk, Amaral, da Silva Ribeiro, Vieira, do Prado and Gadioli2020) typical of traditional ceramic products (Fig. 5) due to the oxidation of Fe compounds present in the green clay. Knowledge of the drying and firing shrinkage values of tiles (Equation 2) is important. If desired, the drying and firing shrinkage values can be calculated for each galette using the values from Table 3. Unfortunately, because the firing temperatures of the galettes were different, their average firing shrinkage could not be calculated.

(2)$${\rm Firing\ shrinkage\ } = \displaystyle{{W_{\rm d}\;-\;W }_{\rm f} \over {W }_{\rm d}}\;\times \;100\% $$

Water absorption

A dry galette was weighed (W _d) and immersed in water for 24 h. The weight of the tile was then measured after immersion (W _w). Finally, the water absorption (W _a; Equation 3) of the tile was determined (Bureau of Indian Standards, 2002; ASTM C1492, 2003; ASTM C830-00, 2016; De Silva & Mallwattha, Reference De Silva and Mallwattha2017). The average W _a of the Muttalip green clay was calculated to be 16.35%.

(3)$$W_{\rm a} = \displaystyle{{W _{\rm w}\;-\;W }_{\rm d} \over {W}_{\rm w}}\;\times \;100\% $$

When tiles exhibit significant water impermeability, small plants and algae may grow on them, thereby decreasing their durability and aesthetic values. The water-holding capacity of clay mixtures should not exceed 18% (De Silva & Mallwattha, Reference De Silva and Mallwattha2017). The water impermeability of the Muttalip green clay complied with the water-holding capacity limit (18%) of clay roof tiles (Bureau of Indian Standards, 2002; De Silva & Mallwattha, Reference De Silva and Mallwattha2017).

Flexural strength

The most important parameter of a tile is its flexural strength. The breaking flexural strength of the tiles was determined by the maximum load they could withstand using a three-point bending test. Here, the tile was fixed at both ends and various forces were applied at the midpoints of these ends. Then, the breaking flexural strength of each tile was measured using a sensor (Table 5). The effects of various firing temperatures on the flexural strength of the samples were also investigated. The strengths of the galette tiles fired at 900 and 1000°C were similar. Compared with these tiles, the galette fired at 950°C demonstrated increased flexural strength. Specifically, the strength of the galette tile fired at 950°C was 4.12% greater than that of the galette tile fired at 900°C.

Table 5. Flexural strength and converted flexural strength values of galette tiles at various firing temperatures.

In this study, the applicability of de-aerated rectangular galette tiles as roof tiles was investigated. Galette tiles are usually flat. Because Marseille-style tiles, which are frequently used on house roofs, vary in size and shape, a conversion coefficient is required to obtain accurate strength values. A factory production setting is obtained by multiplying the strength value of the Marseille-style tile produced in the laboratory by the factory conversion factor. Tile manufacturers use unique conversion coefficients for laboratory samples (rectangular galettes) and tile production. Thus, the flexural strength of the Muttalip green clay was determined according to the conversion coefficient of Hatipoglu Günes Tile & Brick Industry, Inc. (Table 5). According to the TS EN 1304:2016 and EN ISO 10545-4:2014 standards in Turkey, the breaking load of fired tiles should be >120 kg cm^–2. The Muttalip green clay tiles prepared in this study met this standard.

Guimarães et al. (Reference Guimarães, Santos, Thedoldi and Lima2022) reported red clays with flexural strengths of 10.19–91.00 kg cm^–2, while Babisk et al. (Reference Babisk, Amaral, da Silva Ribeiro, Vieira, do Prado and Gadioli2020) obtained clays with flexural strengths of 10–130 kg cm^–2. The converted flexural strength of the galette tiles obtained from the Muttalip green clay was greater than these values. Moreover, the flexural strength of the tiles prepared in this work was significantly greater than those obtained from schist materials (156.15 kg cm^–2; Kuru Mutlu & Mutlu, Reference Kuru Mutlu and Mutlu2022). Thus, the material alternatives available to the tile industry can be expanded by developing such clay mixtures, which will reduce the energy costs of industrial processes (Comin et al., Reference Comin, Zaccaron, de Souza Nandi, Inocente, Muller and Dal Bó2021).

Absorption and reflection measurements

Absorption and reflection measurements were performed on the galette tiles obtained from Muttalip green clay in three different regions of the electromagnetic spectrum (UV, NIR and MWIR) using various detectors, light sources and beam splitters. A vacuum was not required for the absorption–permeability measurements of the tiles in the visible and NIR regions because no atmospheric absorption occurs in these regions. By contrast, because atmospheric absorption occurs in the 2.5–25.0 μm region, optical measurements were performed under vacuum in this region to yield accurate results.

The reflection spectra of the galette tiles (Fig. 6) revealed that their reflection in the UV-B (280–320 nm) region was greater than that in the UV-C (200–280 nm) region. Most UV-B and UV-C rays are absorbed by Earth's atmosphere, and very few of these rays reach Earth's surface. In addition, the amount of light reflected by the tiles in the UV-A region (320–400 nm) was greater than that in the UV-B and UV-C regions (Fig. 6).

Fig. 6. Reflection spectra of the galette tiles in the UV and visible regions.

Most UV-A radiation from the sun reaches Earth's surface. UV-A exposure causes cell damage in plants (Cavusoglu et al., Reference Cavusoglu, Kalefetoglu Macar, Macar, Cavusoglu and Yalcın2022) and the skin (Radrezza et al., Reference Radrezza, Carini, Baron, Aldini, Negre-Salvayre and D'Amato2021); it also has negative impacts on the eyes (Sherin et al., Reference Sherin, Vyšniauskas, López-Duarte, Ogilby and Kuimova2021) and can cause melanoma (Wood et al., Reference Wood, Berwick, Ley, Walter, Setlow and Timmins2006; Barnes et al., Reference Barnes, Williamson, Lucas, Robinson, Madronich and Paul2019). However, when UV-A radiation from the sun reached the Muttalip green clay tiles, these tiles reflected the UV-A rays due to their surface properties.

The galette tiles showed consistent absorption in the NIR (1.0–2.5 μm) region (Fig. 7). By contrast, the absorption of the galette tiles in the MWIR region (Fig. 8) decreased as the wavelength increased. Previous studies (Prager et al., Reference Prager, Köhl, Heck and Herkel2006; Aboud et al., Reference Aboud, Altemimi, Al-HiIphy, Yi-Chen and Cacciola2019) have indicated that the ability of tiles to absorb radiation in the IR region is useful for the heating of buildings.

Fig. 7. Absorbance spectra of the galette tiles in the NIR region.

Fig. 8. Absorbance spectra of the galette tiles in the MWIR, LWIR and FIR regions.