Chapter V: Aluminosilicate Degradation in Acidic Environments
5.3 Results & Discussion
5.3.2 SO 2 Exposure
5.3.2.1. Firing Temperature
From an archeological and conservation science perspective, the fundamental difference between kaolinite and metakaolin in their interactions with S-rich pollutants is an important finding that motivates further studies on the degradation of low-fired pottery. Primarily, it is important to investigate whether the dealumination process could impact not just powders, but also objects fired at different temperatures. To consider this, kaolinite powders and green bodied tiles were fired at temperatures ranging from 300 °C to 900 °C, and later exposed to vapors of SO2.
Figure 5.6. Spectra of heat-treated kaolinite, fired at temperatures ranging from 300 °C to 900 °C, including a) XRD and b) ATR-FTIR. c) ATR-FTIR spectra of as-purchased
kaolinite. Note that XRD spectra shows presence of kaolinite (○) and quartz (■).
X-ray diffraction spectra shown in Figure 5.6a highlight the evolution of kaolinite into metakaolin. Samples fired at 300 °C and 400 °C have preserved the crystalline structure of kaolinite. Samples fired at 500 °C retain small amounts of the kaolinite structure, however at this temperature the majority of the material converts into amorphous metakaolin. Above 500 °C, traces of kaolinite are fully transformed into metakaolin, and the only crystalline
component detected is from the quartz traces that were originally present in the as- purchased kaolinite. Similarly, ATR-FTIR spectra in Figures 5.6b-c indicate that samples fired at 300 °C and 400 °C have a comparable structure to kaolinite, however with less intense Al-OH bands at 937 cm-1 and 907 cm-1. At firing temperatures of 500 °C or higher, the spectrum starts to resemble the fingerprint of metakaolin, previously seen in Figure 5.4b. Notably, the position of Si-O bands at 1053 cm-1 appears to shift towards higher wavenumber as the firing temperature increases, illustrating the gradual conversion of the material into an amorphous structure. The XRD and ATR-FTIR spectra of the fired powders were identical to the spectra of the fired tiles.
The observations seen in Figure 5.6 allow us to categorize structures based on their firing temperature: (i) kaolinite is the raw clay, (ii) partially dehydrated kaolinite is observed at 300 °C and 400 °C, and (iii) metakaolin is observed at 500 °C or higher.
In order to examine how the mechanical properties of the tiles changed with firing temperature, the Vickers hardness of the fired tiles were measured, shown in Figure 5.7a- b. In general, the metakaolin tiles (500 °C and above) have a hardness comparable to talk (1 on the Mohs scale), with values rising exponentially above 800 °C due to sintering and densification effects. The hardness of the partially dehydrated kaolinite tiles (300 °C and 400 °C) are significantly lower, as a consequence of the hydroxylated sheet-like structure that facilitates the compression of the material. The porosity of the tiles, seen in Figure 5.7c, similarly depends on the firing temperature of the tile. The porosity of metakaolin tiles decreases down to 54.6 ± 1.3 % with increasing temperature, with the most porous tile being created at 500 °C. The porosity of the partially dehydrated kaolinite tile (400 °C) is comparatively higher, with values reaching 61.6 ± 2.9 %. Note that it was not possible to measure the porosity of the tile fired at 300 °C, due to it disintegrating in water upon immersion. It is expected that the high open porosities seen in all samples (Figure 5.7c) directly influence the low hardness of the tiles (Figure 5.7b).
In comparison, the expected hardness of low-fired pottery, more specifically that of low- fired earthenware objects, is cited as being close to 4 on the Mohs scale, however this is largely dependent on the composition of the original clay which might additionally contain feldspar and glass-forming materials like lime and quartz to aid with densification [49].
Our findings in Figure 5.7a-b provide a fundamental understanding of how the hardness of pure kaolinite tiles are affected by firing temperatures. The intention of this study is to provide a base assessment that can be built on in future experiments, when eventually more complex low-fired systems (e.g., containing additions of feldspar, mica, silica, iron oxides etc.) will be investigated.
Figure 5.7. Hardness of tiles fired at temperatures ranging from 300 °C to 900 °C: a) micrograph of indentations and b) Vickers hardness. c) Porosity of tiles measured using
Archimedes’ technique.
The fired tiles were exposed to SO2 mixed with air, as illustrated in Figure 5.1. Figure 5.8a shows the resulting ATR-FTIR spectra and compares it to the original spectra before the exposure. The spectrum of the partially dehydrated kaolinite sample (400 °C) is similar to the original, showing identical peak positions and absorption intensities, reminiscent of results seen in Figure 5.4a. In contrast, the spectra of the metakaolin samples (500 °C and above) show a shift in the Si-O bands, located near 1053 cm-1 and 438 cm-1, towards higher wavenumbers after exposures to SO2. This shift indicates that the tile surfaces (within 1 µm from the surface) are richer in silica after the SO2 exposure, a result that could imply the occurrence of dealumination, as previously seen in Figure 5.3.
Figure 5.8. Spectra of heat-treated kaolinite, fired at temperatures ranging from 300 °C to 900 °C, before (|) and after (:) exposures to SO2: a) ATR-FTIR and b) XRD. c) Vickers
hardness of tiles before (×) and after (●) exposures to SO2, including a rehydroxylated sample in red.
Note that the largest shift is observed at 500 °C, where high porosities were observed that could facilitate interactions between SO2 and the aluminosilicate surface area. However, such a shift is not observed in Figure 5.4b, potentially due to the set-up of the acid bath
experiment where constant stirring caused the silica-rich passivation layer to break down, forming a white precipitate that became separated from the aluminosilicate powders. The absorption intensities of Al-O-Si bands in Figure 5.8 remain largely intact after the SO2
exposure, suggesting that the pH of the SO2 environment is approximately equal to 6, based on previous results from Figure 5.4b.
Nevertheless, Al2(SO4)3 is not detected in the ATR-FTIR (Figure 5.8a) nor XRD (Figure 5.8b) spectra. It is not fully understood why Al2(SO4)3 is not visible, however this might be due: 1) the sulfate salt is present in ppm quantities, which are difficult to detect, 2) the SO2 concentration was too low, or the environment too dry, for dealumination to take place, 3) a different silicifying process is occurring that is not dealumination. Figure 5.8c shows the hardness of the tiles after the exposure, where it was found that the dry, acidic environment did not alter the hardness in any statistically significant way.
5.3.2.2. Rehydroxylation
In addition to probing the effects of firing temperature, porosity, and environmental pH, it was decided to investigate the effect of environmental humidity on the properties of a fired tile. A tile fired at 900 °C was immersed in water for a week, after which the hardness of the tile was remeasured, seen in Figure 5.8c in red. It was found that the hardness of the tile nearly halved in size after being exposed to extreme humidity. The intake of water into the porous body caused the sample to rehydroxylate and thus revert back to a clay-like consistency as denoted by the softening of the body.
The rehydroxylated tile was subsequently exposed to SO2 for the duration of 3 days. Figure 5.8a shows the ATR-FTIR spectra of the tile before and after the exposure, demonstrating that the spectrum remained unchanged after the exposure. This is likely a result of preferred interactions between SO2 and H2O molecules to form H2SO4, temporarily protecting the aluminosilicate from reacting with SO2 during this short 3-day long exposure. As a result, the rehydroxylated tile did not show any evidence of dealumination. In addition, the exposure to SO2 mixed with air did not alter the hardness of the rehydroxylated tile in any statistically significant way.