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Scanning electron microscopy (SEM) images reveal the type and location of clay minerals in MG and CV_O (Fig. 3a–d). Swellable smectite, muscovite and kaolinite are located in both pore space (Fig. 3a) and in the matrix (Fig. 3b,d) as well as on grain boundaries (Fig. 3c). Smectite has the ability to cause volume increase by swelling, especially when it is located on grain contacts. The alteration of feldspar to clay minerals often leads to the formation of intracrystalline porosity and therefore increases the effective porosity (Fig. 3c).

The scanning electron microscopy (SEM) images (Fig. 3e–h) reveal very small mordenite needles and clinotilolite laths in CV_E, which cause a high specific surface area (Fig. 3e–h).

The zeolite-rich samples CV_O, CV_E and CA_E show the highest hygroscopic water sorption and hydric expansion of all samples (see Kück et al. 2020a in this issue). During thermal expansion, the zeolite-rich samples (CV_O, CV_E and CA_E) show a large difference between the first and the second heating cycle, with high residual strain (Fig. 4). After the first wet cycle the samples show a decrease in residual strain. CA_O and CA_E do not recover from contraction after the second drying cycle. The Mitla samples and QB are rather unaffected by both thermal and thermohyric dilatation (see Kück et al. 2020a).

Figure 3: a–d: Different appearances of swellable clay minerals. a): smectite appears as a ‘spiderweb’ in the pore space as relict of weathered pumice clast in MG, b): kaolinite booklet and smectite in densely packed matrix, c): dissolved feldspar with clay minerals on the grain boundary reveals intracrystalline porosity, d): smectite in the matrix of CVO; e–h): Zeolites in CVE cause a high specific surface area. e): flaky zeolite in the matrix, f): zeolites grow into pore space, g): clinoptilolite laths in the matrix, h): 100 nm thin mordenite needles in a pore.

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Figure 4: Residual strain [mm/m] of CVE in the X- and Z-direction for two heating cycles from 20–90 °C. Noticeable are intense shrinking and non-reversible thermal dilatation.