Abstract
A perlite sample representative of an operating mine in Milos was investigated with respect to the type and spatial distribution of water. A set of different methods was used which finally provided a consistent view on the water at least in this perlite. Infrared spectroscopy showed the presence of different water species (molecular water and hydroxyl groups / strongly bound water). The presence of more than 0.5 mass% smectite, however, could be excluded considering the cation exchange capacity results. The dehydration measured by thermal analysis occurred over a wide range of temperatures hence confirming the infrared spectroscopical results. Both methods point to the existence of a continuous spectrum of water binding energies. The spatial distribution of water and/or pores was investigated using different methods (CT: computer tomography, FIB: scanning electron microscopy including focused ion beam technology, IRM: infrared microscopy). Computer tomography (CT) showed large macropores (20 – 100 μm) and additionally revealed a mottled microstructure of the silicate matrix with low density areas up to a few μm in diameter. Scanning electron microscopy (FIB) confirmed the presence of μm sized pores and IRM showed the filling of these pores with water. In summary, two types of pores were found. Airfilled 20 – 100 μm pores and μm-sized pores disseminated in the glass matrix containing at least some water. Porosity measurements indicate a total porosity of 26 Vol%, 11 Vol% corresponding to the μm-sized pores. It remains unsolved wether the water in the μm-sized pores entered after or throughout perlite formation. However, the pores are sealed and no indications of cracks were found which indicated a primary source of the water, i.e. water was probably entrapped by quenching of the lava. The water in these pores may be the main reason for the thermal expandability which results in the extraordinarily porous expanded perlite building materials.
Highlights
Perlite is a hydrous volcanic material dominated by alumosilicate glass
The difficulty of distinguishing different types of perlite water results from the fact that the primary water of the volcanic glass varies in both amount and composition, with variable mixtures of hydroxyls and molecular water (Stolper 1982; Eckert et al 1988; Dobson et al 1989; Silver et al 1990; Pandya et al 1992)
Summary and conclusions Perlites are believed to form upon post-formational hydration often leading to smectite formation
Summary
Perlite is a hydrous volcanic material dominated by alumosilicate glass. As an industrial material, perlite is mostly used in its expanded form, i.e. after heat treatment resulting in a light-weight macroporous product. According to Friedmann et al (1966) both primary magmatic and meteoric water causing the post-formational hydration of the glass can be found in perlites. This water, in contrast to the hydration shell, was supposed to be located within the Al-Si-framework. The difficulty of distinguishing different types of perlite water results from the fact that the primary water of the volcanic glass varies in both amount and composition, with variable mixtures of hydroxyls and molecular water (Stolper 1982; Eckert et al 1988; Dobson et al 1989; Silver et al 1990; Pandya et al 1992)
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