The influence of porosity on the release of water from perlite glass by thermal treatment

The water release from perlite from the three most important deposits in Central Europe was investigated with the aim to improve the understanding of the expansion process of perlites. Several attributes: water content, temperature, porosity, particle size, and mineralogical and chemical composition were evaluated. Detailed measurements of the water release by thermal treatment at a temperature range between 150 and 850 degrees C, and at various times from 1 min to 100 h were performed. The highest differences were observed between porous and dense perlites at 150 degrees C. Porosity and pore characterisation were performed by X-ray mu-computed tomography and N-2-adsorption. The porosity of perlite with large pores and homogenous pores distribution was decreased and the porosity with heterogeneous distribution was able even increased after grinding. These findings were determined on three mutually perpendicular sections of 3D images of the three studied porous perlites and explain the highest water releasing from perlite with the lowest porosity among them. Slovak crude perlites include higher amount of loosely bound water (45-60% of H2Ot, released in temperature range 0-250 degrees C) and lower amount of strongly bound water (1-7% of H2Ot, released in temperature over 550 degrees C) in comparison with Hungarian and other world perlites (18-50%, respectively, 6-23% of H2Ot). Slovak crude perlites also contain higher ratio of K2O to Na2O (2-2.5) when compared to other world perlites (0.7-1.4). These can be important factors that cause discrepancies in the expansibility of perlites among Slovak and world perlites.