Microstructure Ceramic Changes Resulting from Sodium Sulphate Subflorescence

Abstract

In this article, the influence of sodium sulphate crystallization on microstructure changes in traditional ceramic was discussed. The tested elements in form of brick halves underwent capillary moistening in sodium sulphate solution with concentration of 5, 10, 15, 20, and 25%, and then their bottom walls were protected with polyethylene covers in order to ensure drying only through side walls. After drying, samples were taken from face walls (5mm) and 2 subsequent lower layers (each 10mm thick). Prepared samples were used for comparative analysis of ceramic microstructure changes caused by salt crystallization and material damage visible after salt extraction. In the tests, mercury porosimetry and confocal microscopy were used. Because of necessity to dry the samples to solid mass, the temperature of 80°±5°C was defined which enabled removing free water without damaging of crystallized salt structure. Based on the ceramic analysis, the initial salt concentration when macroscopic damage starts as a result of subflorescence (face layer delamination) was found. The analysis of pictures from a confocal microscope enabled to define the mechanism for ceramic material damage. A characteristic granular diffusion was observed which is the effect of pushing out ceramic by building out the sulphate crystals.