Determination of soluble salts and efflorescences in ceramic roofing tiles

The appearance of efflorescences constitutes a serious problem for ceramic roofing tile quality, since efflorescences affect roofing tile technical and aesthetic qualities. This paper describes a method of determining the soluble salts present in the raw materials used in ceramic roofing tile manufacture and in the resulting roofing tiles, as well as an analysis of the phases in roofing tile efflorescences. The study was divided into three parts. First, a method of determining the soluble salts was developed, optimising the extraction method and fine-tuning an anion measurement method by ionic chromatography. Secondly, the changes in solubility, as well as the chemical and structural changes that occurred when the raw materials were subjected to firing, were studied. Finally, the efflorescences of a group of unfired and fired ceramic roofing tiles were characterised, in order to relate them to the quantity and type of soluble salts contained. The study concludes that it is necessary to conduct solubility tests on fired samples under real conditions, since important structural changes occur in the firing process, which lead to considerable variations in the soluble salts. Furthermore, in addition to analysing Na(I), K(I), and Mg(II) as indicated in the standard, it is advisable to determine Ca(II) and SO(4)(2-), since these are the major components in the efflorescences.