BACKGROUND: This paper discusses the formation of thaumasite in the mortar sub-flooring used as a base material for laying tiles, and the influence of this mineral in relation to deterioration of building materials. An investigation of a mortar sub-flooring constructed with dolomitic limestone sand with high sulfate content used to settle a ceramic pavement was carried out. RESULTS: The mortar sub-flooring underwent an important deterioration process due to the formation of expansive compounds in the environmental conditions of temperature and humidity of the installation area in Murcia (Spain). Expansion appeared essentially at the interface between the ceramic tiles and the mortar sub-flooring, an area consisting primarily of cement paste. CONCLUSION: Materials present in the soft pulpy mass were examined by X-ray diffraction and scanning electron microscopy with energy-dispersive X-rays. The presence of thaumasite was detected, which had been formed under conditions of temperature higher than temperatures reported in the literature and with a high degree of humidity. (C) 2008 Society of Chemical Industry