Thermal conductivity of traditional ceramics. Part I: Influence of bulk density and firing temperature

The thermal conductivity of traditional ceramics is known to be a function of their porosity or bulk density. However, the scatter in the thermal conductivity–bulk density data in certain studies, particularly when data from industrially processed brick are involved, suggests that thermal conductivity depends, apart from porosity, on other characteristics such as phase composition, microstructure, humidity or the presence of soluble salts. A red-firing clay used in brick manufacture has been used in this study with a view to systematising the impact of the different variables that could influence thermal conductivity and mechanical strength. This first paper presents the results obtained when the dry bulk density of the pieces and their firing temperature were modified. It was confirmed that thermal conductivity did not solely depend on the total porosity of the fired pieces, but that pore size distribution and pore interconnectivity need to be considered, which depend on the degree of firing attained, and may also be influenced by the phases present in the pieces, whose content and/or composition could vary with firing temperature.