Thermal conductivity of traditional ceramics: Part II: Influence of mineralogical composition

The thermal conductivity of traditional ceramic materials 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 mineralogical composition, microstructure, humidity, and the presence of soluble salts. A standard 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. Part I of the study presented the results obtained when the dry bulk density of the pieces and their firing temperature were modified. Part II examines the influence of the mineralogical composition of the starting raw materials mixture on the thermal conductivity and mechanical strength of clay brick products. The findings suggest that to manufacture traditional ceramics with high thermal insulation and appropriate mechanical properties, it is advisable to use illitic-kaolinitic clays. Large-sized potassium feldspar and quartz particles adversely affect fired mechanical strength. In addition, quartz has high thermal conductivity. The addition of carbonates or the use of calcareous clays has a positive effect on mechanical strength, because carbonate acts as a pore-forming agent and generates crystalline phases during firing that enhance mechanical strength.