Thermal properties of cement-based composites.

Abstract

Heat transfer through a solid building element is primarily controlled by its thermal conductivity in the steady-state, and by an association of thermal conductivity and specific heat in transient situations. The thermal properties of the construction materials are some of the main responsible factors for the habitability and energy demand of a building. Therefore, the present work assesses how the chemical composition, physical properties and microstructure influence the thermal properties (specific heat and thermal conductivity) of cement-based composites. For this purpose, we investigated three mortar mixes with cement and hydrated lime (1:3, 1:1:6 and 1:2:9); three types of aggregates (river sand, iron ore tailings, friable quartzite); and three dosages of air-entraining admixture (0%, 0.05% and 0.5%). Physical and chemical characterisation of the aggregates were performed; along with physical, thermal and microstructural evaluation of the resulting mortars. Lastly, the correlation between thermal conductivity and ultrasonic pulse velocity (UPV) was also investigated. As result, mortars with iron ore tailings (IOTm) presented the lowest thermal conductivity and highest specific heat among all materials tested. The air-entraining admixture reduced the thermal conductivity of the mortars up to 40% but did not significantly influenced the specific heat. It was observed that the microstructure of the resulting matrix is more relevant to the thermal properties of mortars than the chemical composition of their components. Also, the pore system generated by the aggregates in the mortars is more influential to their thermal conductivity and UPV than their specific gravity. A satisfactory determination coefficient (R² > 0.9) was obtained between thermal conductivity and UPV of the mortars when they were grouped under similar components and pore structure. These results demonstrate the importance of using methodologies that keep the microstructure of the matrices mostly intact. Based on the thermal properties obtained, residue-based mortars are promising alternatives to improve the energy efficiency of buildings. In conclusion, this work hopes to contribute to the technological and sustainable development of cement-based composites.