Fractal Nature of Cementitious Systems Exposed to Sustained Elevated Temperatures

This paper quantifies the air-void network in cement-based systems and relates the same to changes within, as resulting from exposure to sustained elevated temperatures. Using the Transient Plane Source technique, the thermal conductivity and specific heat capacity were evaluated for a series of plane and fibre reinforced mortars. Polymeric microfibres were introduced at up to 0.2% by volume. Specimens made from these mortars were subjected to elevated temperatures up to 500 °C, and their thermal constants were evaluated in situ. At each sustained temperature, the fractal dimension of the pore phase was evaluated from backscattered electron (BSE) imaging, and is seen to increase with an increase in the temperature of exposure. As well, the thermal conductivity decreased with an increase in the fractal dimension. The pore sizes were evaluated in the range of 0.5 to 100 nm, based on the well-known nitrogen adsorption porosimetry. There was an increase in the mean pore size of all systems when exposed to elevated temperatures. These findings agree well with a predictive model for the thermal conductivity of cement-based mortars that combines a rule-of-mixtures approach for the solid phases with the fractal dimension of the pore phases.