Recycled additions for improving the thermal conductivity of concrete in preparing energy storage systems

Two types of concretes were prepared in order to build thermal storage units for solar plants having as primary aim to improve thermal conductivity. The first type consists of concrete for casting on site (A), whereas the second for moulding upon vibration (B). Samples of both typologies were prepared changing type of additions or aggregates. The use of recycled materials into concrete (e.g. polyamide fibres from post-consumer textile carpet waste, metallic powders or shavings and steel fibres) was investigated. Fibre-reinforced concretes were tougher (up to 300%) than ordinary ones. All the concretes show high thermal conductivity and are good candidates for an efficient thermal storage unit, but the performances of type B concretes are better than those of type A. Moreover, the morphology of type B concretes appears compact and less cracked, even after thermal treatment at temperature higher than 300 °C. The thermal conductivity of the mix containing polyamide fibres and metallic shavings was 2.74 and 2.13 W/m °C, before and after a thermal treatment of 4 h at 300 °C, respectively.