Integrative Development of Mechanical Strength and Thermoelectric Properties in Cement Composites Incorporating Reduced Graphene Oxide and Manganese Dioxide

Buildings constantly consume energy to maintain internal temperature, and the thermal difference between the interior and exterior of a wall panel provides an opportunity for energy generation through the Seebeck effect. Due to their strength and durability, cement-based thermoelectric materials are gaining interest among materials scientists for their potential applications in thermal energy harvesting in buildings. Though thermoelectric cementitious materials have been developed, there has been little research into the mechanical properties of these materials with the addition of conductive fillers. This study investigates the incorporation of the carbon-based additive, reduced graphene oxide (rGO) and the transition metal oxide, manganese dioxide (MnO2) to improve the thermoelectric properties of cement composites while maintaining adequate compressive strength. In order to test both thermoelectric and mechanical properties with the same sample, cube samples measuring 50x50x50 mm were prepared instead of the typical cuboid shape samples. The Seebeck effect was assessed by heating one side of each sample to 65°C using a hotplate and recording the temperature and generated voltage. The same samples were then used for compressive strength testing, and the samples with the best properties were then subjected to microstructural analysis. The findings indicate that while both rGO and MnO2 enhance the thermoelectric properties of cement, their reactions with the cement phases produce distinct relationships with compressive strength specially when rGO and MnO2 added together. It was found that increasing rGO from 0.025 to 0.075 wt.% decreased compressive strength as workability was reduced, whereas increasing MnO2 from 2.5% to 7.5% wt.% increased compressive strength by more than 60% as workability improved. Thermoelectric properties were best in the composite sample with 0.075 wt.% rGO + 7.5% wt.% MnO2. To evaluate the integrative properties a new index called “Thermoelectric Strength Index (TSI)” is developed, which showed MnO2 is better in the integrative development of mechanical strength and thermoelectric properties in cement composites with 7.5 wt.% giving the best results.