Facile synthesis of graphite-and graphene-based hybrid additives by silanization and effects of these additives on the thermal conductivity of cementitious grouts used in shallow geothermal systems

Official URL: https://risc01.sabanciuniv.edu/record=b2553612 _(Table of contents)

The thermal conductivity of grout backfilling boreholes and pipes has been considered as an important issue for the improvement of the efficiency of shallow geothermal systems. Especially preserving the heat through the boreholes in the ground without temperature difference is achieved by formulating grout composition having high thermal conductivity. In this thesis, the main objective is to develop hybrid silica-carbon additives to enhance the thermal conductivity of the grout and thus increase the effectiveness of the heat transmission and prevent the aggregation of treated hybrid additives in grout mixture. Three main materials, graphene from waste tire, expanded graphite and rice husk ash, were hybridized by using silane coupling agents and building chemical bridges with silica particles in grout composites. This functionalization provided to enhance the dispersion and solubility of carbon materials and adjust their water uptake during grout mixing since there is a close relation between water demand and thermal conductivity. According to optimization study on the formulation development of grout mixtures, as the amount of graphene-based hybrid additive increased from 3 to 5 wt%, water uptake increased from 660 to 725 g resulting in the reduction of thermal conductivity by 20.6%. Furthermore, the highest thermal conductivity of 2.656 W/mK was achieved by adding 5 wt% expanded graphite-based hybrid additive compared to reference grout having thermal conductivity of 2.373 W/mK. Consequently, this study shows noticeable potential of hybrid additives produced from virgin and recycled sources to be used in the grouts of geothermal heat-exchange boreholes. This research will be more discernible since renewable energy sources come into prominence by ever-increasing energy-demand and global pollution.