Synergistic Effect of Expanded Graphite-Silane Functionalized Silica as a Hybrid Additive in Improving the Thermal Conductivity of Cementitious Grouts with Controllable Water Uptake

Abstract

<jats:p>Recently, a growing demand for geothermal applications has led to the exploitation of energy efficiently by developing grouting materials in the borehole between pipes and the ground. Therefore, the current study developed newly formulated cementitious grouts by the integration of expanded graphite (EG)-based hybrid additives synthesized by building chemical bridges between silica particles and EG in the presence of amino functional silane coupling agents. These produced hybrid additives with controlled EG and silica ratios were utilized in grout mixtures used in borehole heat exchangers to enhance the thermal conductivity. According to the optimization study on the formulation development of grout mixtures with bentonite, silica sands, cement, and superplasticizer by adding neat EG and EG-based hybrids, the relationship between the carbon amount and water demand was found to have a significant impact on thermal conductivity. The highest thermal conductivity value of 2.656 W/mK was achieved by the incorporation of 5 wt% hybrid additive with the ratio silica/EG of 1:5 compared to the reference grout, which showed a thermal conductivity of 2.373 W/mK. Therefore, the enhancement in thermal conductivity was dependent on the increase in the EG content and also the additive loading ratio, resulting in a slight increase in the water demand.</jats:p>

Silanization of SiO2 Decorated Carbon Nanosheets from Rice Husk Ash and Its Effect on Workability and Hydration of Cement Grouts

“…At the loading of 10 wt% f-RHA, a sharp loss in thermal conductivity was detected which was consistent with 10 wt% RHA based grout. This also supports that there is a direct relation between thermal conductivity and the amount of water uptake in the grout [ 28 , 29 ]. Furthermore, f-RHA becomes more hydrophilic after the functionalization by APTES and retains more water during the hydration of grout.…”

“…The standard mud balance technique was used to measure the density of the grout sample. Further details regarding the preparation and testing for cementitious grout composites can be found in our previous studies [ 28 , 29 ].…”

“…In our previous studies, thermal conductivity properties of cementitious grout were improved by the incorporation of graphene nanoplatelet/silica and expanded graphite/silica additives in which carbon materials were connected to silica in the presence of silane coupling agents [ 28 , 29 ]. This study focuses on the hydration kinetics and workability of cement paste prepared by the incorporation of 3-aminopropyltriethoxysilane (APTES) treated RHA and also its effect on thermal conductivity.…”

Application and Design Aspects of Ground Heat Exchangers

“…In combination with silica hybrid additive, used to enhance the solubility and dispersion of carbon material, at the 5 wt% of hybrid-graphene, thermal conductivity increased to 1.82 and 2.34 W m −1 K −1 for the Si:graphene ratios of 1:5 and 1:10, respectively. In a study by Berktas et al [138], expanded graphite base additives made by the synthesis of silica particles and expanded graphite were added to cementitious grout. With 5 wt% of additives in the mixture and functionalized silica aqueous solution to expanded graphite ratio of 1:5, the thermal conductivity of 2656 W m −1 K −1 was recorded.…”

Research on Fresh and Hardened Sealing Slurries with the Addition of Magnesium Regarding Thermal Conductivity for Energy Piles and Borehole Heat Exchangers

“…Appropriately designing the slurry is not easy, it requires long and meticulous research in terms of both the additives used and the ingredients' proportions. The current state of the art includes the use of ready-made industrial mixtures, as well as cement with additives such as graphite, as sealing slurries for borehole heat exchangers [66][67][68]. According to the manufacturers, the thermal conductivity of ready-made industrial mixtures is approx.…”