Synergistic effect of expanded graphite-silane functionalized silica as a hybrid additive in improving the thermal conductivity of cementitious grouts with controllablewater uptake

• 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.

Nyckelord

Cement-based composites

Cementitious grout

Expanded graphite

Ground source heat exchangers

Hybridization

Silanization

Thermal conductivity

Additives

Boreholes

Coupling agents

Geothermal energy

Graphite

Grouting

Mixtures

Mortar

Silanes

Silica

Borehole heat exchangers

Formulation development

Functionalized silica

Geothermal applications

Grouting materials

Optimization studies

Silane coupling agent

Synergistic effect

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