Influence of swelling and non-swelling clays on the thermal properties of grouting materials for borehole heat exchangers

Abstract

Grouting materials for borehole heat exchangers often contain rheological additives. Typically, swelling clays are added to benefit from their thixotropic behavior, helping to adjust the stability and flowability of the grouting suspension. Clay compounds without swelling clay minerals but with similar control effects in case of suspension stability and flowability due to their surface charges and structure are usually not used specifically in such geotechnical applications. However, it is conceivable that the thermal conductivity of a water-saturated grout might be higher with non-swelling clays than with swelling clays due to the higher solid phase thermal conductivity of non-swelling clay minerals. In this work, thermophysical, hydraulic and mechanical properties of geothermal grouts containing swelling or non-swelling clays were investigated. It was aimed not to negatively affect the important grout properties despite the replacement of swelling clays by non-swelling clays. The results of this laboratory study showed an increase in thermal conductivity of 5% by substituting swelling clays with non-swelling clays and can be an approach to further optimize grouting materials.