Alternative binders to improve hempcrete material properties and microbial viability

Abstract

Conventional insulation materials have high embodied energy and carbon. Hempcrete is a carbon-neutral to carbon-negative insulation material commonly made from hemp hurd and lime and offers a sustainable alternative to conventional insulation materials. Hempcrete achieves its low carbon footprint through carbon sequestration during hemp growth and binder carbonation over its material lifespan. However, hempcrete's low compressive strength limits its utilization primarily to wall infill insulation. Improving hempcrete's low compressive strength could expand its utility and durability. Efforts to improve the strength of hempcrete have included using mechanical compaction and a reduction in hurd content; however, while these methods can improve the compressive strength of hempcrete, they increase the thermal conductivity. Increased thermal conductivity increases the material requirements needed to achieve the same level of insulation. Additionally, partial replacement of lime with ordinary portland cement (OPC) can increase the compressive strength of hempcrete. However, the inclusion of OPC reduces hempcrete sustainability due to the high embodied carbon of cement and the reduction in the binder carbonation potential. Pozzolanic materials like silica fume, metakaolin, fly ash, etc., could confer strength benefits to hempcrete. Additionally, it may be beneficial to leverage microbial functionalities such as biomineralization, crack healing, or environmental sensing in hempcrete by creating "living building materials." This work investigated how hempcrete binders rich in silica fume, an industrial byproduct, and metakaolin affect hempcrete's pH, microbial viability, and strength. High silica fume ~39% and moderate metakaolin ~8% binder compositions generated hempcrete with strengths comparable to hempcrete with higher OPC content. These results signify that silica fume can strengthen hempcrete without diminishing its sustainability. Additionally, high silica fume content decreased the pH of both pastes and hempcrete. Reducing pH is relevant for microbial inclusion, as pH has been identified as an environmental stressor affecting microbial viability in cementitious materials. For very high silica fume mixes, which have the lowest pH, microorganisms had higher viability in paste samples immediately after mixing, compared to pastes with higher lime or OPC content. However, the viability at 24 hours was not maintained in paste samples compared to hempcrete, which maintained viability at 24 hours, demonstrating that other factors beyond pH continue to challenge microbial viability in these materials. Future work within this area should evaluate how these additives impact hempcrete's thermal conductivity and sustainability. It would also be beneficial to investigate other methods for incorporating microbes into this material and determine how microbes contribute to the observed strength differences, i.e., through biomineralization or other mechanisms.