Silica Fume Rich, Low-Cement Mixes to Extend the Applications and Sustainability of Hempcrete

Building-related operations, such as heating and cooling, account for 28% of anthropogenic carbon emissions, alongside the carbon cost of the materials [1]. Hempcrete, a carbon-neutral to carbon-negative insulation material consisting of hemp hurd, lime, and potentially other binder materials, offers a sustainable alternative but has poor load-bearing properties. Including cement and increasing density through compaction can improve strength but reduce bulk porosity, which lowers thermal resistivity and increases the material requirement to achieve the same insulation level. Higher cement content also elevates embodied carbon, reducing hempcrete's sustainability. This research aims to enhance hempcrete's utility by using microbially induced calcium carbonate precipitation (MICP) and examining how cement additives, silica fume and metakaolin improve microbial viability. Incorporating MICP-capable bacteria can potentially increase hempcrete's strength while preserving its thermal properties through targeted biomineralization, expanding its applications without compromising its insulation properties and sustainability. However, modifications need to be made to hempcrete paste to promote microbial viability, such as reduced pH.<br/><br/>This study aimed to investigate the impacts of commonly available, sustainable fillers on paste pH and strength to improve the properties of hempcrete and create environments more suitable for microbial viability (i.e., lower pH). Silica fume, a byproduct of the ferrosilicon industry, and metakaolin, a pozzolana, were chosen for their potential to reduce pH by decreasing free portlandite and increasing the relative amount of calcium silicate hydrate. Central composite designs of experiments were used to determine silica fume's and metakaolin's impact on hempcrete paste pH and strength. We found that increased cement replacement with silica fume and metakaolin reduced the pH of hempcrete paste, reaching as low as ~10 at the highest replacement levels. Silica fume was the primary driver of pH reduction. Higher cement replacement inversely affected strength. Our results demonstrate that increased silica fume replacement decreased strength while incorporating metakaolin slightly improved strength in high silica fume replacement mixes. Additionally, we used the most probable number (MPN) method to assess the viability of the MICP-capable bacterium <i>Sporosarcina pasteurii</i> in cement and low pH mixes both immediately and 24 hours after mixing. Our preliminary results showed increased bacterial viability immediately after mixing, rising from ~10^4 MPN/g paste in ordinary Portland cement (OPC) to ~10^6 MPN/g paste in high silica fume mixes. This is significant because high pH, temperature, and nutrient depletion challenge bacterial viability in cementitious materials, and previous methods to protect microbes, such as expanded clay and glass beads, have been costly and only moderately successful. Increased viability was not observed at 24 hours after mixing compared with OPC, demonstrating that other stressors still impact microbial viability in these pastes.<br/><br/>These results are significant for several reasons, including (1) generating hempcrete where sustainable fillers replace significant amounts (up to 80%) of cement, (2) improving early microbial viability in paste compared with OPC, and (3) producing building materials with less potential for leaching of high-pH fluid to the surrounding environments. The next steps are to explore further how cement additives influence microbes' metabolic and biomineralization activities.<br/><br/>[1] Abergel T, et al., Global Status Report, 2018