Towards Sustainable Construction: Innovations in Hemp-Lime Composite Production for Carbon-Negative Building Materials

Abstract

Hemp-lime composites (HLC) have captured significant attention in the construction industry due to their sustainability, availability, and excellent hygrothermal performance. These bio-based insulating materials are produced by mixing a lime-based binder with hemp shivs and water. Despite their positive attributes, the properties and performance of the resulting composite depend on various parameters such as the ratio of constituent materials, manufacturing method, binder type, and shiv characteristics (e.g., particle size and purity). Therefore, its widespread adoption has been impeded by the notable variation in hemp-lime formulation and manufacturing methods, leading to inconsistent performance. In this research, new methods are proposed to enhance the homogeneity and uniformity of hemp-lime composites while minimizing their environmental impact to produce a carbon-negative insulating material with predictable and reproducible properties. The main objectives of this research are: 1) to improve the consistency and repeatability of hemp lime composites performance by reducing the hemp shiv particle size and introducing vibration as a compaction method; 2) to investigate the effect of impurities (e.g., hemp fibres) on the mechanical and hygrothermal properties of hemp lime composites; and 3) to investigate the effectiveness of producing hydrated lime from different CaCO₃ sources, including organic waste, using an electrochemical decarbonation process to formulate carbon-negative hemp lime composites for sustainable construction.

In accordance with the above objectives, modifications are proposed to the hemp shiv particle size, binder source, and placement method used to make hemp-lime composites. The experimental results demonstrate a marked improvement in hygrothermal properties compared to values reported in available literature, with thermal conductivity in the range of 0.053-0.060 W/m K and moisture buffering values between 2.21 to 2.53 g/m² RH, as well as a very low coefficient of variation of dry density between 0.37-0.73%. The results also demonstrate a noteworthy reduction in the directional difference of thermal conductivity, reduced to less than 3% when employing fine hemp fragments. Furthermore, using finer particles reduces the amount of binder needed which significantly lowers the carbon footprint when using conventional hydrated lime produced through calcination. An alternative process is also introduced in which hydrated lime is produced through electrolysis, which facilitates direct carbon capture. This process is evaluated using three different limestone sources and two organic sources (mussel and eggshells) as feedstock. The precipitated materials are mainly comprised of Ca(OH)₂ with a suitable chemical composition and particle size distribution required for Type N hydrated lime. The environmental assessment of wall assemblies revealed that hemp-lime composites incorporating electrochemically decarbonated hydrated lime (ED-HLC) offer comparable thermal and moisture performance to conventional binders while achieving superior mechanical strength. Life-cycle analysis demonstrated a markedly lower embodied footprint and a carbon-negative balance, reaching -23.1 kg CO₂ eq. when carbon sequestration was included. Compared to conventional lime-based hemp-lime composites and glass wool insulation, the ED-HLC wall showed 15.7% and 457% lower global warming potentials, respectively, confirming its strong potential as a carbon-conscious insulation solution.