The compressive strength of earth-hemp blocks tested with different densities, earth types, and cementitious binders

The building sector needs to shift toward the use of materials that have low-embodied energy, minimize operational-energy, and minimize the amount of waste upon disposal. Here, we report on a series of experiments on low-density earth-hemp blocks, which can be implemented as an insulation for buildings. Earth-hemp finds a similar usage to hempcrete/hemp-lime, yet the use of raw earth as a binder allows to dramatically decrease the embodied energy. The set presented here evidences that pure earth-hemp with high content in clay minerals reaches higher compressive strength (0,33 MPa) than equivalents with hydraulic binder, for similar thermal conductivity (0,07 W/m.K).

Earth-hemp samples were characterized in terms of compressive strength in order to test the influence of density, earth type, incorporation of mineral additives, and amount of water used for creating the blocks. Two types of natural earths were investigated, which differ in their clay content: a surficial loess with 25 wt.% clay minerals and a quarried paleosoil with high clay content (65 wt.%). For each earth type, 4 types of mineral additives were investigated in order to test whether they can have a stabilizing effect: Portland cement, aerial-lime, gypsum-plaster and a MgO-based cement. The binders (i.e. earth + additive) were created with replacement of earth by mineral additives at 0, 4, 8, and 20 wt. %. For each type of binder, 3 densities of the resulting earth-hemp samples were produced (250, 280, 340 kg/m3). Additionally, two series of this set of samples were produced using a low amount of added water (150 wt.% water/hemp) and high amount of added water (370 wt.% water/hemp).

Samples using the earth with high clay content have compressive strengths up to twice as high as those with low clay content. This result is expected since clay minerals are the main agent of binding in earth materials. Also expected was the increase in compressive strength with sample density, which is directly correlated to the amount of binder. More interestingly, the dataset also exhibits the negative effect of mineral additives: a trend of decreasing compressive strength with amount of incorporated mineral additive is visible, independently of the type of additive and earth type. In between additive types, the compressive strengths of samples mixed with MgO-based cement and gypsum-plaster are better than those mixed with Portland cement and aerial lime. Additionally, samples produced using a low amount of added water are much less resistant than those with a high amount of added water for every sample tested. Finally, samples using pure earth with high clay content and high amount of incorporated water are the most resistant, and reach compressive strengths of 0.33 MPa for a density of 340 kg/m3, which is slightly stronger than existing commercial lime-hemp blocks.