Hempcrete is low carbon footprint building material, which consists in a mix of a cement and/or lime binder and vegetal particles (shiv). Its high porosity and low density give it good insulation, hygrothermal and acoustic properties. On the other hand, the mechanical strength of hempcrete is very poor, which limited its used like a filling material in a load bearing structure. Its limited mechanical properties are due, among other things, to an Interfacial Transition Zone (ITZ) around the shiv, which is not fully hydrated like the rest of the matrix. Two phenomena are assumed to be responsible for the formation of ITZ: a lack of water due to the high water absorption rate of hemp and/or a diffusion of sugar and other molecules from the vegetal to the cement paste.
In order to evaluate the contribution of sugar and water absorption in the formation of ITZ, tests were conducted on hemp pellets, as a reference, and on polymer foam pellets with a water absorption capacity close to that of hemp. The latter were also impregnated with saccharose solution at different concentrations in order to visualize the effect of sugar release on ITZ formation. Two types of cements were also used, a Portland cement and a Prompt cement, in order to evaluate the impact of the binder's setting kinetics on ITZ. For this purpose, a new experimental test protocol, based on image analysis, was also developed to achieve repetitive and robust visual observations of the formation of ITZ, by the present of a halo around the pellet. This method makes it possible to follow the evolution of ITZ surface over time and thus to determine its kinetics.
Investigations on polymer foam pellets demonstrate that the water absorption is not the main cause responsible for the halo. On the other hand, the foam pellet soaked in saccharose solution generated halo even at very low concentrations. A correlation between the concentration in saccharose and the surface of the halo was underlined. This shows that the halo is mostly due a diffusion of sugar and other molecules from the pellet into the cement paste. The use of Prompt cement seems to delay the formation of the halo compared to Portland cement.
In the long term, the objective is to extend these tests to different binders and vegetal species in order to establish the most relevant binder/vegetal combinations for the development of plant-based concrete.
This work was supported by the ANR BIO-UP project of the French Agence Nationale de la Recherche (ANR-21-CE22-0009).