NMR and MRI observation of water absorption/uptake in hemp shives used for hemp concrete

Abstract : Hemp concrete used as a construction material is known to provide excellent 15 thermal insulation and hydric regulation, and prevents condensation. Initial water content in 16 the hemp and water exchanges between hemp and binder play a major role in these processes. 17 Here we study how hemp absorbs liquid water. In that aim we rely on 1 H NMR (Nuclear 18 Magnetic Resonance) measurements which make it possible to distinguish water situated 19 inside from water situated outside the hemp. Then, following the evolution of the distribution 20 of NMR relaxation times we are able to quantify the effective amount of water entering the 21 hemp as a function of time. We show that such a measure is better controlled than usual 22 techniques such as weighing an immersed sample or the TC (Technical Committee) RILEM 23 (Réunion Internationale des Laboratoires et Experts des Matériaux, systèmes de construction 24 et ouvrages) method which cannot easily distinguish water inside hemp from water situated 25 outside or wetting the external hemp surface. The water absorption in hemp occurs in two 26 steps: about half the water enters the material in a time of the order of a minute, while the 27 second half of water slowly penetrates over a time of the order of three days. Finally, from 1 H 28 micro NMR imaging we show that the first step corresponds to water entering the pith while 29 the second one corresponds to water diffusing in the wood part of the shiv. 30 31 32