Water Transport in Bio-based Porous Materials: A Model of Local Kinetics of Sorption—Application to Three Hemp Concretes

Abstract

The classic models describing the hygric mass transfers inside porous materials seem unsuitable in the case of bio-based materials. They are based on the assumption of instantaneous local equilibrium between relative humidity and water content (Künzel 1995). These two parameters evolve according to the diffusive fluxes following the sorption isotherms. This study shows that it leads to predict much shorter times of stabilization than those experimentally obtained. A new approach is presented here, it frees from the local instantaneous equilibrium introducing a local kinetics to describe the transformation of water from vapor state to liquid state and vice versa. The local kinetics of sorption is coupled with the well-known hysteresis phenomenon. It is adjusted from bibliographic data (Collet et al. 2013) giving mass evolution of three hemp concrete under adsorption / desorption conditions. 1D cylindrical simulations allows an excellent fitting on the experiments. Finally, a semiempirical model is proposed, allowing to determine the kinetics parameters more easily.

Hygrothermal effects and moisture kinetics in a bio-based multi-layered wall: Experimental and numerical studies

“…As demonstrated and validated in the study of Reuge et al [9] based on three different hemp concretes, the following expression for the sorption rate seems appropriate for the bio-based materials:…”

“…Note that the coupling of eqs (8,9) with a hysteretic model (e.g. [4]) can easily be done: the reversal points (i.e.…”

“…Note that heat and mass transfer phenomena have to be properly modeled and coupled, as highlighted in [5]: heat transfers in walls can have a significant effect on the hygric transfers and inversely. But despite the reliability of these models, it seems that inside some porous materials, and particularly in bio-based materials, simulations largely underestimate the time required for the water content to reach the equilibrium: this has been established for bio-based materials in [6,7,8] and by Reuge et al [9], but also for more traditional material such as cement compounds or concretes [10,11,12]. These considerations take on their full meaning in the cases of hygrothermal transfers with permanent fluctuations such as in materials of building envelopes, in soils or in food materials.…”

“…Classic simulation tools predicting hygrothermal transfers in building materials are based on the assumption that for a given local relative humidity ϕ, the associated equilibrium local water content w is reached instantaneously. In our previous studies [9,13], three hemp-lime concretes were studied and it was demonstrated that such an assumption led to serious inconsistencies. Thus, as reported in the aforementioned literature, our previous studies have established that a local kinetics of sorption exists (from water vapor to liquid water and inversely) which can be slow compared to the diffusive fluxes.…”

“…From the knowledge of these classic properties, a simple steady-state sensitivity study is performed to determine the most sensitive layers in the ISOBIO reference wall in term of thermal transmittance and resistance to water vapor diffusion. Then, the theoretical background necessary to model the water sorption / kinetics of sorption is presented, relying on the study [9]. Thanks to this theoretical background, the kinetics constants of some of the studied ISOBIO materials are determined numerically and analytically by adjustments on sorption experiments.…”