Temperature-dependent sorption models for mass transfer throughout bio-based building materials

Abstract

Hemp and rape straw concretes are bio-based building materials with excellent hygrothermal behaviour and low environmental impact compared to conventional concretes. Their hygroscopic capacity can improve indoor comfort by regulating temperature and relative humidity levels. This buffering ability can be expressed by sorption curves which correlate moisture content at different relative humiditiy levels. However, sorption isotherms vary according to temperature revealing the importance of taking into account heat transfer in sorption phenomena. Therefore, aiming to accurately model sorption curves considering temperature and hysteresis influence, this study compares four different temperature-dependent models (Poyet, Milly, Staudt and modified GAB) to better describe the hygroscopic behaviour of bio-based materials. Results show that, when compared to experimental values, the modified GAB model exhibited the best statistical agreement. Poyet and Milly remained coherent with empirical results, nevertheless, Staudt’s approach is far from experimental data. The isosteric heat of sorption and the theory of compensation are used to shed a new light on results discrepancy. This work also points out that the robustness of analysis is highly related to the initial parameters of accuracy given by GAB fitting.