Mathematical Description of Sorption Isotherms of Autoclaved Aerated Concrete

Abstract

The article presents the results of measurements and developed on their basis sorption isotherms with mathematical description of the graphs obtained. The research was conducted on samples of three kinds of autoclaved aerated concretes with similar bulk density of approximately 600kg/m3, but provided by different manufacturers. The tested concretes were also different by composition of raw materials. One of the examined concretes had fly ash in its composition. The moisture sorption studies were conducted at temperature 25oC and at six relative humidity levels φ ≈ 11, 33, 53, 75, 84, 97%. The levels of temperature and relative humidity conditions correspond to a typical temperature/humidity conditions in which in practice commonly used autoclaved aerated concretes can be found. The moisture sorption content was obtained after several months of measurements at the equilibrium state. This data was then used to prepare sorption isotherms of the tested materials. The experimentally obtained data were then applied to calculate mathematical description of graphs. For this purpose three commonly known and used equations were applied: equation of Chen Y. and Chen Z., equation of Jovanovič’s for multi-layer adsorption, and Hüttig's equation. Using statistical methods the compliance of the adsorption isotherm equations was compared with experimental data. For that purpose the STATISTICA programme was used. To estimate the coefficients of the equations the authors used one of the programme modules, i.e. the non-linear estimation. To assess how well the data fit a statistical model the coefficient of determination R2 was used.