Effect of Carbonation on Long-Term Measurements of Sorption Isotherms of Autoclaved Aerated Concrete

The laboratory measurement of water vapour adsorption/desorption curves is a standard tool for determination of the moisture capacity of porous building materials in hygroscopic region. However, in case of cementitious materials, additional processes like carbonation can occur during the measurement. Generally, process of carbonation affects the pore structure of the material and results in an increase of its bulk density. Therefore, for determination of correct sorption curves the carbonation caused bulk density increase has to be eliminated. In this work the both mentioned effects of carbonation on the measured water vapour sorption isotherm were quantified for autoclaved aerated concrete (AAC). Within the monitored 9-year period, the carbonation caused noticeable changes in microstructure of the tested AAC samples, namely a decrease of the specific surface area of pores and the portion of very small pores. These microstructure changes affected the moisture storage capacity too – the obtained equilibrium moisture contents of the partly carbonated AAC were lower than the ones of the AAC close to the original state. It was also confirmed that in case of continuous measurement of the water vapour sorption curve the carbonation induced bulk density increase could result in an overestimation of the equilibrium moisture contents.