Sorption and Desorption Isotherms of Lightweight Alkali-Activated Materials Modified with Silica Aerogel

The moisture content in a building material has a negative impact on its technical parameters. This problem applies in particular to highly porous materials, including those based on aerogel. This paper presents moisture tests on a new generation of alkali-activated materials (AAMs) with different aerogel contents. Silica aerogel particles were used as a partial replacement for the lightweight sintered fly ash-based aggregate at levels of 10, 20, and 30 vol%. The experiment included four formulations: R0 (without the addition of aerogel) and the recipes R1, R2, and R3, with an increasing content of this additive. The level at which moisture stabilizes in a material in contact with the environment of a given humidity and temperature depends on whether the equilibrium state is reached in the process of moisture absorption by a dry material or in the process of the drying out of a wet material. The equilibrium states achieved in these processes are described by sorption and desorption isotherms, determined at a given temperature, but at different levels of relative humidity. The SSS (saturation salt solution) method has been used for years to determine them. Unfortunately, measurements carried out using this method are difficult and highly time-consuming. For this reason, a more accurate and faster DVS (dynamic vapor sorption) method was used in this study of R0-R3 composites. The research program assumed 10 step changes in humidity in the sorption processes and 10 step changes in humidity in the desorption processes. As a result, the course of the sorption and desorption isotherms of each of the four composites was accurately reproduced, and the hysteresis scale was assessed, which was most evident in the cases of the R0 composite (made without the addition of aerogel) and R1 composite (made with the lowest aerogel content). Studies have shown that the increased addition of aerogel resulted in an increase in the amount of water absorbed. This was true for all ten relative humidity levels tested. As a result, the highest values in the entire hygroscopic range were observed in the course of the sorption isotherm determined for the R3 composite with the highest aerogel content, and the lowest values were for the sorption isotherm of the R0 composite without the addition of aerogel.