Assessment of Moisture Influence on the Thermal Properties of Autoclaved Aerated Concrete of Four Density Classes

Autoclaved aerated concrete (AAC) is currently one of the most popular building materials used for the construction of external walls in modern buildings. Current global market forecasts indicate that the value of the AAC sector will increase from USD 28.91 billion in 2026 to USD 50.04 billion by 2034. This trend coincides with the progressively more stringent thermal performance requirements imposed on external building envelopes. In response to these trends, a comprehensive investigation was undertaken to determine the basic thermal parameters of the most commonly used aerated concretes with densities ranging from 400 to 700 kg/m3. The values of these parameters primarily depend on the pore structure of these materials, but even more so on their moisture content. For the studies, AAC400, AAC500, AAC600, and AAC700 aerated concretes from the same manufacturer and with the same raw-material composition were used. For each of the four selected concretes, initial microstructural tests were carried out to obtain detailed data on pore distribution in each of the tested materials. In the main experiment, first, prismatic specimens of all four classes were subjected to controlled capillary absorption, which led to varying moisture distribution. After one month of capillary uptake, these specimens were cut along their height into 6 component samples. Then, on these component samples, with different moisture levels, thermal parameters were measured: the thermal conductivity coefficient λ and the volumetric heat capacity cv. The measurements were done using a non-stationary technique with the ISOMET 2104 apparatus. Immediately after testing, the samples were dried to determine the moisture level of each sample: their mass moisture content (Um) and volumetric moisture content (Uv). The results of the conducted studies demonstrate that even a moderate increase in moisture content leads to a substantial rise in both thermal conductivity λ and volumetric heat capacity cv for all tested materials (AAC400, AAC500, AAC600, AAC700). This confirms the necessity of incorporating moisture effects into calculations related to both the thermal insulation performance and the thermal dynamics of AAC-based building envelopes. The study shows that these relationships can be effectively described using the proposed linear functions, which provide high accuracy for λ(Uv) and λ(Um) with R2 ≥ 0.99, as well as for cv(Uv) and cv(Um) with R2 ≥ 0.92.