Microstructure of autoclaved aerated concrete after several decades in service – SEM, EDX, and XRD analysis

Abstract

Uptake of CO ₂ into mineral building materials is linked to carbonic acid attack (CAA), i.e., carbonation. In general, the higher the porosity of building materials, the faster the CAA will proceed. CAA can cause the destruction of initial Ca‐bonds and, therefore, changes in the mineral composition and microstructure of Ca‐rich building materials. From a thermodynamic point of view, the CAA on CH, CaCO ₃ , and CASH/CSH is spontaneous as all reactions have a negative reaction enthalpy. Given the high porosity, it is questionable whether AAC is durable under normal CO ₂ conditions. Therefore, in this study, a comprehensive microstructural characterization of AAC with different nominal densities and strengths from various buildings, i.e., different types of applications, with an age of up to 72 years, is conducted. The results are also discussed in view of the compressive strength of the AAC investigated. For the microstructural characterization, REM, EDX, and XRD analysis were performed. Based on the results, it is evident that CAA causes degradation of CASH/CSH minerals and the formation of a notable amount of CaCO ₃ in AAC, leading to a long‐term loss of strength for certain AAC types/applications.