Recarbonation matters—CO₂ absorption of AAC during its use phase

Rebounding of CO₂ during service life and recycling in building materials reduces the CO₂ content of the atmosphere. The CO₂ was formerly released from limestone through deacidification during the production of cement and quicklime. Binder‐bound building materials later reabsorb this CO₂ again. This process is called recarbonation. In contrast to concrete, detailed knowledge about the CO₂ cycles of autoclaved aerated concrete (AAC) is missing. Standards concerning adequate sampling and measurement methods as well as suitable calculation methods are currently under development.

According to the valid EN 15804:2012+A2:2019, life cycle assessments and environmental product declarations (EPDs) concern the entire product life cycle and thus also the CO₂ reabsorption by building materials. For AAC, this amount can be calculated from the raw material input. The reduction bases on the active CaO contained in the AAC, is absolute and related to the binder mass used. Thus, it can be applied comprehensively and without the influence of the used database system of the background data in EPDs.

Unfortunately, the amount of originally active CaO cannot be measured directly in AAC samples. As a first approximation, it was obvious to assume that all chemically determinable CaO in AAC was formerly active. However, this method underestimates the degree of recarbonation because the raw materials also contain non‐reactive CaO, for example, in the form of calcium silicates. Nevertheless, all diagrams concerning recarbonation of AAC are based on this assumption and the values are too low.

In the presentation, we will introduce a new method to determine the degree of recarbonation of AAC—forced carbonation. Forced carbonation is carried out under defined temperature, humidity, and CO₂ conditions in a climate chamber. The measurement is bases on the detection of CO₂ in the sample and considers the proportion of non‐reactive CaO and thus leads to realistic degrees of recarbonation.