Forced carbonation – A new method to determine the degree of recarbonation of calcium silicate units

Rebound effects in building materials reduce the CO₂ content of the atmosphere. This reabsorption of CO₂ during service life and recycling is called recarbonation. In contrast to concrete, detailed knowledge about the CO₂ cycles of autoclaved aerated concrete and calcium silicate units is missing. Moreover, standards concerning adequate sampling methods as well as suitable calculation methods do not exist yet.

Autoclaved aerated concrete is a highly porous material and recarbonizes evenly. Hence, samples for examination can be drilled out. In contrast, calcium silicate displays lower porosity and decreased CO₂ permeability. Therefore, its recarbonation starts on the surface and penetrates inwards. The units are inhomogeneously recarbonized. Consequently, bulk samples – not drill samples – must be taken.

In addition, calcium silicate contains considerable amounts of CaO bound in silicates. For this reason, the degree of recarbonation must not be calculated on the basis of the total CaO and CO₂ content. The newly developed calculations are based on the maximum CO₂ uptake determined using forced carbonation. It is carried out under defined temperature, humidity and CO₂ conditions in a climate chamber. Thanks to this method, the degree of recarbonation of calcium silicate can be determined more precisely.