Low-sulfate autoclaved aerated concrete (AAC): A recyclable AAC with calcined clay

Abstract

Autoclaved aerated concrete (AAC) is a construction material, which is mainly used for exterior walls thanks to its excellent thermal properties. Compared to concrete it has less than one-tenth the thermal conductivity. One main disadvantage is the high sulfate leaching of AAC that normally jeopardizes or even prevents its recyclability. This article introduces calcined clay (CT) as a measure to overcome this drawback and provide significant additional advantages. First, calcined clay is used as a replacement for the sulfate carrier in AAC and in a second step, it is used as supplementary cementitious material replacing up to 30 wt% of the cement in the AAC mixture. This stepwise replacement reduces the leachable sulfate by up to 75 % compared to reference samples. This permits for the first time the AAC to be used pure as recycled construction waste at the end of its lifetime according to German environmental regulations. The suitability of low-sulfate AAC products using calcined clay is verified in full-scale production and analyzed with respect to mechanical properties such as compressive strength and shrinkage, mineral composition and morphology as well as leaching behavior. The results show that the AAC made with calcined clay is suitable for the production of AAC blocks in the density classes 0.4 and 0.5 according to the relevant European standard.