Recycling of autoclaved aerated concrete in screed and stabilized sand

Autoclaved aerated concrete (AAC) is a lightweight cellular concrete that has been used for more than 80 years. Currently, however, no good recycling options for AAC from construction and demolition waste exist. The amount of AAC waste that can be recycled in the production of new AAC is limited because of quality issues. Furthermore, recycling AAC into traditional concrete or as unbound aggregate causes both technical and environmental problems because of the low compressive strength (2-8 MPa) of AAC and its high amount of leachable sulfate: typically > 10,000 mg/kg dm (L/S = 10).
In this paper, recycled AAC waste was evaluated as a replacement of sand in a traditional screed (subfloor) and in cement stabilized sand products. A range of cements (CEM I, CEM II and CEM III), were used in combination with the crushed AAC waste aggregate (0-8 mm). During hydration a reaction of the AAC leachable sulfate and the aluminate contained in the cement resulted in the formation of (insoluble) ettringite. The main conditions influencing the formation of ettringite, and hence the leaching of sulfate, were examined in cement stabilized sand products. A sufficiently high pH was found to be crucial to meet sulfate leaching standards. The presence of additional sulfate as gypsum impurities in the AAC waste proved detrimental towards sulfate leaching.
Finally, the replacement of sand by recycled AAC improved the thermal properties of the screed, rendering the product suitable for niche products that target improved thermal insulation.