Belite cement clinker from autoclaved aerated concrete waste fines with high sulfate content

The processing of belite cement clinker in a rotary kiln at about 1000°C in a CO₂ atmosphere is a new recycling option for Autoclaved Aerated Concrete (AAC) waste that otherwise must be landfilled. Waste fine fractions from a sorting facility enriched in sulfate due to intermixing with waste plaster have been processed. During clinkering the cement clinker phase belite (Ca₂SiO₄) besides technical ellestadite, (Ca₁₀(SiO₄)₃(SO₄)₃Cl₂), or ternesite, (Ca₅(SiO₄)₂SO₄), are formed, depending on the addition of flux minerals. However, not all phases of the novel clinker react hydraulically. Whereas ternesite reacts with water, ellestadite forms complex solid solution series (Ca/Pb, SO₄/PO₄), which may be used as an insoluble reservoir mineral for undesirable constituents, such as phosphates and chlorides. The produced clinker has been successfully used to partially substitute OPC in AAC production in technical trials. Waste quantities and landfill costs are minimized, while at the same time, CO₂ emissions and the primary resource consumption of AAC production are reduced. Joint work with industrial companies is underway to increase technology readiness. Particularly large reduction effects on CO₂ emissions can be achieved through electrical heating of the rotary kiln.