Effect of waste ash on clinker phase composition and microstructure

In most kiln systems the coal ash is incorporated into the kiln feed, thus affecting the final compound composition of clinker. This work examines the effects of waste ash on clinker phase composition and crystalline structure (microstructure). Clinkers were prepared from mixtures of industrial raw mix with coal ash and waste ash, respectively. The coal ash was substituted on a calorific basis by 50% and 100% of waste ash originating from refuse-derived fuel (RDF). The thermal treatment of the mixtures was monitored by differential thermal analysis (DTA). The phase composition and microstructure of the produced clinkers were investigated by X-ray diffraction and optical microscopy and the level of trace element content was measured. According to the results of DTA, the addition of waste ash shifts clinkering reactions to lower temperatures. The unit cell volume calculations of clinker phases suggest only limited alterations in the crystal lattice of alite and cubic tricalcium aluminate compared to the reference clinker. Free lime tends to increase when moving from coal to RDF ash but decreases with increasing RDF addition. The microscopic study of clinkers did not reveal significant differences. Finally, clinkers produced with waste ash addition meet environmental criteria relating to trace element content.