Scientific and technical behaviour of blended Cement made from coal mining waste

Abstract

Cement industry traditionally uses in its productive process a huge quantity of waste and by-products. The use of supplementary cementing materials in the manufacture of eco-efficient cements is focused in industrial by-products and wastes. Research studies founded on kaolinite based waste, are being followed with special attention from the scientific community to obtain recycled metakaolinite. One of these lines of research is based on the industrial waste from coal mining. A coal gangue waste coming from a landfill at an opencast mine (Sociedad Anónima Hullera Vasco-Leonesa, Spain) was used for this investigation. As all kaolinite based waste, it was previously activated at 600ºC for two hours from environmentally, economic and energy viewpoint. This paper focussed on the scientific advances (chemistry, pozzolanic properties, hydrated phases, etc.) of activated coal mining waste as well as technical advances (physical and mechanical) up to 365 days of hydration in 20% pozzolan blended cement system. Finally, it assesses the requirements according to European standard on the manufacture of common cements. The results clearly show that these industrial wastes, once subjected to controlled thermal activation process, had high pozzolanic properties comparable to metakaolinite obtained by activating a natural kaolinite. The blended cement produced with 20% of pozzolan showed a similar mechanical behavior to the reference OPC, as a consequence of the formation of hydrated phases from the pozzolanic reaction between pozzolan and lime generated during the reaction of hydration of cement particles. The presence of crystalline hydrated phases were a) tetracalcium aluminate hydrate (C4AH13); b) stratlingite (C2ASH8) (stable phase); c) monosulfoaluminate hydrate (C3A·SO4Ca·12H2O) and d) LDH compounds (phyllosilicate/carbonate) (metastable phase). The resulting 20% blended cement complies with the requirements of the current standard for cements CEM II-A. Therefore, this residue could be used in developing the future eco-efficient and innovative cements, constituting an alternative source for obtaining of recycled metakaolinite, pozzolan collected in the existing standards as Q (calcined pozzolan).