Advances in the understanding of alkaline waste materials as potential eco-pozzolans: Characterisation, reactivity and behaviour

This paper explores the potential of three industrial alkaline waste materials – concrete construction and demolition waste (CDW-C), white ladle furnace slag (LFS) and biomass ash (BA) – for use as secondary raw materials in the manufacture of future eco-cements with a reduced carbon footprint. Circularity is one of the key strategies behind the circular economy, the cement industry roadmap and the climate neutrality targets set for 2050. The three materials were characterised using various instrumental techniques (XRF, laser, BET, XRD–Rietveld, SEM/EDX, FTIR, TG/DTA and NMR) and their chemical reactivity, the changes in their mineralogical phases and the thermodynamic stability within the pozzolan/lime system were determined. Finally, their physical and mechanical behaviour in binary cement pastes at replacement proportions of 7 % and 20 % over 90 days of curing were analysed. The results obtained show that these alkaline waste materials are different in nature and composition to standard pozzolans, with the LFS containing fluorine and the BA containing sulphates, potash and chlorides. The standard combined water test showed different levels of chemical reactivity (BA > LFS > CDW-C). Analysis of the materials’ composition, mineralogical phases and thermodynamic stability over 90 days of reaction in the pozzolan/Ca(OH)₂ system revealed that hydrogarnet was the stable phase in the LFS cement paste, while in the BA and CDW-C pastes ettringite and CSH and C-(A)-SH gels, among others, were the stable phases. All the blended cement pastes with 7 % and 20 % replacement content met the physical requirements and maintained the strength category of the starting cement.