Fly ash/paper sludge as constituents of cements: hydration phases

Ternary cements prepared with equal amounts of thermally activated paper sludge and fly ash in proportions of 35% and 50%, in substitution of ordinary Portland cement, are mixed to study their phase stability. The underlying intention is to increase the proportional substitution of waste products in these types of cement, thereby consigning less waste to landfill sites. The hydration phases of the cement specimens were analysed using thermogravimetric/derivative thermogravimetric (TG/DTG) analysis, X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) spectroscopy. The phases identified in the cement blends include Portlandite, monocarboaluminate compound C4AC̅H12, and layered double hydroxides–type compounds (LDH), as well as ettringite and calcium silicate hydrate gels. Increased formation of both tetracalcium aluminate carbonate 12 hydrate (C4ACH12) and LDH-type compounds and reductions in ettringite and portlandite were observed when the proportional substitution of cement by waste products rose from 35% to 50%. A characteristic compound of pozzolanic reactions, bicalcium silicate (C2ASH8), was not observed, suggesting that it is inhibited in these types of substitutions.