Solidification/stabilization of a heavy metal sludge by a Portland cement/fly ash binding mixture

A Portland cement/Class F fly ash binder was used to solidify a heavy-metal sludge containing Cr, Ni, Cd, and Hg. The cement/fly ash ratio was 0.4, and the sludge/solids ratio was 1.43. Samples were cured at room temperature for a minimum of 28 days, then examined by scanning electron microscopy, energy dispersive x-ray spectrometry, optical microscopy, and x-ray diffractometry. The sludge was composed of the hydration products of cement/fly ash mixtures, and impure, complex compounds of the waste metals. The sludge formed a simple mixture with the binder, retarded hydration, and affected microscopic phase composition; calcium hydroxide did not form. Reaction products near the surfaces of fly ash particles contained minor amounts of Cr and Ni, suggesting some chemical stabilization of these toxic metal ions