Recycling of calcium fluoride sludge as additive in the solidification–stabilization of fly ash.

Recycling of calcium fluoride sludge as additive in the solidification of fly ash with cement or calcium hydroxide was studied. Fly ash solidified with calcium hydroxide and calcium fluoride sludge failed to achieve a target compressive strength of 200 kN/m2 over a reaction period of 10 d. White Portland cement had produced good results in comparison with high early strength cement or Portland cement. Compressive strength of the material solidified with white Portland cement was found to be 39% higher than the material solidified with Portland cement. The optimum mix was found to be that with 80% fly ash, 5% white Portland cement, and 15% calcium fluoride sludge. Heavy metal ions and fluorides found in the leachate of stabilized matrix were below the discharge limit. Recycling of calcium fluoride as additive reduced the cost of solidification of fly ash to 41% while helping to transform hazardous sludge into an environmentally friendly material. [ABSTRACT FROM AUTHOR]