Using Waste-to-Energy Fine-Combined Ash as Sand or Cement Substitute in Cement Mortar

Abstract

US waste-to-energy (WTE) plants generate about 7 million tons of fly ash and bottom ash annually, mixed to combined ash (CA), which, after metal recycling, is disposed of in landfills. Fine-combined ash (FCA), the sandy fraction (<2 mm) of CA after water washing, crushing, and size separation, amounts to about 25% of total CA. This study examines two uses of FCA in cement mortar based on tests of mechanical properties, workability, mineral transformation, and leachability of heavy metals: (1) directly using FCA as a sand substitute by up to 50% by volume (37% by weight); and (2) milling the FCA to powder (MFCA) and using it as a cement substitute by up to 25% by volume (24% by weight) substitution of portland cement. When FCA was used as a sand substitute, water was added during mixing to improve workability and increase the FCA replacement. When MFCA was used as a cement substitute, it chemically reacted with the hydration system and contributed to the formation of more amorphous phases. The calcite in MFCA reacted with hydrated cement and transformed into hemicarboaluminate and monocarboaluminate. Metallic aluminum in the ash can cause hydrogen gas expansion during cement hydration, which limited the substitution level of FCA or MFCA in cement mortars to 50% and 25% by volume, respectively. The study showed that all FCA or MFCA cement mortars were effectively stabilized/solidified and transformed into nonhazardous materials.