Geopolymer-based solutions for coal combustion product solidification and stabilization
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Coal combustion products (CCPs) such as fly ash have been used for years as a sustainable substitute for cement in concrete, however, significant quantities of CCPs are currently discarded into ash ponds and landfills each year. CCPs can contain heavy metals that have the potential to leach into surface and ground waters when improperly disposed of. Of further concern is that in recent years power plants have been injecting sodium carbonate compounds such as trisodium hydrogendicarbonate dihydrate (trona) into their flue gas streams to reduce SO[subscript x] emissions. Trona injection has been shown to alter the characteristics of collected fly ash and increase leaching of heavy metal compounds from the ash, posing a higher environmental threat. Geopolymers that utilize CCPs as an aluminosilicate precursor are under consideration as an alternative to conventional Portland cement. These materials, produced by activating aluminosiliceous powders (e.g., fly ash) with highly alkaline solutions, may reduce the carbon footprint of infrastructure materials and stabilize heavy metal wastes. While previous work indicates that geopolymers can successfully stabilize heavy metal wastes under lab conditions (i.e. reagent water), this work aims to evaluate the potential for geopolymer stabilization of heavy metal wastes in landfill environments and to identify characteristics that reduce leaching potential of geopolymers produced from trona impacted fly ash. Leaching tests on trona impacted CCP based geopolymers were performed with a simulated landfill leachate to mimic conditions within ash ponds and landfills. Batch extraction tests were used to characterize leaching at different pH’s and different liquid/solid ratios (L/S) using LEAF protocols. It was found that geopolymers made with 4M NaOH had higher reactivity than those made with 8M NaOH, and increasing the silica modulus from 0-1.5 also increased reactivity of specimens. Cements were observed to better bind oxyanions at high pH than geopolymers, but geopolymers were capable of reducing leaching for a number of elements over a broad pH range. The extent of leaching depended on both the element and the geopolymer composition. Results also indicated that leaching of copper and selenium are affected by landfill leachate properties, which could have impacts on wastes with larger quantities of these elements.