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Coal combustion products (CCPs) contain heavy metals that have the potential to leach into surface and ground waters when disposed improperly. 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 SOx 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 for solidification/stabilization (S/S) of CCPs prior to disposal. These materials, made by activating aluminosiliceous powders (e.g. fly ash) with highly alkaline solutions, may improve stabilization of heavy metal wastes. The design of geopolymer mixtures for optimum S/S of CCPs is not straightforward, however. This study represents an advancement upon the traditional trial and error mixture design process by using thermodynamic phase equilibria models to predict the phases present in the solidified materials, including porosity, and experimentally validating these models. Characterization of the CCPs was conducted to provide parameter inputs to the thermodynamic models. Mixtures were designed for experimental testing by minimizing model predicted porosity, thereby maximizing predicted solid phase formation. Geopolymer mixtures were compared against portland cement-stabilized mixtures, and both were experimentally tested for solidification (compressive strength, phase formation, and porosity) and stabilization of heavy metals and oxyanions following EPA standard LEAF protocols at different liquid/solid (L/S) ratios and pH. Another unique aspect of this work was the comparison of the standard reagent water used for leaching tests to a simulated landfill leachate fluid that more accurately represents landfill conditions. 

Thermodynamic modeling suggested testing geopolymer activating solutions for CCPs of: 4M NaOH, 8M NaOH, and 4M NaOH with added fumed silica to increase the silica modulus (SiO2/Na2O) of the solution to 1.5. It was found experimentally that geopolymers made with 4M NaOH had higher reactivity than those made with 8M NaOH, and increasing the silica modulus to 1.5 also somewhat increased reactivity of specimens. Portland cement-based mixtures were observed to bind oxyanions more effectively 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 the element, the geopolymer composition, and the test method used. Testing with simulated groundwater/landfill leachate indicated that leaching of copper and, to a lesser extent, selenium is affected by landfill leachate components, which could have impacts on wastes with larger quantities of these elements. A comparison of the cost of S/S strategies with their effectiveness demonstrated that CCPs activated by a 4M NaOH solution, without modification of silica modulus, provide the lowest cost solution with excellent performance. 

This project demonstrated that an iterative approach of thermodynamic modeling and experimental testing proved to be invaluable as the models helped in the design of the experiments and the experimental results will help inform and improve future modeling efforts. It can be concluded that S/S of CCPs using a 4M NaOH activating solution is a cost-effective strategy, providing excellent mechanical and chemical performance, which can likely be broadly applied. For example, future work could examine a 4M NaOH solution to geopolymerize combinations of CCPs and brines for plants seeking zero-liquid discharge.