To examine the geopolymer matrix suitability for solidification-stabilization (S/S) of coal combustion wastes, a thermodynamics-based approach using the Gibbs Energy Minimization Software is demonstrated for model geopolymer formulations based on alkali-activated fly ash (AFA) and alkali-silicate-activated fly ash (ASFA). First, special focus is placed on computing the equilibrium phase assemblages using the bulk reactant compositions as an input, to ascertain hydrate mineralogy as a function of binder type and activation solution concentration so as to identify: (i) formulations that produce hydrates with the greatest potential for stabilizing heavy metals by structural incorporation, and/or, (ii) formulations which yield the lowest porosity – and diffusion coefficients – for the transport of heavy metals (and other contaminants). Secondly, the matrix’s stability is assessed by simulating leaching that is controlled by progressive/sequential dissolution and/or precipitation of the solids across a range of liquid-to-solid ratios (l/s, mass basis) and pH levels of the leachant; e.g., consistent with LEAF methods 1313 and 1316. Thus, the rate of degradation of hydrated solids (i.e., the matrix), as a function of l/s and pH, are assessed to select formulations with the best resistance to dissolution under leaching exposure. The approach enables rapid, parametric assessment of a wide range of cementitious systems. It provides a useful means to, <i>a priori</i>, assess the suitability of a given binder system for the immobilization of contaminants of coal combustion residues (CCR’s), by effectively assessing the evolution of hydrated phase assemblages and porosity that are relevant to select host matrices which: (i) display the best characteristics for physical and chemical stabilization of contaminants, and, (ii) which also offer the best matrix stability in terms of their resistance to dissolution under leaching exposure.

Thermodynamics-based modeling method to evaluate geopolymer formulations for solidifying and stabilizing coal combustion wastes

ACS Spring 2022

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