Fly Ash-Ca(OH)2 Reactivity in Hypersaline NaCl and CaCl2 Brines

The disposal of highly concentrated brines from coal power generation can be effectively accomplished by physical solidification and chemical stabilization (S&S) processes that utilize fly ashes as a reactant. Herein, pozzolanic fly ashes are typically combined with calcium-based additives to achieve S&S. While the reactions of fly ash-(cement)-water systems have been extensively studied, the reactivity of fly ashes in hypersaline brines (ionic strength, I-m > 1 mol/L) is comparatively less understood. Therefore, the interactions of a Class C (Ca-rich) fly ash and a Class F (Ca-poor) fly ash were examined in the presence of Ca(OH)(2), and their thermodynamic phase equilibria were modeled on contact with NaCl or CaCl2 brines for 0 <= I-m <= 7.5 mol/L. At low ionic strengths (<0.3 mol/L), reactivity and stable phase assemblages remain effectively unaltered. However, at high(er) ionic strengths (>0.5 mol/L), the phase assemblage shows a particular abundance of Cl-AFm compounds (i.e., Kuzel's and Friedel's salts). Although formation of Kuzel's and Friedel's salts enhances the Class F fly ash reactions in both NaCl and CaCl2 brines, NaCl brines compromise Class F fly ash reactivity substantially, while CaCl2 results in the reactivity remaining essentially unchanged. Thermodynamic modeling that accounts for the fractional and noncongruent dissolution of the fly ashes indicates that their differences in reaction behavior are provoked by differences in the prevalent pore solution pH, which affects phase stability. The outcomes offer new insights for matching fly ashes, Ca additives, and brines, and accounting for and controlling fly ash-brine interactions as relevant to optimizing physical solidification and chemical stabilization applications.

Automated summary

The study investigated the use of fly ash for treating highly concentrated brines, finding that reactivity and phase assemblages are generally unaffected at low ionic strengths, but show specific compounds abundance at higher ionic strengths. Combining calcium salts with NaCl significantly reduces fly ash reactivity, while it remains essentially unchanged with CaCl2.