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

Abstract

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 C 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.

Rapid Elemental Extraction from Ordered and Disordered Solutes by Acoustically-Stimulated Dissolution

“…Initially, the FAs were washed (in deionized water, at a solid-to-liquid (s/l) ratio of 1:1000 [mass basis], while being shaken at 200 rpm, 50 °C, 50 min) prior to the dissolution experiments. These conditions were sufficient to remove readily soluble components (e.g., CaO: free lime) as evidenced by XRD in a previous work . Both washed FAs exhibited small native particle sizes (<20 μm) and thus were examined as-received.…”

“…These conditions were sufficient to remove readily soluble components (e.g., CaO: free lime) as evidenced by XRD in a previous work. 19 Both washed FAs exhibited small native particle sizes (<20 μm) and thus were examined asreceived.…”

Thermochemical data and phase equilibria of halide (Cl−, Br−, I−) containing AFm and hydrotalcite compounds

“…LDH compounds are of interest for anion immobilization in the context of waste management, particularly of liquid wastes 5 . For example, the formation of Friedel's salt during the pozzolanic reaction between fly ash, portlandite, and Cl‐enriched brines was shown to retain a significant fraction of the Cl − in the solids formed 16,17 . As such, LDH phases could be of interest to retain other less common anions (e.g., other halide anions present as salts in brines or as radionuclides in radioactive wastes).…”

“…Halide anions may display a competitive behavior: they have been observed to compete for uptake in hydrotalcite phases following in the order F − > Cl − > Br − > I − 18 or Br − > Cl − > I − 19 . The stability range of these phases is also dependent on the aqueous environment which may induce the formation of other phases instead (e.g., ettringite) 16 . It is thus necessary to assess how the presence of halide anions in the interlayer positions of LDH phases affects their solubility and their range of chemical stability.…”

Solubility behavior and thermodynamic modeling of sodium monosulfoaluminate (“U‐phase”) in cementitious systems

“…1,[3][4][5][6] For this reason, it has been utilized in multiple applications, including civil engineering, 7,8 nuclear waste management, 2,9 and/or liquid wastewater stabilization. [10][11][12] Although any of the common cementitious (e.g., ordinary Portland cement, OPC) hydration products can be accurately predicted, 1,4,6,13,14 modeling of uncommon systems remains challenging since the measured thermodynamic properties of less prominent phases may not be available in current databases.…”