Fly ash degree of reaction in hypersaline NaCl and CaCl2 brines: Effects of calcium-based additives

The current work highlights our capacity to accurately model a broad range of solidification/stabilization systems in order to select the best performing matrix as a function of the brine composition and concentration. [Display omitted] •The reactivity of 2 fly ashes is assessed as a function of the additive and the brine type.•Al- and Si-containing additives hinders the reactivity of the fly ashes.•Additives with no Al or Si content induce an increase in fly ash reactivity.•We propose a machine learning model that can predict the fly ash reactivity for accurate phase assemblage prediction. The pozzolanic reaction of fly ashes with calcium-based additives can be effectively used to solidify and chemically stabilize (S&S process) highly concentrated brines inside a cementitious matrix. However, complex interactions between the fly ash, the additive, and the brine typically affect the phases formed at equilibrium, and the resulting solid capacity to successfully encapsulate the brine and its contaminants. Here, the performances of two types of fly ash (a Class C and Class F fly ash) are assessed when combined with different additives (two types of cement, or lime with and without NaAlO2), and two types of brine (NaCl or CaCl2) over a range of concentrations (0 ≤ [Cl−] ≤ 2 M). The best performing matrices – i.e., the matrices with the highest Cl-containing phases content – were identified using XRD and TGA. The experimental results were then combined with thermodynamic modeling to dissociate the contribution of the fly ash from that of the additives. All results were implemented in a machine learning model that showed good accuracy at predicting the fly ash degree of reaction, allowing for the robust prediction of extended systems performance when combined with thermodynamic modeling.