Topological controls on aluminosilicate glass dissolution: Complexities induced in hyperalkaline aqueous environments

Abstract

Fly ash, an aluminosilicate composite consisting of disordered (major) and crystalline (minor) compounds, is a low‐carbon alternative that can partially replace ordinary portland cement (OPC) in the binder fraction of concrete. Therefore, understanding the reactivity of fly ash in the hyperalkaline conditions prevalent in concrete is critical to predicting concrete's performance; including setting and strength gain. Herein, temporal measurements of the solution composition (using inductively coupled plasma‐optical emission spectrometry: ICP‐OES) are used to assess the aqueous dissolution rate of monophasic synthetic aluminosilicate glasses analogous to those present in technical fly ashes, under hyperalkaline conditions (10 ≤ pH ≤ 13) across a range of temperatures (25°C ≤ T≤45°C). The dissolution rate is shown to depend on the average number of topological constraints per atom within the glass network (nc, unitless), but this dependence weakens with increasing pH (>10). This is postulated to be on account of: (a) time‐dependent changes in the glass’ surface structure, that is, the number of topological constraints; and/or (b) a change in the dissolution mechanism (eg from network hydrolysis to transport control). The results indicate that the topological description of glass dissolution is most rigorously valid only at very short reaction times (ie at high undersaturations), especially under conditions of hyperalkalinity. These findings provide an improved basis to understand the underlying factors that affect the initial and ongoing reactivity of aluminosilicate glasses such as fly ash in changing chemical environments, for example, when such materials are utilized in cementitious composites.

Analytical model of the network topology and rigidity of calcium aluminosilicate glasses

“…First, the angular BB constraints associated with four-fold Al atoms have been suggested by many studies to be notably weaker than those associated with Si atoms. 27,28,[69][70][71][72] This has been attributed to the fact that the nearby presence of a charge-compensating cation tends to destabilize the angular environment of four-fold coordinated Al atoms. 28 Second, ionic Ca-FO constraints are expected to exhibit a low bond energy and, hence, to break at low temperature.…”

“…This ranking is based on the following observations. First, the angular BB constraints associated with four‐fold Al atoms have been suggested by many studies to be notably weaker than those associated with Si atoms 27,28,69–72 . This has been attributed to the fact that the nearby presence of a charge‐compensating cation tends to destabilize the angular environment of four‐fold coordinated Al atoms 28 .…”

Development of structural descriptors to predict dissolution rate of volcanic glasses: Molecular dynamic simulations

“…Both the AMODE and ASDC parameters have been shown to outperform the NBO/T parameter in predicting the relative reactivity of CMAS and CAS glasses in different alkaline environments 34 . These investigations have highlighted the importance of incorporating detailed structural information in developing reliable structural descriptors 30,31,32,33,34 (as opposed to those empirical parameters based on composition alone).…”

“…28 For this purpose, molecular dynamics (MD) simulations have been increasingly used to generate detailed structural representations for different glass systems. 29 Based on MD simulation results and topology constraint theory, an important structural descriptor, that is, the average number of constraints per atom in the glass network (an estimation of the network rigidity), has been developed and used to predict the dissolution rates of aluminosilicate and borosilicate glasses, [30][31][32] with better performance than NBO/T 18 . However, estimation of topology constraints for complex CMASTFNK glasses has not yet been reported in the literature as far as the authors are aware.…”

Influence of Accelerating Admixtures on the Reactivity of Synthetic Aluminosilicate Glasses

“…An "initial dissolution" stage over the first 360 min due to the hydrolysis of the network forming species (Al-O-Si and Si-O-Si bonds). As the concentration of the different elements in solution increased, the dissolution rate decreased and a "steady-state dissolution" was reached that can be explained by the local saturation of the solution with respect to the glass or by the formation of alteration layers on the glass surface [26,27]. The amount of Al, Si and Ca released in the dissolution experiments carried out in 0.1 M NaOH were significantly lower (20-60%) with respect to 1 M NaOH, as the reactivity of the glasses increases with the pH of the solution.…”