Impact of Fe, Mg and Ca elements on glass alteration: Interconnected processes

In this study, the effects of Fe, Mg and Ca elements on the International Simple Glass (ISG) alteration were studied throughout 511 days of aqueous leaching experiments. The aim was to determine the processes involved and the additive or competitive nature of their effects on glass alteration. The three elements were introduced separately or together in solution as XCl2 chloride salts (X = Fe, Mg or Ca) with monthly additions to compensate for their consumption. Glass alteration was determined by leachate analyses (ICP-AES), and the alteration products were characterized in terms of composition, morphology and microstructure (SEM, TEM-EDX, ToF-SIMS and XRD). Two main processes were identified: (i) the precipitation of secondary phases integrating the added elements and (ii) the incorporation of the added elements into the gel layer. The first process enhances glass alteration and leads to a decrease in pH. The precipitation occurs in the presence of Fe and/or Mg. When these two elements are added simultaneously, their effects are additive. This phenomenon is self-limited; when a pH threshold value is reached (7.8 for Mg-silicates and 6.2 for Fe-silicates), the precipitation stops. The second process is mainly illustrated by the behavior of Ca. As this element is retained/incorporated into the gel layer, the gel layer becomes denser, and its protective properties increase. In lower proportions, Fe and Mg can also be incorporated into the gel layer. In the presence of each other, there is a competition between Ca, Fe and Mg to be incorporated into the gel layer. The quantity of element incorporated into the gel layer follows the order Ca >> Fe > Mg. These two processes are tightly linked to the evolution of the pH. As the secondary phases precipitate, the pH decreases, making Ca more soluble and less retained in the gel. When the precipitation stops, Fe and/or Mg can be incorporated into the gel in replacement for Ca. From these conclusions, an organogram was established that can be used to predict the effects of chemical elements on glass alteration depending on their behavior. (C) 2018 Elsevier Ltd. All rights reserved.