The fate of silicon during glass corrosion under alkaline conditions: A mechanistic and kinetic study with the International Simple Glass

Abstract

International Simple Glass -a six oxide borosilicate glass selected by the international nuclear glass community to improve the understanding of glass corrosion mechanisms and kinetics -was altered at 90°C in a solution initially saturated with respect to amorphous 29 SiO 2 . The pH 90°C , was fixed at 9 at the start of the experiment and raised to 11.5 after 209 d by the addition of KOH. Isotope sensitive analytical techniques were used to analyze the solution and altered glass samples, helping to understand the driving forces and rate limiting processes controlling long-term glass alteration. At pH 9, the corrosion rate continuously drops and the glass slowly transforms into a uniform, homogeneous amorphous alteration layer. The mechanisms responsible for this transformation are water penetration through the growing alteration layer and ion exchange. We demonstrate that this amorphous alteration layer is not a precipitate resulting from the hydrolysis of the silicate network; it is mostly inherited from the glass structure from which the most weakly bonded cations (Na, Ca and B) have been released. At pH 11.5, the alteration process is very different: the high solubility of glass network formers (Si, Al, Zr) triggers the rapid and complete dissolution of the glass (dissolution becomes congruent) and precipitation of amorphous and crystalline phases. Unlike at pH 9 where glass corrosion rate decreased by 3 orders of magnitude likely due to the retroaction of the alteration layer on water dynamics/reactivity at the reaction front, the rate at pH 11.5 is maintained at a value close to the forward rate due to both the hydrolysis of the silicate network promoted by OHand the precipitation of CSH and zeolites. This study provides key information for a unified model for glass dissolution.As a result of the complex suite of corrosion processes listed above (hydration, interdiffusion, hydrolysis, condensation, precipitation), the dissolution of glass is rarely congruent. This means that, in addition to soluble species directly released into the solution, solid products are also formed. The 157 167 SiGlass surface at t = 0

Corrosion of the International Simple Glass under acidic to hyperalkaline conditions

“…Formation of C-S-H phases during the alteration of ISG has also been reported by Gin et al 14 and Abdelouas et al, 31 however these authors observed the precipitation of C-S-H when ISG was altered in a KOH solution at pH 11.5 14 or the formation of tobermorite (a crystalline C-S-H phase) when ISG was altered under different relative humidities. 31 In both of these cases the Si and Ca consumed in forming C-S-H must originate from the glass.…”

“…Gin et al have observed the presence of a Zr-rich film on the outer surface of ISG altered at pH 11.5 in KOH solution; they suggested that this is due to the reprecipitation of less soluble components following complete glass dissolution, however they did not report the banding seen here. 14 The formation of a ZrO 2 rich layer on the surface of the zirconia containing glasses developed for the reinforcement of cements is believed to be a critical in ensuring the improved durability of these glasses in cementitious environments 34,35 although again, banding has not been reported. While ZrO 2 is generally thought to improve the durability of glasses, especially in alkaline environments, in work on nuclear waste glasses it has been concluded Table 2.…”

“…(7)- (8); c Si forward rate data compared to other studies; d, e particles after 100 days SPFT at pH(RT) 11, 40°C & log 10 [(q/S)/(m s )] = -6.81, features A in (d) are indicative of localised preferential attack; features B in (e) appear to be dehydration cracks with an evaluation of the rate law parameters. In addition we report on the performance of ISG, at essentially the same pH as that studied by Gin et al, 14 but in a Ca(OH) 2 imposed high pH which is more representative of likely disposal conditions in the UK than a KOH imposed elevated pH.…”

“…[8][9][10][11] These results together with those of Andriambololona et al 12 and Chave et al 13 demonstrate the importance of Ca in determining the behaviour of the glass studied under such pH conditions. Meanwhile Gin et al 14 have shown that at pH(90°) = 11.5 in a KOH solution the international simple glass (ISG) dissolves congruently. However Ca (OH) 2 solutions will result in precipitation in both the inlet and outlet SPFT experiments and thus the effects of Ca have been assessed using the PCT and MCC-1 protocols.…”

Determination of the Forward Dissolution Rate for International Simple Glass in Alkaline Solutions

“…Static tests in YCWCa at 30°C showed a continuous increase of the silica concentrations up to more than 1200 mg·L -1 for SON68 [18] and even more than 3000 mg·L -1 for the ISG [19], while the glass was still dissolving. Because the glass dissolution would proceed by a dissolution/precipitation mechanism at this high pH [20], the formed layer would be less protective than a gel layer formed by condensation at near-neutral pH. It could nevertheless constitute a diffusion barrier, leading to a dissolution rate decreasing following the square root of time, as proposed for SON68 in [18].…”