Calcium silicate hydrate (C-S-H) gel dissolution and pH buffering in a cementitious near field

Abstract

A cementitious backfill has been proposed in many geological disposal concepts for intermediate-level waste and low-level waste in the UK and elsewhere. In this paper, the main features of the chemical evolution of backfill and the associated changes in the near-field pH are illustrated with results from recent work. For example, interaction of the groundwater with calcium silicate hydrate (C-S-H) phases in a backfill is expected to play an important role in the long-term pH-buffering behaviour. Existing experimental data for the dissolution of C-S-H gels are compared with recent experimental results from leach tests on gels of a lower calcium to silicon ratio (C/S) to provide a consistent set of data across the full C/S range. The results confirm that a congruent dissolution point around C/S = 0.8 is approached by leaching from below (i.e. for gels with 0.29 < C/S < 0.8), as well as from above, as reported elsewhere. In addition, a spreadsheet model has been developed to calculate the volume of backfill required at the vault scale to meet specified pH performance criteria. This model includes the major reactions of the backfill with the groundwater, waste encapsulants and waste components. It can also consider the effects of specific waste packages on local pH performance to allow comparison with the vault-scale calculations.

A thermodynamic and kinetic model for paste–aggregate interactions and the alkali–silica reaction

“…This behavior is consistent with the experimental data of Harris et al [35] and Chen et al [36], which show that at high Ca:Si ratios (above~1.0) C-S-H dissolves incongruently effectively buffering the pH above 12; however, as shown by Baston et al [37], at lower Ca:Si ratios, pH can be lower, with C-S-H buffering pH~10 at Ca:Si ratios of~0.2-0.7.…”

A long-term experimental study of the reactivity of basement rock with highly alkaline cement waters: Reactions over the first 15 months

“…Cement-equilibrated water will migrate into the surrounding host rock forming a plume of elevated pH, alkaline water, forming a region known as the 'alkaline disturbed zone' (ADZ) (Atkinson, 1995;Savage, 1995Savage, , 2011. The pH of the cement porewater and of the alkaline plume will decrease over time (Atkinson, 1985;Baston et al, 2012) as differing cement components buffer the pH (Figure 1). The alkaline cement leachate will react with the rock causing dissolution of some minerals and precipitation of others, and this will potentially modify both the geochemical and hydrogeological properties of the host rock within the ADZ prior to any eventual radionuclide release (e.g.…”

Studying the Hydration of a Retarded Suspension of Ground Granulated Blast-Furnace Slag after Reactivation

“…[85,86]). The high intensity of the reflections illustrates the effect of the two activator concentrations.…”