A review of analogues of alkaline alteration with regard to long-term barrier performance

Abstract

Cement and concrete will be used as fracture grouts, shotcrete, tunnel and borehole seals, and as matrices for waste encapsulation inter alia in geological repositories for radioactive wastes. Alteration of the host rock and/or swelling clay in waste package buffers and tunnel backfills by hyperalkaline solutions from cement/concrete may be deleterious to system performance through changes in the physicochemical properties of these barrier materials.Analogue systems (and timescales) relevant to the understanding of the alkaline disturbed zone include: industrial analogues, such as alkaline flooding of hydrocarbon reservoirs (up to 30 y), cementaggregate reactions (up to 100 y) and the Tournemire tunnel (up to 125 y); and natural analogues, including the hyperalkaline springs at Maqarin, Jordan (more than 100,000 y), saline, alkaline lakes (more than 1,000,000 y) and certain fracture fillings in granites (more than 1,000,000 y).These systems show that alkaline alteration can be observed for thousands of years over distance scales of hundreds of metres under extreme conditions of hydraulic gradients in fractured rocks (Maqarin), but may be limited to a few centimetres over tens to a hundred years in mudstone (Tournemire). Important reaction mechanisms for retardation of alkaline fluids include: fluid mixing (alkaline oil floods, Maqarin), ion exchange (alkaline oil floods, Tournemire) and kinetic mineral dissolution-precipitation reactions (all systems studied). Qualitative and quantitative kinetic data for mineral reactions are available from cement aggregate reactions and the Searles Lake analogue, respectively. Short-term alteration observed in cement-aggregates is characterized by calcium silicate hydrate (C-S-H) minerals and incipient zeolite formation, whereas evidence from the Tournemire tunnel shows the growth of K-feldspar after relatively short time intervals (tens to a hundred years). There is a tendency for alkaline alteration to result in porosity decreases, but locally, porosity may be enhanced (e.g. near-injection well interactions in alkaline oil floods, or at fracture margins at Maqarin, Jordan). Data from industrial and natural analogues may thus supply some key data for bridging time and space scales between laboratory and in situ experiments on one hand and the requirements for safety assessment on the other.

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

“…One of the main purposes of the cement is to maintain high pH conditions (>pH 12.5) that will reduce radionuclide solubility and microbial activity. 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).…”

“…Thomassin and Rassineux, 1992;Savage 2011). Predictive computer modelling is one way to achieve this, but the outputs from such modelling are highly dependent upon the databases used, and the codes may not adequately represent the complexities of real systems.…”

“…These minerals have been found to occur within a range of natural alkaline environments (e.g. Hay, 1986;Savage, 1998Savage, , 2011Sand and Mumpton, 1978 and references therein), as a product of reaction of aggregate within Portland cement (Marfil and Maiza, 1993), and as products within alkaline fluid-rock laboratory experiments at a range of temperatures (Adler et al, 1999;Chermak, 1992Chermak, , 1993Knauss, 1987;Knauss et al, 1985). They have also been predicted to form within alkaline fluid-mineral experiments (Bateman et al, 1999) and the ADZ (Gaucher et al, 2004;Savage, 2011;Savage et al, 1992;Savage and Rochelle, 1993;Soler, 2003;Soler and Mäder, 2007;Watson et al, 2009) -especially as the pH of the alkaline plume decreases.…”

Herbert's Quarry, South Wales – an analogue for host-rock alteration at a cementitious radioactive waste repository?

“…Pertinent to this study, examination of cements in historic structures and natural systems, such as Maqarin in Jordan and a number of saline lakes, have been studied as analogues for high-pH mineral alteration (Tinseau et al, 2006;Techer et al, 2012;Hodgkinson and Hughes, 1999;Milodowski et al, 1998;Surdam, 1977;Taylor andSurdam, 1981, in Savage, 2010;Chermak, 1992Chermak, , 1993. These studies have also shown high pH causes silicate mineral dissolution and secondary C-S-H phase formation but also suggest that over long timescales (10 s to 10 6 years) C-S-H may evolve to Kfeldspars or zeolites (Savage, 2011). However, analogue systems are generally poorly constrained and increasing the number of sites studied is important to increase confidence in their applicability to CDZ scenarios (McKinley and Alexander, 1992).…”

“…However, these studies are necessarily short in timscale with few lasting longer than 1-2 years (Gaucher and Blanc, 2006;Fernandez et al, 2010;Moyce et al, 2014). To improve understanding of the potential mineral alteration processes at longer timescales, high-pH systems in the natural and anthropogenic analogues have also been studied (IAEA, 1989;Miller et al, 2000;Savage, 2011). Pertinent to this study, examination of cements in historic structures and natural systems, such as Maqarin in Jordan and a number of saline lakes, have been studied as analogues for high-pH mineral alteration (Tinseau et al, 2006;Techer et al, 2012;Hodgkinson and Hughes, 1999;Milodowski et al, 1998;Surdam, 1977;Taylor andSurdam, 1981, in Savage, 2010;Chermak, 1992Chermak, , 1993.…”