Cement – clay interfaces
As in geological settings, interfaces of materials with contrasting chemical composition will evolve over time due to chemical interactions, giving rise to changes in transport properties at and across the material interfaces. In that sense, interfaces between cement and clay and the chemical interaction are of relevance, and process understanding was greatly enhanced by the CI field experiment in the Mont Terri rock laboratory, Switzerland, where cement – Opalinus clay or bentonite interactions, respectively, were investigated over several years (see Bernard et al. 2020, Yokoyama et al. 2021, Mäder et al. 2017). A detailed review on cement – clay interaction is presented in Chapter 2 of NAB 22-34 (Prasianakis et al. 2022).
Cement – clay interactions and long-term interface development have also indirectly benefited from the studies of the fractured rock complex at Maqarin, in northern Jordan, as a valuable natural analogue (see Maqarin Natural Analogue Site Study Group 1992, Linklater 1998, Alexander & Smellie 1998, Pitty & Alexander 2011 for details). There, materials around open fractures were assumed to have once been part of sealed fracture faces (Linklater 1998). Along these sealed fracture faces, mineral alterations into the limestone host rock (biomicrite) interface as a result of the hyperalkaline plume was observed to have limited penetration extent (mm) as demonstrated by Martin et al. (2016). i.e., the maximum alteration extent amounted to 25 mm. Importantly, generated interfaces where not totally clogged as e.g., an alteration rim of a vein with microscopically clogged pores was identified to contain a remaining porosity of about 19 vol.-% (Martin et al. 2016).
These observations are well aligned with what Kosakowski et al. (2023) noted in Section 3.3.2 of NTB 23-03: “Interfaces between cement and clay materials can be characterised by strong (geo)chemical differences, which influence the diffusive transport of solutes into both materials and may result in the formation of alteration fronts. Typically, dissolution of cement and clay phases is observed in combination with precipitation of secondary mineral phases at or near the material interface.” This statement derived from a synthesis and assessment of various short and long-term laboratory and URL experiments (e.g., Lalan et al. 2016, Gaboreau et al. 2020, Yokoyama et al. 2021), as well as archaeological (e.g., Lee 2001) and natural (e.g., Pitty & Alexander 2011, Martin et al. 2016) analogues.
Iron – clay interfaces
Mineralogical and chemical studies of the interfaces between clay and iron (magnetite) ore have been carried out at the Kiruna bentonite natural analogue in the Kiirunavaara mine (Alexander et al. 2024). The smectite clay is made of significant amounts of a Fe-poor montmorillonite with a generally Ca-rich exchangeable interlayer composition. Preliminary results show that magnetite ore and smectite have been in direct contact at a temperature of between 50 and 150 °C for an extended period of time (ca. 350 Ma). Little or no uptake of Fe in the clay has been observed under these conditions, which are similar to those in a deep geological repository. Thus, the observations suggest minimal to no incorporation of iron into the clay matrix. Analysis of dioctahedral smectites demonstrates only marginal fluctuations in Fe2O3 content, ranging from 0.93 to 2.46 wt.-%. Interestingly, these variations do not appear correlated with the presence or absence of residual magnetite in the altered rocks or their proximity to magnetite ores.