The long-term durability of low alkali cements. Evidence from new natural analog sites in Europe and North Africa

The long-term durability of low alkali cements is of interest where they are under consideration as repository tunnel and exploration borehole seals and plugs. It is essential to have an appropriate understanding of their longevity to inform decisions on their potential use in a repository environment. Archaelogical analogues of low alkali cement have been studied for some time. Thomassin and Rassineux (1992), for example, reviewed some of the literature on Gallo-Roman cement-based materials and noted that one of the most impressive examples is the 1700 year old Roman mortar used in Hadrian's Wall (UK) which still contains substantial amounts of CSH (calcium silicate hydrate) compounds. These mortars were studied specifically with the behaviour of an ILW repository in mind (Jull and Lees 1990). However, plugs and seals will generally be required to be durable for longer than the few thousand years which can be accessed via archaeological analogues, so it is essential to turn to natural systems for evidence of longer term durability. To date, there have been no reported studies on natural low alkali cements. In principle, however, such cements should exist and the Bituminous Marl Formation, which hosts the natural OPC cements in Jordan (Pitty and Alexander, 2011), is a likely source. This Formation constitutes a widespread terrain which stretches from Syria in the north, through Israel and Jordan to Saudi Arabia in the south. The natural cement was formed by the combustion of organic rich limestones, a process which continues today. In Syria and northern Jordan, for example, the Formation is punctured by Late Oligocene to Quaternary volcanics so sites which include pozzolanic ash mixed with the Bituminous Marl exist and, on combustion, should produce natural low alkali cements. A site in northern Jordan is currently under investigation for evidence of long-term fresh groundwater/low alkali cement interaction and the preliminary results of the study will be reported here. In addition, a second site in Europe has been identified where natural low alkali cements exist in combination with both saline groundwaters and brines, allowing examination of saline groundwater/low alkali cement interaction. Once again, the preliminary results of the study will be reported here and a comparison between the two sites will be made, highlighting the different impact of the differing groundwaters on the cement. Although precise dates for the systems examined are not yet forthcoming, this is a focus for future efforts at both sites, allowing more precise definition of the probable longevity of low alkali cements in repository environments.