Geochemical characterisation of Norwegian Cambro-Ordovician black mudrocks for building and construction use

Norwegian black mudrocks of Cambro-Ordovician age contain sulfides that can oxidise when exposed to air and water, generating acidic water with high concentrations of sulfates, and including, where present, heavy metals, uranium and aluminium (so-called acid rock drainage or ARD). In addition to the environmental risk they pose, these reactions can lead to acid and sulfate attacks on concrete, which may severely damage foundations and buried structures. Secondary precipitations can also cause the rocks themselves to swell, resulting in heave. Relevant identification and characterisation of these black mudrocks is therefore necessary for building and construction projects to enable the selection of the most appropriate design for rock support and foundation materials. Rock masses that are potentially reactive may also need to be characterised in order to select the most appropriate and environmentally safe disposal option. The assessment of black mudrocks can, however, be complex, and there is a demand among professionals (building owners, consultants and contractors) for a practical and widely applicable characterisation method that can be used to anticipate and select adequate measures during a construction project. To this end, an innovative characterisation method for black mudrocks has been developed for routine use. This practical and simple approach is based on a study of data from more than 400 samples of Cambro-Ordovician black mudrocks, calcareous mudrocks, and grey mudrocks from construction projects in the Oslo region of Norway. The method, which only requires whole-rock analyses, is based on a combination of elemental composition analyses, the estimation of acidification and neutralisation potentials, and the calculation of iron:sulfur mole ratios. The method is fast, economical, reliable, independent of the weathering grade, and can be used indiscriminately for intact, blasted or crushed rock or core samples.