The Donegal mica scandal, the tip of an iceberg?

The Donegal mica crisis began in 2011 and was initially linked to the presence of excessive free mica in the binder of aggregate concrete masonry units (ACMU) used for construction of dwellings and commercial premises in the decade prior. Free mica was identified to be abraded from the muscovite rich aggregate source, a low-grade phyllite. This free mica increased the microporosity of the cement binder leaving it highly susceptible to secondary degradation processes, including moisture ingress, potential freeze-thaw degradation and internal or external sulphate attack. As well as weakening the binder overall strength the ability to adhere to the flakey mica-rich aggregate was also reduced. Further work since then has highlighted the presence of elevated levels of pyrrhotite within the predominant problematic aggregate, a highly reactive sulphide responsible for internal sulphate attack. Taken together, ACMU’s produced using phyllite aggregate has produced abundant defective concrete block within County Donegal. These ACMU’s have rapidly deteriorated producing the present crisis within County Donegal.

Whilst the most defective phyllite-bearing ACMU has typically already suffered critical deterioration, work undertaken as part of I.S. 465 has also highlighted other metasedimentary aggregates in use within County Donegal which are of lower or higher metamorphic grade. Sulphide abundance within these aggregates is lower and sometimes absent, depending on the grade and phases of deformation they have been subjected to. In addition, when sulphides are present pyrite tends to predominate, with pyrrhotite largely absent except for at the higher metamorphic grades transitioning from the problematic phyllite (i.e. schist, hornfels and amphibolite). These aggregates also produce slightly lower levels of free mica in the binder, either due to stronger annealing at higher metamorphic grades or less abradable mica and splaying along crenulations at lower grades. These aggregate types, and the houses built from the ACMU’s are nevertheless showing signs of degradation and point to the need to consider the longer-term impacts and risks of ACMU’s made from the different aggregate types that may be prevalent within County Donegal.

This abstract proposes a three-fold metric for identifying the most defective ACMU’s at risk of premature deterioration dependent on the metamorphic grade and deformation characteristics of the aggregate used, the amount of free mica produced in the binder and the abundance of sulphides, particularly pyrrhotite.