Responding to feedback - High Rick concrete from County Donegal

Our recent publication covering the geology of defective concrete block from County Donegal received some significant feedback, some public and some private, that it is worth publicly responding to for clarification (see points below). It is noted from the outset that we welcome critical feedback of our work. No progress is possible without critical feedback of some form and the importance of the defective concrete problem requires it.

1. For the most deleterious phyllite aggregate too much is made of the "free mica" within the binder. The argument confuses the situation as this is from 1st to last a problem derived from internal sulphate attack (ISA).
2. In contrast, for others too little is made of the "free mica" within the binder. The presence of pyrrhotite within the aggregate is a secondary issue, that may be causing localised reaction but the widespread failure relates to poor quality mica-rich binder and subsequent leaching / reaction of this.
3. The referencing of EN 12620 is inappropriate as this wasn't formerly transferred as a legal constraint on aggregate production until 2013.

The last of these referring to EN 12620 is the most straightforward to give an opinion on. Below is a screenshot for an upcoming paper submitted for ICISR referring to our reasoning on this, that essentially EN 12620 did apply, or should have applied from 2003, and homeowners can expect as such from Ireland's membership of CEN.

With respect to a role for free mica. Our argument is best envisaged through a thought experiment. If the pyrrhotite was magically removed the concrete would still be poor quality in and of itself. The free mica content of the binder is a useful measure of this. The high free mica content means the binder has high microporosity - along with this the aggregate hosting the pyrrhotite is relatively weak and easily breaks along cleavage planes (see image D for e.g.) - this matters because both these metrics make the concrete susceptible to secondary processes.

The fact that a relatively poor quality concrete block also then hosts excessive quantities of pyrrhotite has been devastating for 1000s of homes. The oxidising pyrrhotite, and the reactions that proceed from this, have the capacity to "rip" through the blockwork which has little internal resistance.

The fact that free mica alone can act as a measure for poor quality concrete block is seen in the WMQ and PGV aggregate types, most clearly seen in poor binder to aggregate cohesion and high microporosity binder (see Fig below). Nonetheless, damaged properties built from these latter sulphide-free aggregate types currently number in the 10s, not the 1000s. Whilst there is no certainty on the final numbers, this difference in scale, and severity, of free-mica only affected properties against ISA affected properties is a strong indication of the difference in importance of the two factors.

As a final thought experiment - imagine taking excessive sulphide content and placing it in stable aggregate and good quality binder - whilst not an entirely like for like comparison the Penlee dolerite concrete block in Cornwall is a good example to consider. This blockwork has been stable for >100 years and the sulphide largely unreacted despite being present in concentrations unsuitable for aggregate now because of the sulphide content - this illustrates that high quality stable aggregate may provide resistance to ISA in the right circumstances.

Moving back to the Donegal context the HFS aggregate also contains pyrrhotite content, often in excess of compliance, though not to the level seen in the most deleterious phyllite block. In reality it is only just above compliance, but this is reason enough to monitor it. As an aggregate it is inherently better quality as the low level of metamorphism has led to the growth of minerals across pre-existing foliation (biotite hornfels, mafic hornfels) forming fragments that are inherently more coherent. Pyrrhotite within HFS aggregate is also typically encapsulated and unreacted (see Fig below).

In conclusion then, the phyllite aggregate is poor quality, which can be seen from the high microporosity binder and low-strength aggregate. The fact that this aggregate then contains excessive pyrrhotite means internal sulphate attack is able to propagate rapidly once it starts - essentially it's able to degrade concrete block which has little internal resistance. There are contrasting examples of aggregate containing elevated sulphide content that are nonetheless stable, notably Penlee dolerite, but also potentially HFS aggregate from Donegal itself. These need to be considered as an important illustration of the importance of the nature of the aggregate in susceptibility to, or resistance against, ISA.

References

Brough, C, Staniforth, B, Garner, C, Garside, R, Colville, R, Strongman, J & Fletcher, J. (2023): High risk concrete blocks from County Donegal: The geology of defective aggregate and the wider implications. Construction and Building Materials, 408, 133404. https://www.sciencedirect.com/science/article/pii/S0950061823031215?via%3Dihub

Leeman A., Lothenbach B., Münch B., Campbell T., Dunlop P. (2023): The “mica crisis” in Donegal, Ireland – A case of internal sulfate attack? Cement & Concrete Research, 168, (107149). https://doi.org/10.1016/j.cemconres.2023.107149