Where did the mica narrative come from?

For years, the damage to homes in Donegal was called the "mica crisis". This page explains where that idea came from, what the laboratory tests went on to show, and what the research now says. For a shorter overview, see the science and what defective concrete is.

What did the 2017 report say?

In June 2017 the government published a desktop study, the Report of the Expert Panel on Concrete Blocks. It was published by the Department of Housing on 13 June 2017 and last updated on 4 December 2020.

The report found that high levels of muscovite mica were the cause of the cracking and breakdown of the blocks in Donegal. Muscovite mica is a non-swelling silicate mineral found in the defective blocks.

The theory was that the mica absorbed water. That water could then freeze and thaw as temperatures changed, cracking the blocks.

How did that shape testing?

In 2020, homeowners in Donegal began to apply to the government's defective concrete block scheme. The scheme required geological laboratory tests to confirm that a home was built with defective blocks.

The tests followed I.S. 465, a protocol developed by the National Standards Authority of Ireland (NSAI). In Donegal, it required the tests to identify and measure the muscovite mica, and engineers to assess the risk this posed to the property.

What did the lab reports actually find?

As lab reports came back to homeowners in 2020, most confirmed high levels of mica. But they also found iron sulphide minerals, which are widely accepted and understood to be destructive for concrete.

The main one is pyrrhotite. Pyrite, marcasite and chalcopyrite have also been reported in defective Donegal blocks.

What does the research say now?

Research has found that pyrrhotite, a highly reactive iron sulphide mineral, is causing internal sulfate attack, and that this is the primary cause of the failure seen in Donegal homes. The science has moved on considerably since the 2017 report.

In June 2023, Geological Survey Ireland funded a €500,000 research project into the role of pyrrhotite in the Donegal defective blocks. It is led by Ulster University, with partners at:

• Empa, Switzerland
• Laval University, Canada
• the University of Connecticut, USA
• Concrete Research and Testing LLC, Ohio, USA

This research will be used to inform the review of I.S. 465, which is under way. You can read more about that review on our I.S. 465 page.

How does pyrrhotite damage concrete?

Pyrrhotite is an iron sulphide. When iron sulphides are exposed to water and oxygen, a series of chemical reactions breaks them down and forms new minerals called sulphates.

These sulphates take up more space than the iron sulphides they replace. As they grow, they push against the surrounding material, causing it to swell and crack.

How much pyrrhotite is allowed in concrete aggregate?

The European standard for concrete aggregates, EN 12620, sets a limit of 1% total sulphur by mass. That limit falls to 0.1% total sulphur if pyrrhotite is identified in the aggregate.

Many defective block homes tested in the Donegal area show roughly 2 to 3% pyrrhotite and other deleterious materials. Irish guidance on using this standard was updated in 2026: see S.R. 16.

Where can I read the research?

Published papers

• New research shows foundations containing pyrrhotite are at risk of long term failure (Redress Focus Groups). This summarises the paper Effect of exposure conditions and concrete quality on pyrrhotite oxidation and internal sulfate attack by Andreas Leemann, Beat Münch and Paul Dunlop. It begins: "Thousands of homes in County Donegal, Ireland, are damaged by cracks formed a few years after…"
• The "mica crisis" in Donegal, Ireland – a case of internal sulfate attack? Leemann, A., Lothenbach, B., Münch, B., Campbell, T. and Dunlop, P. (2023), Cement and Concrete Research, Vol 168.
• An Examination of Governance Failure by the Irish State: The critical case of the 'Mica'/Defective Blocks issue. Doherty, E., Carcary, M., Ramsey, E., Kelly, D. and Dunlop, P. (2022). In F. Matos and A. Rosa (eds), Proceedings of the 18th European Conference on Management Leadership and Governance, ECMLG 2022, pp. 154–163 (Vol. 2022-November).
• Pyrrhotite in concrete aggregate. Introduction to mechanism, damage potential and ongoing research. Lindstad, Hallvard; Lindgård, Jan; Pedersen, Bård Magne; Wigum, Børge Johannes; Aasly, Kurt; Ullnæss, Inger Lise; Oberhardt, Nikolas; De Weerdt, Klaartje (2022).
• Concrete Damage Due to Oxidation of Pyrrhotite-Bearing Aggregate: A Review. Duchesne, J., Rodrigues, A. and Fournier, B. (2021), RILEM Technical Letters 6, 82–92.

Seminars, conferences and lectures

• The impact on family homes of defective concrete products, EU webinar, European Parliament, Brussels, 23 March 2022: watch the European Parliament webinar.
• Losing Your Home: The Impact of Defective Concrete Blocks in Ireland, ADEPT Research Group conference, Dr Martina Cleary, Limerick School of Art and Design, 30 May 2023: visit the Losing Your Home conference page.
• The Science and Societal Impacts of Defective Concrete, international conference, Clanree Hotel, Letterkenny, Donegal, 15 November 2022: read the conference schedule or watch all the conference talks.
• Deleterious Material in Concrete Masonry Units – Sharing the Knowledge, Engineers Ireland seminar, 11 April 2023, with presentations by John T Garret (Chartered Engineer), John Paul Farren of Anytime Coring Ltd and Dr Chris Brough of Petrolab: watch the Engineers Ireland seminar.
• Crumbling Concrete Foundations Due to Pyrrhotite Bearing Aggregates in CT, Dr Kay Wille, University of Connecticut, American Concrete Institute Spring Convention, Orlando, USA, 27–31 March 2022: watch Dr Wille's talk.

Useful websites

• The University of Connecticut Crumbling Foundation Research Team has two videos introducing the pyrrhotite problem. They describe the basic chemistry simply, and explain the method the team developed to measure pyrrhotite and what results it gives: see the UConn research team's method.
• Pyrrhotite and Crumbling Concrete in Connecticut, from the Connecticut Department of Energy and Environmental Protection.
• Connecticut's Crumbling Concrete, a case study from FEMA.

As new research is published, we will add it here.