Pyrrhotite and Mica in Donegal Concrete Blocks

The “Mica Crisis” in Donegal, Ireland, is a term that refers to the structural damage observed in thousands of homes built with concrete blocks that contain high levels of mica. This issue has affected not only private residences but also commercial and public buildings, including schools, libraries, hospitals, and government buildings. The severity of the damage ranges from web-like cracking to complete disintegration of the concrete blocks, rendering the buildings uninhabitable.


In 2013, homeowners in County Donegal and County Mayo first reported these structural defects, which prompted the Irish government to appoint a panel to investigate the problem in 2016. The panel concluded that the high mica content in the aggregates was the primary cause of the issue in Donegal, while pyrite was responsible for the damage observed in County Mayo. The Irish government launched a compensation scheme, the Defective Concrete Blocks Grant Scheme, to provide financial support to affected homeowners in 2020. The scheme is estimated to cost the Irish state around €3 billion and rising.However, a recent study conducted by researchers from Empa, the Laboratory for Concrete & Asphalt in Switzerland, the Ulster University in Northern Ireland, and the NTNU Department of Structural Engineering in Norway, suggests that the internal sulfate attack triggered by pyrrhotite oxidation, not the high mica content, may be responsible for the disintegration of the concrete blocks in Donegal.

The study investigated the concrete of four affected homes in Donegal using microstructural and chemical analysis combined with thermodynamic modelling. The results showed that apart from mica, the aggregates contained iron sulfides, mainly in the form of pyrrhotite. The sulfur content of the aggregates considerably exceeded the limit value defined by the European standard for concrete aggregates (EN 12620). The microstructural analysis coupled with thermodynamic modelling demonstrated that the concrete suffered from internal sulfate attack triggered by pyrrhotite oxidation. The comparison of the results of this investigation with the data collected by chartered engineers on almost one hundred damaged homes shows that the four investigated cases are representative of the situation in Donegal.

Mica wrongly blamed

The study challenges the assumption that the high mica content in the aggregates is the primary cause of the disintegration processes observed in the concrete blocks. Instead, it suggests that the presence of iron sulfides, mainly in the form of pyrrhotite, is responsible for the internal sulfate attack that leads to the deterioration of the concrete. This finding is significant because it suggests that the current compensation scheme, which is based on the assumption that the high mica content is the primary cause of the problem, may not be adequate to address the issue fully.

Moreover, the study highlights the need for more in-depth scientific investigation of the mechanisms of deterioration in the concrete blocks. Despite the enormous financial losses and profound impact on people’s lives caused by these issues, there has been no comprehensive scientific investigation or financial funding for such research. This lack of research is surprising given the widespread nature of the problem and the significant financial and social implications for affected homeowners and communities.

The study also sheds light on the source of the aggregates used in the defective concrete blocks in Donegal. The major source of the aggregates is from one quarry that provides phyllites and schist from the Dalradian metamorphic formations. These lithologies contain significant amounts of mica and iron sulfides, mainly in the form of pyrrhotite. The study suggests that more attention should be paid to the quality control of the aggregates used in the construction industry, especially in areas with similar geology to Donegal.

Was it caused by the very cold spells in 2009/2010?

The probability of frost attack on concrete blocks in Donegal, Ireland, has been a topic of concern due to reports of widespread damage to homes and buildings in the region. According to I.S.465, excessive quantities of free muscovite mica in aggregate can make the blocks susceptible to freeze-thaw degradation when saturated. While sub-zero temperatures can occur in Donegal, they are relatively rare due to the Atlantic influence on Ireland’s climate. Only two prolonged cold periods have occurred in the last decade, with below freezing temperatures experienced in many areas across Ireland. The first was in mid-December 2009 to early January 2010, and the second happened after the middle of November 2010. However, such temperatures are far from the norm, and the average temperatures in Donegal are typically well above these extreme events, with only a few days experiencing zero to sub-zero temperatures annually.

The Inishowen Peninsula in North Donegal has been the epicenter of the defective block crisis and has the highest concentration of affected homes and other buildings, numbering in the thousands. Climate records from Malin Head meteorological station, available from Met Eireann‘s online portal, show that only 50 days with zero or sub-zero temperature were recorded there during the last 10 years. Only three other meteorological stations with long-term daily temperature records are available for Donegal on this portal. These are Glenties, Finner, and Ballyshannon, and all have recorded more days with frost than occurred at Malin Head for the previous 10 years. However, these stations are situated over 100 km southwest of the center of Inishowen, and to date, there have been very few reports of affected homes in southwest Donegal in the vicinity of these three stations.

Temperatures as recorded at Malin Head cannot lead to frost damage of the concrete blocks. As the blocks are protected by a 2-3 cm thick layer of plaster, they are hardly exposed to frost when the temperature decreases to a few degrees below zero during the night and reaches temperatures above zero during the day. Within these boundary conditions, the temperature in concrete is a few degrees higher in a depth greater than or equal to 25 mm compared to the concrete surface. Moreover, a high degree of water saturation, a prerequisite for frost damage, is usually not reached on vertical surfaces like walls. When a concrete is not saturated, the freezing temperature of water decreases with decreasing saturation degree. As an example, water in a concrete with a water-to-cement ratio of 0.63 freezes at about -10 and -20 °C at relative internal humidity values of 94 and 90%, respectively. Here, it has to be pointed out that the Inishowen Peninsula receives considerably less precipitation than the western parts of Donegal, where only a few cases of damaged homes are present despite the likely higher moisture content in the outer leaf.

The damage characteristics of the homes are another strong indication against frost as the mechanism. Frost damage on the walls would be expected to manifest as surface parallel cracking and scaling, decreasing in severity with increasing distance from the surface exposed to the environment. This is in pronounced contrast to the cracking and damage observed in affected homes in Donegal. Cracks form perpendicular to the surface of the walls and often extend from the ground up to the roof. Some of them even propagate from the roof down, far above the typical frost zone at ground level. The purported frost damage is in contrast to the strength loss and crumbling of the concrete affecting the entire cross-section of the blocks to the same degree.

Moreover, as HomeCheck Engineers and Surveyors have witnessed while carrying out pre-purchase surveys, the deterioration of the homes has progressed significantly between the last extreme frost event in 2009/10 and 2022, and there are homes that started to crack before 2009/2010. This further supports the conclusion that frost is at best a secondary contributing factor, while the primary cause of the observed damage is the presence of pyrite and inadequate concrete composition and manufacturing.

Conclusion – it was the Pyrrhotite 

In conclusion, the defective concrete blocks found in the affected homes in Donegal are not frost-resistant, but the primary cause of their damage and deterioration is not mica and frost, as previously thought. Rather, the primary cause is pyrrhotite oxidation in the blocks (due to Deficiencies in their manufacturing and composition, combined with exposure to moisture and sulfuric acid), releasing sulphur resulting in a mineral called ettringite being formed which cause expansion in the blocks.