Research sheds more light on the causes of defective blocks in Donegal homes

The latest publication of research by an Ulster University research consortium investigating the causes of the defective blocks crisis in Donegal has reaffirmed the primary cause as internal sulphate attack.

The research consortium has been selected by the Geological Survey of Ireland to investigate the causes of defective concrete failure in thousands of Donegal homes and the publication has been accepted for the International Conference on Sulphide Reactions in Concrete, at  Université Laval in Canada next May. 

Internal sulphate attack (ISA) occurs when pyrrhotite in the concrete oxidises when it comes into contact with oxygen and moisture. It first rusts causing it to swell and releases sulphur. This creates sulphuric  acid which reacts with the concrete to form secondary sulphate minerals that cause the concrete to swell, crack and lose strength.  

Official government research into the issue in Donegal was finally funded in June 2023 by the Department of  Housing, Local Government & Heritage and commissioned by the Geological Survey of Ireland following the publication of independent research in March 2023 in the international peer review journal,  Cement and Concrete Research.

The research Ulster University and Empa in Switzerland provided the first independent scientific evidence that ISA resulting from oxidisation of pyrrhotite in the concrete is the primary cause of the deterioration of the homes in Donegal and not mica freeze/thaw processes. This conclusion has subsequently been confirmed by a paper from researchers from Petrolab UK in the international peer review journal  Construction and Building Materials in October 2023. 

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The commissioned research conducted by Paul Dunlop, Andreas Leemann and Beat Münch, is based on a detailed investigation of four homes from different locations  in the county – three known to be affected by defective concrete and a control home that is not affected. All concrete elements of the homes were sampled for analysis - the outer leaf, inner leaf, rising wall and strip foundations. The research found that pyrrhotite oxidisation and ISA were clearly identified as the  primary process causing the observed damages in the investigated homes. This was as a result of micaceous  phyllite aggregate containing pyrrhotite being used in the blocks as well as the poured foundation concrete.  

The research also confirmed the presence of high quantities of muscovite mica in the concrete, which is known to create poor quality porous concrete. This is due to the high water  demand that occurs during the mixing phase, where free mica flakes attract water to their surface and  more water is needed to make the concrete less stiff and workable. The porous concrete opens up  further pathways for moisture and oxygen to access the pyrrhotite and fuel internal sulphate attack.  In response to the latest research, Lisa Hone, chair of the Mica Action Group said rigorous science must underpin any  defective concrete blocks scheme “to ensure that  the recommended remediation eliminates this issue for the homeowners”. 

“The research findings published  by the Ulster University consortium reaffirm the conclusions of previous independent investigations - the  primary cause of the deterioration of our homes is due to internal sulphate attack as a result of unregulated  quantities of iron sulphides, primarily pyrrhotite present in the aggregate. Iron sulphides are a known risk to concrete. Had the State ensured an effective system of regulatory governance and market surveillance this horrendous situation affecting thousands of homes would never been allowed to develop.”

The conference paper can be accessed here.