High risk concrete blocks from County Donegal: The geology of defective aggregate and the wider implications

•The results of over 1700 concrete petrography tests from County Donegal are summarised.•There are seven high-risk concrete types, each given a unique three-letter code.•The relative importance of excessive free mica and reactive sulphides (pyrrhotite, pyrite & framboidal pyrite) are discussed in each aggregate type.•Implications for remediation options, if any, are presented.•The risk window and implications for conveyancing are discussed. Since 2019, geologists at Petrolab have tested concrete block samples from over 1800 properties in County Donegal and observed deterioration patterns based on the aggregate types used within those concrete blocks. Across these properties more than 25 aggregate types have been used within concrete block, with seven distinct types considered high risk when assessed against relevant standards. With respect to those considered high risk, elevated free mica and elevated reactive sulphides (predominantly pyrrhotite) contents appear to play a critical role, particularly in phyllite aggregates commonly found in defective blockwork. The particular concentration of both of these mineral groups is considered the main cause of the rapid degradation often observed with this aggregate type. Other high-risk aggregates have lower amounts of one or both of these mineral groups, but they are still present in concentrations that are cause for concern and associated in some instances with damaged property. These findings have important implications. Firstly, with two mineral groups to define risk, the extent of risk can be split depending on the concentrations of the different minerals with potentially different remedial pathways followed in each instance. Aggregates with high concentrations of both free mica and sulphides (predominantly pyrrhotite) will pose the highest risk, likely requiring the most significant intervention in the long-term, while aggregates with only free mica and low sulphide content will be lower risk. In support of this, none of the properties tested to date, where only free mica risk is present within the concrete samples, have gone on to display the most serious degradation category. This, coupled with the evidence of internal sulphate attack present within the most high-risk pyrrhotite-bearing aggregate, is further evidence of the importance of internal sulphate attack in the highest risk aggregates. For all seven aggregates of concern the authors provide general recommendations for remedial measures, or further