Internal sulfate attack in concrete caused by framboidal pyrite

Numerous homes in county Mayo, Ireland, built with concrete blocks have developed severe cracking approximately a decade after construction. The culprit of the deterioration is framboidal pyrite present in the argillaceous limestone used as aggregates for the concrete blocks. This type of pyrite is prone to oxidation due to its small size and high specific surface area. Its oxidation leads to the formation of iron oxides and hydroxides as solid phases leading to a volume increase. Furthermore, sulfur is released into the concrete where it reacts with monocarbonate to form ettringite. This part of the reaction causes an additional, pronounced volume increase. It is commonly referred to as internal sulfate attack (ISA). In this study, a home that was demolished due to the pronounced cracking of the concrete blocks is investigated. The changes in the microstructure of the concrete blocks are investigated using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy.