Dr. Andreas Leemann:

I propose that I start. I will quickly say something about the role of pyrrhotite in the concrete blocks from Donegal. Pyrrhotite is the unstable form of iron sulphate. It oxidises, which results in a first phase of expansion. This is relatively small. It is the rust that is formed from the iron. Sulphur is then released, which leads the concrete blocks to expand due to the formation of a new minerals. The first is called ettringite and then, in the second stage, another sulphate mineral that leads to the loss of the concrete's strength, thaumasite, is formed. At a given point, carbonation, that is to say the concrete taking in CO2 from the air, kicks in. This boosts the formation of thaumasite, which means a loss of strength. In the end, these two new mineral phases, ettringite and thaumasite, disappear because gypsum is formed. This is the end stage. With regard to the standards and the levels of sulphur that are allowed in aggregates, Professor Dunlop has looked into this question and can comment.