Internal sulfate attack on concrete – a case study from homes in Inishowen, Northwest Donegal, Ireland

Concrete blocks are widely used in Ireland in the construction of domestic buildings. Homes built from the late 1990’s onward using this method are increasingly affected by cracking and deterioration of the concrete blocks. The degree of deterioration in the several thousands of houses affected ranges from fissures running through blocks and their plaster cover, to complete disintegration of the concrete. The cause of the problem in Donegal has primarily been attributed to the high muscovite mica content present in the aggregates. This is based on the conclusions of a desktop study published in 2017 by a panel of engineers and geologists, who were appointed by the Irish Government to investigate the problem.

Samples of four houses showing a degree of damage on the facades from relatively minor to severe have been investigated using scanning electron microscopy with energy-dispersive spectroscopy, image analysis and thermodynamic modelling.

The aggregates used in affected homes in Donegal are mainly phyllites and schist quarried from Dalradian metamorphic formations that contain a high muscovite mica content. However, these aggregates additionally contain iron sulphides, mainly in the form of pyrrhotite. The hydrate assemblage in the concrete blocks shows differing degrees of alteration that can vary considerably on a centimetre scale. Relatively little affected locations show the presence of ettringite in the typical clusters formerly consisting of monosulfate/monocarbonate. In the strongly affected locations the cement paste is nearly completely transformed into thaumasite. Some severely deteriorated concrete blocks are fully carbonated resulting in the formation of CaCO3, silicon-aluminium gel and gypsum. This process obliterates the chemical reactions leading to deterioration.

The alteration of the hydrate assemblage is a clear indication for internal sulfate attack [1,2] with pyrrhotite as the only likely sulphur source. Expansion is typically caused by ettringite formation [1-3], while the complete strength loss of some concrete blocks can be attributed to thaumasite formation [4].

Literature:

[1] I. Casanova, A. Aguado, L. Agulló, Cem Concr. Res. 1997, 27(11) 1627-1632.
[2] Lee, H., Cody, R. D., Cody, A. M., & Spry, P. G. Cem. Concr. Res. 2005, 35(2), 332-343.
[3] W. Kunther, B. Lothenbach, J. Skibsted, Cem. Concr. Res. 2015, 69, 37-49.
[4] M. Romer, L. Holzer, M. Pfiffner, Ceme. Concr. Comp. 2003, 25(8), 1111-1117.