Université Laval: Laboratory Analysis Services in support of Geological Survey Ireland’s "Irish Construction Materials" Projects: Concrete Products

Northwestern Ireland (mainly County Donegal) has been the cradle of concrete related problems over the last ~ 15 years. The cause of the problem in Donegal, according to a government report published in 2017, has mainly been attributed to the high content of muscovite mica in the aggregates. However, the presence of mica in the aggregates as the cause of the block degradation has not been confirmed by scientific evidence. Therefore, the reason given for the disintegration process must be considered as a hypothesis. On the other hand, the presence of pyrrhotite in the aggregates used for concrete block manufacturing has recently been demonstrated. To allow for scientific evidence to be gathered to document and understand the real problems of concrete degradation, the Geological Survey of Ireland (GSI) (funding) and Ulster University in Northern Ireland formed a research project with a group of internationally renowned concrete experts/scientists. Université Laval (Quebec, Canada) was one part of the four institutions involved in the research, along with the University of Connecticut, Concrete Research and Testing LLC, both in the USA, and EMPA in Switzerland. A research and testing program was both funded and approved by GSI and Ulster University at each research facility. The Université Laval testing program included aggregate testing and characterization (macroscopic and microscopic observations, thin sections, SEM, XRD) along with oxygen consumption testing (OCT) on aggregate and concrete specimens, mortar and concrete testing of freshly supplied aggregates (Quarry A, B and C). In addition to aggregate testing, concrete samples (laboratory cast and field specimens) were also characterized by several techniques (macroscopic and microscopic observations, thin sections, SEM, μXRF, OCT).

Quarry A (phyllite) and C (mica schist) samples are poor quality aggregates that would not meet the criteria for use as concrete aggregate in Canada. In addition, Quarry C has a total sulfur content of 0.81 wt%, which is eight times higher than the EN12620 limit. Quarry C aggregate has a % vol. of pyrrhotite as measured by the μXRF of 1.36%, which correlates very well with its total sulfur content. Quarry C aggregate has an oxygen consumption above the protocol limit, indicating an oxidation potential in concrete. Quarry C consists mainly of phyllite in addition to mica schist. The latter is different from phyllite, which is mainly found in concrete blocks taken from deteriorated houses. Quarry B aggregate is a quartzite with very low total sulfur content (0.03 wt%).

The expansion tests indicate that Quarry B and C aggregates have the potential for alkaline reactivity (ASR). However, given the low cement content and high porosity of Irish field blocks, ASR is unlikely to develop in such concrete blocks in the field. For the Quarry C aggregate, it is possible that some of the expansion in Phase I of the sulfide oxidation expansion test is related to the oxidation/sulfate attack of the pyrrhotite present. This is consistent with the high sulfur content and the presence of pyrrhotite in that aggregate, as well as the high oxygen consumption measured in the oxygen consumption test.

The new electromigration test proposed for evaluating oxidation potential in sulfide-bearing aggregates did not show any problematic behavior (e.g. significant cracking) leading to deterioration of the concrete specimens investigated (no significant loss of modulus of elasticity); the concretes made with Quarry C aggregate showed some traces of rust, but nothing significant enough to lead to a significant loss of mechanical properties of the concrete. This new test still needs to be validated. High O2 consumption values with Quarry C aggregate are a possible indication of oxidation potential.

In this case, the material is finely ground and the sulfides finely disseminated in the rock are more accessible to oxidation.
All the concrete blocks examined contain pyrrhotite, mainly in the phyllite facies. The cement paste consists of well hydrated Portland cement and contains a significant amount of mica. The oxidation state of pyrrhotite varies according to the components observed. Oxidation is very advanced in the outer leaf block with the presence of thaumasite in the pores. In the inner leaf, gypsum is observed due to the strong carbonation of the cement paste. In this case, the pyrrhotite is only locally oxidized. The concrete samples from the foundation show pyrrhotite that is not, or only partially, oxidized.