Pyrrhotite in Portuguese aggregates: a first insight

Abstract

The presence of iron sulfides in aggregates can be responsible for severe concrete degradation. The oxidation of these minerals in the presence of humidity can lead to the formation of sulfate minerals, such as gypsum, ettringite and thaumasite, from the reaction with the hydrated cement paste components, leading to expansion and cracking. A number of severe cases is known in Canada and in the USA. In the Iberian Peninsula, especially in the Catalan Pyrenees region, Spain, several concrete deterioration cases have been reported involving the use of aggregates with iron sulfide-bearing phases, namely pyrrhotite. The affected structures comprised buildings and public works, such as gravity dams and a deep study of the affected concrete and the reactions involved was performed, as documented in several papers and reports. The mechanism led to the settlement of a threshold of maximum total sulfur content of 0,1 % as S in aggregates in EN 12620, referring to special precaution when pyrrhotite is present in the aggregate. The identification of this mineral at the microscopic scale demands that polished surfaces or polished thin sections are prepared and the study is performed using reflected light microscope, and not the transmission light petrographic microscope applied in the identification of non-metallic minerals. The most common Portuguese aggregates are limestone and granitic rocks. The later usually contain opaque minerals and, therefore, for the most recent large dams built in the country with granitic aggregates the chemical analysis, including S, was performed. In some regions of the country, other rocks are used in the construction industry, namely schist, diorite and gabbro. The present work presents the state-of-art about the occurrence of pyrrhotite in Portugal and a research plan for successfully investigating the potential oxidation of aggregates containing this iron-sulfide phase.