Technical bulletins
May 2010

Did you say pyrite? But what about pyrrhotite?

By Frédéric Gagnon Eng., M.Sc.,

Pyrite, also known as fool's gold, is a mineral that has been under direct attack from the diverse players in the housing industry. Approximately 10 years ago, medias regularly reported severe problems associated with the presence of pyrite in the backfill material used in the housing industry. The impact of pyrite was such that a Quebec technical committee established a sampling and testing protocol (CTQ M200).

Lately, the incidence of foundation and structural problems associated with the presence of pyrrhotite in the backfill material or in the concrete has been on the rise. What is pyrrhotite? A form of pyrite? A synonym of pyrite? A more aggressive form of pyrite? Everyone in the industry has its take on this phenomenon and this technical article will try to demystify these issues.


Discovered in the middle of the 1930's by scientists, the pyrite problem phenomenon then identified was the swelling of the backfill material that contained pyrite, a mineral that is an iron sulfide compound (FeS2), which is found in several rock formations, such as schist or shale. In the presence of humidity and oxygen, oxidation
of the pyrite takes place and results in the production of sulfuric acid. The acid can then react with the calcium carbonates (lime) found in the crushed stone, such as limestone, commonly used in backfill and concrete. This chemical reaction produces sulfate and gypsum and results in the swelling of the backfill along with the associated movements and cracking of slabs on grade as well as of concrete foundations, both in basements and parking garages. In some cases, the chemical reaction that results from the oxidation of the ferric ions, also known as sulphatation, can also attack the concrete itself, causing delamination to varying degrees. This swelling phenomenon of the backfill material is rather well known and well documented, especially since the end of the 1990's in the province of Quebec. However, the effects caused to foundations by pyrite are not only associated with its presence in backfill material, but can also emanate from the fact that the crushed stone used in the concrete contained the same reactive materials.

Figure 1: Example of cracking at corners of foundations

Since concrete is porous, oxidation of the pyrite that could be contained in the aggregates used for its preparation is possible. Usually, this reaction is rather slow, given the oxygen poor environment within the lattice structure of the concrete. The speed of the reaction depends on the concentration of pyrite present in the aggregates as well as the overall quality of the concrete. Particularly, the quality of the hydrated cement paste will be a major impact on the reaction. Concretes with low quality cement paste result in a more porous structure and thus the oxidation of the pyrite is more rapid. First symptoms of the presence of pyrite in the concrete of the foundations are usually the formation of cracks. The cracking pattern is generally polygonal or usually referred to as scales. These fine cracks come at the surface in a grid, which is usually square in shape. The cracking is the result of the internal pressure caused by the swelling, itself resulting from the chemical reaction. Other symptoms of the presence of pyrite in concrete are discoloration on its surface as well as the formation of craters or pop-outs due to the breakdown of the cement paste. As a result, the concrete will become delaminated and will lose its mechanical properties, such as its compressive and tensile strength, which will be considerably reduced. Finally, the first symptoms of the presence of pyrite, which is reacting inside the concrete, can be observed after several years following the concrete placement.


Pyrrhotite, like pyrite, is an iron sulfide component but its crystalline form is unstable (Fe1-xS). Even though its crystallography is different from pyrite, the reaction mechanism in the concrete is essentially the same as described above. In fact, what distinguishes pyrrhotite from pyrite is the speed at which the oxidation or sulphatation of the compound starts and progresses in time. When present in concrete for foundations, the cracking and deterioration of the concrete can start as early as three to five years following the construction of the building. The early onset cracking observed is generally near the corners of the walls (Figure 1), in areas where the stresses associated with the swelling of the structure are more important due to exposure.

Figure 2: Close-up photograph of a concrete sample retrieved from a foundation

When deterioration of the concrete indicates presence of pyrite and/or pyrrhotite, the investigative measures that must be used consist in sampling the concrete for laboratory analysis (Figure 2). Even though signs such as cracks or rust stains can be visible under the naked eye on the concrete cores obtained from the sampling, is it only a petrographic analysis (identification of the types of rocks and minerals in the concrete mix) carried out in a lab that can confirm the presence of pyrrhotite bearing aggregates inside the concrete mix.

Unfortunately, once the reaction has started, it is next to impossible to stop it and the only solution for building owners is often to demolish and to reconstruct the impacted portion of the structure.


Whichever the form of the reaction and that it be called pyrite, pyrrhotite or by other names, the iron sulfur compounds can cause damage to the foundations and to concrete in general. The first signs appear as cracks and spalling, which can also be associated to other phenomena that can impact the concrete, such as freeze-thaw, alkali-silica reaction and deterioration due to sulfates in general, which can also be found in soil. An investigation is necessary for the identification of the exact cause since the ongoing process deteriorating the concrete must be established in order to identify the proper course of action, since the symptoms can be misinterpreted from a visual inspection.