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François Hamel, geologist



Patrick Usereau, geologist



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Pyrite associated swellings

The oxidation of pyrite

Pyrite is a very common mineral found in most types of rocks used in backfill for residential constructions.

The oxidation of pyrite in the combined presence of humidity and other common calcium carbonates, will produce new minerals with a larger molecular structure (gypsum, etc.). These larger minerals combined with the force of crystallization will, in some cases, cause the break up of the stone and the “swelling” of the backfill, and in other cases, the swelling of the concrete floor itself.

Backfill swelling and concrete deterioration

The progression of the chemical reaction causes the backfill to swell, thus heaving the concrete floor along with structural elements supported by it. The oxidation of pyrite can also cause the deterioration and the swelling of the concrete itself. In fact, it can be responsible for very significant uplift of the concrete slab (up to 15 cm). Damage to the upstairs floors and partitions can also be perceptible.

In many cases, the swelling will be directly proportional to the thickness of backfill underneath the concrete slab. In fact, damages often increase with thickness. This is why, in most cases, the most significant uplifts are observed in garage and other ground level sections, where the backfill layers are thicker than in basement sections. In some cases, lateral pressure generated by the backfill can cause the cracking and the movement of the foundation walls of these sections.

Distinctive signs

Distinctive signs of a problem backfill underneath the concrete floor are the following:

  • Open vertical cracks located at the corners and between corners of foundation walls of garage and ground level sections. Lateral pressure can cause the exterior wall to detach and to move.
  • Star or cross shaped cracks in the concrete floor

  • Concrete floor heaving, often associated with the cracks

  • White powder composed of needlelike crystals near the cracks

  • When false floors and/or partition walls rest on the concrete floor, the following can be observed: bulge in the false floor, buckling of partition walls, cracks on gypsum facings, door malfunction, etc.

  • Similar damage may appear upstairs when heaving is transmitted to the floor and interior partitions.

What is bad backfill material?

Before the application of the CTQ-M100 protocol (voluntary control procedure) put in effect on April 15th 1999, no specific legislation applied to backfill material used for residential construction. Low cost all purpose category backfill comprised of easily crushed, water and oxygen absorbing stone, could easily be found on the market for commercial and residential use. Types of rocks such as shale and mudstones, known to cause swelling, could be found in this category of backfill.

Distribution of cases

In the Montreal metropolitan area, problem backfills may originate from limestones but they particularly come from shale and mudstone formations found in the Basses-Terres-du-St-Laurent geological region.

The majority of cases are located in the Monteregie and Montreal island regions. However, some cases have also been reported on the island of Laval, in the Repentigny/Legardeur region and in the Quebec city suburbs.

Generally, affected buildings are aged between 8 and 20 years. However, buildings 30 years of age and even 40 years of age have been reported to show signs of swelling problems. At the opposite, buildings of less than 5 years of age have also been known to show characteristic signs.

Health and pyrite

The pyrite oxidation phenomenon itself cannot be directly related to health problems. However, health problems could be indirectly caused by pyrite related damage. Cracks in the concrete floor and walls could favor the increase of humidity in the basement therefore causing air quality problems with the appearance of traces of mold.

Where human health is concerned, pyrite associated swelling of the backfill does not seem to pose greater problems than any other types of damage to the foundations and should require similar precautions.