Pyrite and pyrrhotite

Two distinct problems, often confused, and both costly. Pyrite is found in the stone backfill under the basement slab; pyrrhotite is found in the aggregate of the concrete itself. In both cases, on contact with air and moisture, the iron sulfides oxidize and expand, which lifts, cracks and degrades.

Pyrite, under the slab

The contaminated backfill swells under the basement slab and lifts it. Typical signs: a bulging slab, radiating cracks, partitions or door frames that stick in the basement. Several areas of the South Shore and Montérégie have been affected. The diagnosis is confirmed by a petrographic analysis (IPPG index) in a laboratory.

Pyrrhotite, in the concrete

Here it is the foundation concrete that degrades from the inside: the pyrrhotite in the aggregate swells and causes a network of cracks (map cracking), efflorescence, crumbling concrete. This problem is documented mostly in the Trois-Rivières and Mauricie region. It is a serious structural issue that can lead to rebuilding the foundations.

What we look at during the inspection

Lifting or bulging of the slab, characteristic cracks, condition of the foundation concrete, the history and region of the building. A pre-purchase inspection does not replace a laboratory analysis, but it spots the signals and tells you whether to look further, before you sign.

Why it matters before buying

The work tied to pyrite (replacing the backfill and slab) or pyrrhotite (rebuilding the foundation) runs into the tens of thousands of dollars, and the issue can affect insurability and resale. For a building, a building condition assessment quantifies the impact. Knowing it beforehand means negotiating or walking away with full knowledge.