In Quebec, two words worry homeowners: pyrite and pyrrhotite. They're often confused because both are iron sulfides that damage foundations. But they are two distinct problems, with different mechanisms, affected regions and solutions. Here's how to tell them apart.
Pyrite: swelling under the slab
Pyrite is an iron sulfide found in certain crushed stones used as fill under the basement concrete slab. On contact with moisture and oxygen, pyrite oxidizes and triggers a chemical reaction that produces sulfates. These new minerals take up more volume than the original stone: the fill swells and lifts the slab from below.
Typical signs: a basement slab that bulges, cracks opening in the floor, partitions that distort. The problem mainly affected the Montreal and Montérégie region from the 1980s–1990s onward. Diagnosis is done in a lab from a sample of the fill (petrographic analysis, swelling-potential index). The usual repair is replacing the fill and the slab.
Pyrrhotite: concrete that crumbles
Pyrrhotite is another iron sulfide, but this time present in the aggregate used to make the concrete itself. When crushed stone rich in pyrrhotite goes into the concrete mix, oxidation of the mineral, followed by an internal sulfate attack on the cement paste, causes expansion that cracks and disintegrates the concrete from the inside — foundation walls, footings.
This problem is concentrated in the Trois-Rivières and Mauricie region, where concrete poured between the late 1990s and mid-2000s was made with stone from quarries associated with the issue (pyrrhotite content sometimes above 1%). Diagnosis requires a concrete core sample and petrographic analysis. Advanced cases often require full replacement of the foundations.
In short
| Pyrite | Pyrrhotite | |
|---|---|---|
| Where | In the fill under the slab | In the concrete of the foundation |
| Effect | Lifts the slab (swelling) | Cracks and crumbles the concrete |
| Region mainly affected | Montreal / Montérégie | Trois-Rivières / Mauricie |
| Diagnosis | Analysis of the fill aggregate | Concrete core sampling |
The Régie du bâtiment du Québec (RBQ) documents both issues and their remedies. In either case, a structural engineer plays a key role: confirming the diagnosis, assessing the extent of the damage and designing the repair plans. Laboratory results should be interpreted by qualified professionals.
Suspect a problem?
A heaving slab, active cracks or crumbling concrete deserve a professional opinion — not to be confused, by the way, with simple shrinkage cracks, which are harmless. Our residential structural expertise and our foundation design services let us assess the situation and guide the corrective work.
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Sources: Régie du bâtiment du Québec (RBQ), "Pyrite" and "Pyrrhotite" files. This article is provided for general information purposes.