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Pyrite is a sulphite mineral. Traces are found in sedimentary rock used to make crushed stone for backfill. In the presence of humidity and oxygen, pyrite oxidizes and produces sulphuric acid. This acid reacts with calcium carbonates found in crushed stone and concrete based products. This chemical reaction called sulphation forms gypsum whose crystallization will cause the stone to swell.
Sulphation causes the concrete slab itself to swell and crumble. It can produce swelling up to 15 cm. The humidity under the slab tends to creep upward, transporting the sulfuric acid produced by oxidation. When sulfuric acid comes into contact with the bottom of the slab, the concrete swells from sulphation and starts flaking and disintegrating from the bottom. The swelling concrete expands. The foundation walls restrain horizontal expansion; so heaving usually occurs upwards from the centre of the slab, creating large bulges.
The forces of the swelling concrete may cause cracks in your foundation walls, jeopardizing the stability of your home.
The majority of cities affected are located in the Montérégie area and on the island of Montreal. For residential buildings constructed before 1970, the layer of crushed stone is either non-existent or quite thin. This is why we see very few pyrite related concerns for buildings constructed prior to 1970.
Below we have included some common procedures and best practices for pyrite removal.
Not sure if your foundation is crumbling due to pyrite? Seeing the signs but looking for an expert oppinion? Time for a foundation intervention, our team of pyrite experts are available to answer any of your questions.
