Pyrite removal

Protect your concrete slab with Fondabec

Pyrite is a mineral naturally present in certain backfill materials used during house construction in Quebec, particularly between 1980 and 1995. When it comes into contact with water and oxygen, pyrite can trigger a chemical reaction that causes swelling and pressure under the concrete slab, resulting in cracks, lifting and structural damage. At Fondabec, we assist homeowners affected by pyrite at every stage: diagnosis, certified testing, complete removal of contaminated backfill and restoration of your concrete slab. Our goal is simple: to restore the safety, value and stability of your property.

What is pyrite and why is it a problem?

Pyrite (FeS₂) is an iron sulphide mineral present in certain sedimentary rocks and backfill materials used in foundation construction. In the presence of moisture and oxygen, pyrite oxidises, causing:

This chemical reaction may take several years to manifest, but once it starts, damage progresses slowly but steadily, compromising the building’s structure.

What are the signs of pyrite?

Pyrite is subtle: damage is not immediately visible and can take many years to appear. Nonetheless, characteristic signs eventually show up, especially in garage or basement slabs sitting on contaminated backfill. Recognising these early indicators allows action before the situation becomes critical.

Cracks or lifting of the slab

The swelling caused by pyrite oxidation exerts upward pressure, which results in cracks in the concrete slab. These cracks are often in the form of a road map (irregular mesh), star-shaped, multiple and spread over a wide area. In some cases, you will notice localised or general slab lifting, sometimes accompanied by a change in level noticeable to the touch or sight.

White efflorescence on the concrete

The chemical reaction of pyrite releases sulphates that migrate to the surface, leaving powdery whitish marks called efflorescence. They often appear around cracks or at the base of walls. These deposits are an early sign that moisture is being absorbed into the concrete and that a chemical reaction is underway deep down.

Doors or partitions that warp

As the swelling increases, it can deform the interior structures. You may observe doors that do not close properly, partitions that crack or shift, floors that become uneven. These deformations are often mistaken for minor problems, but they signal that the slab is moving under the pressure of the expanding backfill.

How do you confirm the presence of pyrite?

Certified pyrite test (IPPG index)

The only reliable way to detect pyrite is to have a laboratory test carried out on a sample of backfill taken from beneath the slab. This test calculates a swelling potential index (IPPG):

0 to 10 = low risk
11 to 20 = moderate risk
21 and above = high risk

This test is often required during a real estate sale, a claim or an insurance request.

Pyrite removal: how does the work proceed?

Once pyrite is confirmed by an IPPG test and the risk assessed, complete removal of the contaminated backfill becomes essential. It’s not enough to seal cracks or pour a new slab: to solve the problem at its source, all swelling materials beneath the slab must be removed and rebuilt on sound foundations.

At Fondabec, we follow a proven, safe process in line with industry best practices to ensure a durable correction wherever you are in Quebec.

Demolition of the existing slab

The first step is to break and remove the affected concrete slab, whether it is a garage, basement or crawl space. This operation is carried out carefully to avoid compromising foundation walls, columns or other structural elements. Debris is then responsibly removed.

Excavation and removal of the contaminated backfill

Once the slab has been removed, our teams carry out mechanical excavation of the backfill to a depth generally between 30 and 60 cm, depending on the thickness of the contaminated material. All backfill is carefully removed until stable soil or a suitable layer is reached. This pyritic fill is then transported to authorised landfill sites, in accordance with Quebec’s environmental standards.

Replacement with a DB certified backfill (concrete slab)

We then replace the excavated backfill with a “DB” certified material (designed for concrete slabs), approved for corrective pyrite work. This backfill has low reactivity and high stability, ensuring that no future swelling reactions will occur. It is compacted in several layers to provide a solid base before rebuilding the slab.

Pouring a new concrete slab

Finally, a new concrete slab is poured, with or without insulation, depending on your needs (basement use, comfort, renovation project). We follow best installation practices:

A doubt or a question about a pyrite problem?

For more than 20 years, Fondabec has assisted homeowners in Greater Montreal and across Quebec with pyrite removal projects. Whether you are in Laval, Longueuil, Terrebonne, on the North Shore, the South Shore or in rural areas, our team travels quickly to assess your situation and offer the most durable solution.

We hold all the certifications required in excavation, civil engineering and underpinning, and we work with partner engineers and surveyors to guarantee compliance and quality for every project.

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FAQ

How much does pyrite removal cost?

The average cost depends on several factors and varies according to the surface area of the slab, backfill depth, site accessibility and the type of finish desired. A precise estimate is possible after inspection.

Is it covered by homeowners’ insurance?

Not systematically. Some insurers cover the cost if pyrite causes a recognised loss (serious cracks, settlement). It is advisable to check with your insurer or broker before starting the process.

Can you simply seal or inject a crack caused by pyrite?

No. Cracks linked to pyrite are caused by constant pressure from the soil. Repairing them without removing the source of the swelling is ineffective and temporary.

Do you need to replace the entire slab if pyrite is present?

Yes, in most cases. To eliminate pyrite at the source, the slab must be removed and the contaminated backfill excavated. Partial or local repairs are not enough.

Is an IPPG test mandatory before selling a house?

It’s not mandatory, but strongly recommended, especially if the building was constructed between 1980 and 1995, or if there are visible signs (cracks, lifting). In addition, many buyers and financial institutions now require it at the time of sale.

For over 20 years, Fondabec has been assisting homeowners in Greater Montreal and throughout Quebec with their excavation projects.

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