When New France’s first settlers discovered deposits of iron pyrite in Quebec, they thought they had hit the jackpot. Unfortunately for them, this "fool's gold" had little value.

Even today, pyrite is sometimes considered a contaminant that can cause various problems, especially when it is present in the fill used in building construction.

But is pyrite-contaminated soil a real danger? Our contaminated soil management experts give you more information.

What is iron pyrite?

Pyrite is an iron sulphide mineral of the sedimentary rock family. It occurs naturally in several types of rock formations, particularly in Quebec and the northeastern United States. 

Iron pyrite has a slightly golden colour, a bright metallic luster and it is opaque. It tends to crystallize, so its surface reflects light like gold. Its appearance therefore contributed to early settlers confusing pyrite with real gold. However, around the year 50, the Greeks were already using it as a fire stone, hence the name pyrite from the Greek "pyros".

In contact with oxygen, water and moisture, pyrite oxidizes and produces sulphuric acid. Pyrite oxidation also leads to the formation of gypsum crystals, which increases its volume.

Unlike asbestos and lead which can be harmful to human health, pyrite is not considered a hazardous material by law. On the other hand, it can cause other types of significant problems, especially when it is present in the backfill material used under the slab of a building.

Problems caused by a pyrite contamination of soil

Before pouring a concrete slab, crushed stone (backfill) is commonly spread to even out the surface to be covered. However, due to the chemical properties of pyrite that cause it to expand in contact with air and moisture, a fill that contains traces of this mineral can increase in volume. 

This process is quite slow and is sometimes only noticeable 10 years after the building has been constructed. Many owners only realize it when star or cross-shaped cracks appear on their concrete floor and release fine gypsum dust. 

In a finished basement, since cracks and damage to the concrete are concealed by the building materials, it is even more difficult to detect a pyrite problem in the backfill. However, the danger is still there. Cracks in the slab can allow moisture from the soil to penetrate your home, promoting mold growth and the degradation of floor coverings. In extreme cases, swelling can even weaken the load-bearing elements of the building. 

A house’s market value will therefore decrease considerably if pyrite problems have been detected and confirmed by a pyrite test. 

How to solve a pyrite problem

Pyrite problems can be solved, but this intervention requires a lot of time, effort and resources, especially in a basement or a converted garage. 

The first step is to break the concrete slab covering the fill containing pyrite, then excavate all contaminated soils.

The DB crushed stone (certified non-swelling by the supplier) then has to be put back before pouring a new concrete slab. 

In conclusion

To answer the initial question, yes, pyrite-contaminated soils are dangerous, but only when used as fill material. Otherwise, pyrite does not present any major health risk.

Contact us if you have any questions about hazardous material management or to know more about a material’s risk level. Our environmental services specialists will gladly answer your questions and offer eco-friendly decontamination solutions.