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Pyrite And Your House

Pyrite (FeS2) is a very common mineral. Traces of it are found in the sedimentary rock used to make crushed stone. This stone has been used as backfill for house construction in some areas.

Swelling of the Backfill

A chemical reaction can cause pyritic backfill to swell. This can cause heaving problems when the backfill is under the concrete slab of a basement or garage. In the presence of humidity and oxygen, pyrite oxidizes and produces sulfuric acid. This acid reacts with calcium carbonates found in the crushed stone. The chemical reaction results in the formation of sulphate and possibly gypsum. Gypsum crystallizes and causes the backfill to swell. This swelling may heave the slab and interior structural supports resting on it. Swelling is usually worse in garages, where the layer of backfill is thicker. On average, it will take about ten years before any perceptible damage is produced.

Sulphation Causes Concrete to Swell and Crumble

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.

Damage to Foundation Walls

In some cases, the lateral forces of the swelling backfill will produce cracks in the foundation walls around the garage. This does not usually occur in basement walls, since the pyritic backfill is found only at or below the footings.

What Areas are Affected

Pyritic backfill comes from quarries with rock containing pyrite or from excavations of public works. Most cases occur in buildings from 8 to 20 years old. A few older buildings (30 - 40 years old) have also been affected. For residential buildings constructed before 1970, the layer of crushed stone is either non-existent or quite thin. This explains why there are very few problems for buildings constructed before 1970.

Tracking Progress of the Swelling

Damage attributable to swelling caused by pyrite usually appears 10 or more years after construction. Some swelling may occur sooner if oxidation of the backfill has already started during construction. This may happen if the crushed stone has been exposed to a lot of moisture. Pyritic swelling of backfills will stop when all the sulfur has been oxidized. The speed of reaction will depend on several factors; it takes between ten to forty years to fully oxidize.

Tips to Slow the Rate of Swelling

Reduce ground level humidity. Absence of humidity can slow down the swelling of pyritic backfill. In practical terms, the process of oxidation cannot be totally stopped but oxidation over a long period of time could cause less damage than rapid oxidation. Ground level humidity can be reduced by better surface drainage as follows:

  • Provide gutters and extend downspouts to divert water at least six feet from the foundations.
  • Make sure that grading slopes away from foundations. The slope should be at least one inch per foot. If the required slope is not possible, drainage ditches, swayles and/or catch basins may be needed.
  • When possible, check to make sure the foundation perimeter drain is working well (not clogged or damaged).
  • Sealing cracks
  • Cracks allow water and air to reach the backfill and this could speed up any potential reactions. Fill in cracks with a concrete repair product or a flexible polymer sealant.


If the backfill is still in the process of swelling, the only sure solution to resolve the problem is to:

  • Break and remove the slab
  • Excavate and the backfill and any contaminated subsoil
  • Put down new “DB” certified backfill
  • Pour a new slab

The repair should be done by professionals specializing in this type of work. It is the most expensive and disruptive option and is usually considered a last resort.

This article is thanks to: www.homeinspectionmontreal.com

For more information, please contact a Royal LePage TEAM REALTY Sales Representative.