Is Pyrite causing your garage floor to heave and crack? The only way to be sure it’s pyrite is to have it tested by a local laboratory. In Quebec we have a serious issue in older homes located around Montreal, Laval and the South Shore which may have Pyrite in the backfill under their garage floors.
Pyrite or "Fool's Gold" is a type of mineral that can sometimes be found in the gravel used under garage floor slabs. The problem occurred because there were trace amounts of pyrite in the granular fill used as a base for the concrete floor. When exposed to water, oxygen and a certain bacteria, the Iron Sulfide in the granular mix produces Ferric Iron that continues to be produced until one of the elements is exhausted. The concrete slab of your garage floor traps the elements in the Pyritic backfill and eventually the excess Ferric Iron expands until the slab lifts and cracks.
The only way to repair cracks caused by pyrite under the garage slab is to follow the same steps as the garage floor replacement but instead of just removing 1 foot of fill we would have the remove all the gravel as deep as 8 feet and replace with new certified DB stone. If someone replaces the slab without removing all the pyrite it is possible the slab could crack again.
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