Pyrite (FeS2) is a very common mineral found in quarries in Ireland. Pyrite can be found in the sedimentary rock. This material is excavated from quarries and used to make the backfill used under concrete floor slabs and other structures.
Pyrite in its natural state within sedimentary rock is not normally a problem. However when it is quarried and mistakenly used as a backfill under floors or road structures it will combine with moisture and oxygen and expand, therefore becoming a problem.
To be somewhat technical, when pyrite is exposed to moisture and oxygen, the pyrite oxidizes and produces sulphuric acid. The sulphuric acid then reacts with the calcium carbonates (gypsum) which is found in the gravel. This results in the swelling of the backfill and its associated problems..
Swelling of the backfill under floor slabs.
If pyrite within the backfill never got wet, or was not exposed to oxygen, it would not swell. In reality there are methods used to slow down this process but in the long term they are not effective. In time the chemical reaction from the pyrite material leads to the formation of sulphate and gypsum. The gypsum continues to crystallize over time and causes the backfill to swell under the concrete floor slab or other structure. Over time this chemical reaction will result in the flaking and eventual crumbling of the concrete floor slab.
This swelling will lift the floor slab and interior structural supports resting on it. Swelling is usually worse if there is a higher concentration of pyrite in the backfill and the amount of swelling is also determined by the thickness of the backfill.
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