What is pyrite in homes?

Property Health Check does not provide pyrite testing, or any certification in relation to pyrite testing. However, clients often ask when they are thinking about buying or selling a property, should they consider getting a pyrite test carried out before selling or purchasing? The following should hopefully give a little more insight into pyrite and how it can affect homes.

Pyrite In Your Home

The damage that pyrite can cause is stressful to the home owner and an expensive problem to remedy. The possibility that pyrite may have been used in a small number of homes in a development can have a very negative affect on all property values in that development. The presence of pyrite in the back-fill in almost all cases cannot be seen from a pre purchase structural survey. Pyrite in back-fill can take upwards of 40 years to fully oxidise (swell) and become stable. Removing pyrite is an expensive task which involves supporting the existing first floor structure and roof, removing the existing concrete floor slab, partition walls resting on the floor slab, removal and replacement of all services in the affected area, removing all existing contaminated gravel and then replacing same with proper materials.

Pyrite affected back-fill has been used in house construction for possibly up to 20,000 homes in Ireland.

What is pyrite?

Pyrite (FeS2) is a very common mineral. Traces of it can be found in the sedimentary rock (quarries) used to make crushed stone for back-fill.
By itself pyrite is not normally a problem, but when you combine it with moisture and oxygen in a confined space it can become a problem.
In the presence of moisture and oxygen, pyrite oxidizes and produces sulfuric acid. The acid reacts with the calcium carbonates found in the crushed stone and swells.

Swelling of the back-fill under floor slabs

If pyrite affected back-fill never got wet, or had no access to oxygen, it would remain mostly static.
In the presence of moisture and oxygen, pyrite oxidizes and produces sulfuric acid. This acid reacts with calcium carbonates found in the back-fill. The chemical reaction results in the formation of sulfate and gypsum. The gypsum crystallizes and causes the back-fill to swell under the concrete floor slab. Over time this chemical reaction will cause the concrete slab to flake and crumble.
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 back-fill and the amount of swelling is also determined by the thickness of the back-fill.
This is usually a slow process. On average, experience from similar problems in Canada suggests it can take about ten years before any noticeable damage is noted.

Frequently Asked Questions about Pyrite

If I suspect pyrite what evidence do I look for?

Depending on the type of construction, back-fill is often placed under concrete floor slabs. The presence of Pyrite in the back-fill can result in a section or the entire floor slab to lift several centimeters, causing cracks in the slab. Internal and external doors may also stick and/or catch on the floor.

Needle-like white crystals mixed with the concrete near the cracks may be observed.

Cracks caused by Pyrite can be seen by sections of the floor rising in the corners of rooms or in the form of a cross or a star towards the centre of the floor. These types of cracks can take one to 1 to 10-20 years to appear.

Does sulphation result in concrete slab floor to swell and crumble?

Sulphation will result in the concrete slab swelling and crumbling. It can result in swelling up to 150mm. The moisture under the concrete floor slab rises upward, lifting the sulfuric acid which has resulted by oxidation of the pyrite. When sulfuric acid comes into contact with the underside of the concrete floor slab, the concrete swells from sulphation and starts flaking and crumbling.

The swelling concrete expands. The strip foundations and rising walls may prevent horizontal expansion; so the swelling usually occurs upwards from the centre or corners of the floor slab.

Does pyrite cause damage to foundation walls?

In limited cases, the lateral forces of the swelling back-fill will produce cracks in the foundation rising walls around the property.

What age properties are affected?

The majority of properties affected have been built during the Celtic tiger years, however some older properties are also affected and occasionally the defect was not correctly diagnosed.

Is Pyrite a health issue?

The swelling of back-fill does not have harmful effects on the health of the occupants of the house, save for the stress it leads too. The vast majority of expert opinion is that the quantities of sulfuric acid are too minute to cause health problems. It is possible that the radon barrier maybe compromised leading to ingress of radon gas into the dwelling.

How quickly will I see the affects of pyrite?

Damage attributable to swelling caused by pyrite can appear very soon after construction is complete. Swelling may occur sooner if oxidation of the back fill has already started during construction. This may happen if the crushed stone has been exposed to a moisture. The speed of reaction will depend on several factors. Pyrite can take between ten to forty years to fully oxidize.

Can I slow down the rate of swelling?

Absence of moisture can reduce the speed of the swelling of pyrite back-fill. The process of oxidation cannot be prevented but oxidation over a longer period of time could result in less damage than rapid oxidation. Ground level moisture can be reduced by better surface drainage immediately outside the exterior walls.

What if I have a long fine crack in my concrete floor?

Most concrete floor slabs will develop drying cracks. This is normal when floors dry out too quickly. As concrete dries, it shrinks. These cracks are most likely not caused by swelling due to the presence of pyrite.

Who should I call if I have a suspected pyrite problem?

Contact your builder or insurance company if your property is covered under an insurance guarantee. Also check with neighbors in your estate to see if anyone is complaining about a similar problem. The testing for the existence of pyrite is evaluated by specialist laboratories which test for pyrite from sample submitted to them and not by a visual inspection.

There are a number of specialist engineers and contracting companies who specialise in pyrite investigation.

What is the main long-term solution to remedy a pyrite problem?

The only long-term solution is an expensive solution which involves substantial building works as follows.

  • Break and remove the concrete slab floor
  • Excavate the back-fill and any contaminated subsoil
  • Replace back-fill according to Part D of the Building Regulations
  • Pour a new slab
  • Existing services, partition walls and first floors would also need to be replaced and supported during this reinstatement

This work should only be undertaken by competent building contractors.