Pyrite and pyrrhotite: How to protect your investment
Pyrite and pyrrhotite are naturally occurring minerals found in certain construction materials.
Over time, chemical reactions can cause them to expand, leading to severe damage to building foundations.
This article explains the phenomena, how to spot early warning signs, and what steps to take to prevent costly outcomes.
Pyrite issues
Understanding pyrite issues
“Pyrite issues” happen whenever a granular fill under a slab is made of pyrite bearing crushed shale or crushed mudstone. Under humid stress, the pyrite will free SO2 that will eventually combine with water to produce sulfuric acid, which is much detrimental to limestone based minerals such as the surrounding granular fill and which can delaminate the under-slab soaking in acidic water. Swelling and cracking of the concrete slab will ensue and foundation wall dislocation may even be observed. The thickest the granular fill, the worse the reaction will be.
Under specific conditions, moisture and oxygen will oxidize the pyrite. The chemical reaction that ensues causes swelling of the backfill and is likely to lift the concrete slab. In some cases, sulfation can also attack the underside of the concrete slab.
Pyrite: a slow and unpredictable process
Problems associated with pyrite may appear as early as ten years after construction of the house; generally, damages will be consecutive to a French drain malfunction, but it can also be faster or can be much slower, not showing up until 20 or even over 25 years. As chemical reactions of pyrite are difficult to predict, the concerned owner rarely knows what to expect.
Regions affected by pyrite in Quebec
In Quebec, this phenomenon is widespread in the Eastern Township, and also in the Boucherville or Riviere-des-Prairies areas.
It mainly affects the buildings constructed in the 1980s but is also found in homes built before 1970 or even after 1998.
What are the prevention standards and best practices?
Since April 1999, the construction industry voluntarily applies the CTQ-M100 standard which aims to certify the quality of non-swelling backfill used in construction. Owners should require the contractor doing concrete slab construction or renovation to provide their original delivery stub confirming that the crushed stones used are certified “DB”.
To help property owners determine whether their building may be affected by issues related to pyrite, the CTQ-M200 standard also exists to guide the work of professionals specialized in this type of assessment on existing buildings.
Pyrite testing: who should you call?
The Association of Consultants and Experts Laboratories (ACLE) is advising owners who must undergo pyrite testing to choose an acknowledged laboratory, such as Enviroc Groupe Fondasol.
Pyrrhotite issues
What is pyrrhotite?
Pyrrhotite is another iron sulphide, naturally occurring in igneous rock.
Because it oxidizes in air with moisture and because it will then release sulfuric acid, which attacks the cement paste, pyrrhotite should not be used in the aggregate that is poured in concrete.
Documented regional contamination cases
Unfortunately, crushed stone containing 1% and plus of pyrrhotite, from two deleterious quarries near Trois-Rivières, was used to produce several new house concrete slabs or foundation walls between the late 1990s and mid-2000s.
Therefore, hundreds of home owners in the Trois-Rivières area are experiencing major concrete deterioration issues related to the presence of pyrrhotite in the concrete. With time, the consecutive crazing of the concrete will decrease the bearing capacity of the foundation and the slab.
When cracks and concrete issues appear — Enviroc is here to help
Crazing is the first characteristic of reacting pyrrhotite in the concrete. These are shown as crisscross, star-shaped or simply crazing patterns.
Enviroc Groupe Fondasol can diagnose and quantify this kind of problem and can provide the required evidence in court.
Ask Enviroc Groupe Fondasol to assess the reality or extent of pyrrhotite issues in your building.