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What is pyrite?


Pyrite is an iron sulphide that is found in very fine particles in almost all rocks, including in the backfill (the crushed stone) under the concrete floors (the slab) of the basement and garage. It can damage the concrete slab and foundations.


Swelling due to pyrite




















Sectors concerned


Most of the affected properties are located on the South Shore of Montreal, more particularly between Île-Perrot and Sorel, notably in Boucherville, Brossard, Laprairie, Longueuil, Montreal, Repentigny, Saint-Bruno, Saint-Hubert and Varennes. In the east of the island of Montreal (Pointe-aux-Trembles and Rivière-des-Prairies) as well as in the west of the island of Montreal (DDO, Pierrefonds and Pointe Claire). There are other cities affected in Greater Montreal, but to a lesser degree.


Disorders caused by pyrite


Cracks in the concrete slab or foundation are not noticeable until at least ten to fifteen years after the building is constructed. The progression of disorders is slow and can last more than 40 years.There are no consequences for the environment or the health of the occupants. However, cracks in the concrete slab or in the foundations can increase humidity in the basement of the building and cause the appearance of mold and affect air quality.


Heaves on the concrete slab can be considered non-harmful if they do not exceed approximately 10 mm (less than half an inch). Of course, the degree of tolerance of the occupant is decisive. If the uplifts exceed 10 mm, they can push vertically on the raised floor and basement partitions, cracking them and even affecting the ground floor. Basement foundation walls will not be damaged by pyrite swelling because the thickness of the backfill is normally less than 300 mm (12 inches).


There is much more damage in garages located at ground or street level, for two reasons: there are frequently several meters of gravel under the concrete slab of the garage and, moreover, the builders often put gravel of a lower quality than that of the subsoil. The damage is therefore more significant in the garage: in addition to heaving on the slab, the foundation walls can be damaged. Usually, one or two of the four walls of the garage adjoin the basement. This is how damage to certain foundation walls of the garage can damage the residential part.


On April 15, 1999, Minister Louise Harel announced that the Quebec Technical Committee for the study of swelling problems associated with pyrite had developed an analysis method, called the CTQ-M-100 standard, to eliminate the risks of swelling associated with this material. This certification allows producers (quarries) of aggregates (crushed stone) to guarantee their customers that they are providing them with crushed stone which can be used without risk under concrete slabs for construction and renovation work. In fact, it does not cause any damage if these rocks are of good quality. This stone, called “DB certified crushed stone” (DB for Concrete Slab), must have been analyzed by one of the independent laboratories recognized by the technical committee.


For both the CTQ-M-100 (analysis of new backfill from a quarry) and CTQ-M-200 (analysis of backfill taken from under the slab of an existing building) protocols, the laboratories use the IPPG measurement, or Petrographic Swelling Potential Index. For new backfill, if the index is less than 10, the material is immediately DB certified, i.e. without swelling potential for any concrete slab. If the index is higher, the laboratory carries out other tests before giving or refusing a DB certificate to the quarry.


When the backfill analyzed was taken from under a concrete slab, the IPPG index has much less importance. Indeed, a used aggregate which has a high IPPG could never swell or have already experienced complete oxidation, at the time of analysis, without causing damage to the slab. What matters is the judgment of the professional using the CTQ-M-200 standard.


We therefore advise you to work with a professional who uses the CTQ-M-200 standard when you want to establish a link between cracks or heaving and pyrite, or when you want to check if the aggregate that was used under your slab presents a risk of swelling.



Pyrite is not a problem in itself, it is rather the quality of the backfill that must be concerned.



Frequently Asked Questions


Are houses built after the year 2000 safe?


No, pyrite affects all properties, even those built after the year 2000 because there is no law forcing builders to use DB (i.e. non-swelling) backfill. They do this on a voluntary basis only. So be careful!


How many buildings are affected?


Around one in five buildings (20%) has the potential for swelling which can cause damage. Not all of them will suffer significant damage. It is only after forty years following construction that we will be able to realize this.


Repair costs


Depending on the layout of the basement, the costs vary from $15,000 to $40,000 while the garage can cost between $6,000 and $8,000. The work consists of replacing the concrete slab and the underlying fill down to the natural ground. The backfill must be DB quality and a polyethylene canvas, at least, must be placed on the crushed stone before pouring the new concrete slab.


Impact on the value of the building


Regarding the reaction of the real estate market to the problems associated with pyrite, experience has shown us beyond any doubt that the impact is direct on the price of the property, even if there is no apparent disorders.


Is this a construction defect?


Yes, even if the pyrite problem was not known to manufacturers before 1997, the year the public became aware of it. Under the Civil Code, the limitation period is three years after knowledge of a construction defect. There is no limit on the length of time between purchase and discovery of the defect.


Major defects are covered by the APCHQ New Home Guarantee and the APCHQ Renovation Guarantee, for a period of five years. A major defect is a problem that affects the structure and stability of the building. We must therefore be able to observe and denounce a real manifestation of the phenomenon within the time limits of the Guarantees. If you believe you have observed actual damage caused by pyrite and would like to know how to claim the warranty, call (514) 353-9960 or 1 800 361-2037 and ask for Customer Service at the Warranty Division.


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Under certain specific conditions, moisture and oxygen cause pyrite to oxidize. The ensuing chemical reaction forms gypsum crystals which then occupy a much larger volume (up to 100 times its volume), hence the swelling of the backfill likely to cause the concrete slab to rise. Then, it is possible to see cracks appear in the concrete slab, with or without heaving (bumps at the level of the cracks). If the fill is relatively deep like garages located at street level, there may be lateral thrusts on the foundation walls.


There is a second phenomenon which is added to the swelling of the backfill: the sulphation of the concrete slab. Indeed, the chemical solutions formed in the backfill during oxidation are sometimes absorbed by the concrete slab, which can cause cracks, with or without heaving, without however affecting the foundations of the building.


Finally, there is a third phenomenon: the swelling of the natural rock under the property. As some quarries on the South Shore have clay shales (reactive to pyrite) in their soil, the same goes for the natural soil under certain buildings, mainly on the South Shore. Significant damage, however, is much rarer.​

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