Examples of Pyrite Framboids and Crystals when viewed under a 0.05 & 0.02mm grid.
Pyrite is a naturally occurring mineral found commonly in sedimentary or carbonaceous rock types. This type of rock has prevalently been used as the hardcore material beneath ground bearing floor slabs in Ireland.
In its natural state, the pyrite mineral remains un-reactive. However when the virgin rock is broken up to create aggregate the pyrite is exposed to the atmosphere and moisture, the mineral may oxidise causing the hardcore to swell.
Pyrite is a sulphide material that occurs in many sedimentary and metamorphic rocks. In the presence of humidity and oxygen, pyrite oxidises and produces ferrous sulphate and sulphuric acid which causes the first phase of expansive action. The crystalline pyrite also forms into secondary sulphates within the aggregate, causing the rock particles to fracture and expand. These secondary sulphates are predominantly thaumasite and gypsum. Furthermore the sulphuric acid can react with calcium carbonate and other minerals found in the rock, forming gypsum causing further expansion.
The problem of pyritiferous heave in sub-slab fill material is currently evident in five local authority areas, Fingal, Dublin City, Meath, Kildare and Offaly relating back to properties built within the last twelve years. When aggregates containing pyrite are used in confined conditions, such as beneath floor slabs, then the resultant expansion forces can cause structural damage such as cracking and displacement. The chemical reactions are usually a slow process, however if the prevailing conditions are such, then damage can be noticed within two years, and this has been found to be the case with many of the cases found in Ireland.
In addition to the structural damage associated with pyritiferous heave of the sub-slab fill material, porous concrete exposed to thaumasite, water and carbon dioxide may experience sulphate attack which renders the concrete structurally weakened. The amount of thaumasite production is related to the amount of 'unconverted' pyrite present in the fill and therefore the production is finite. Cracked concrete and porous concrete such as medium and normal density masonry blocks are most at risk of sulphate attack when the sulphates become suspended and transported in water. Materials constructed using 'sulphate resisting cement' may not actually prevent thaumasite sulphate attack. They are better suited to resisting ettringite sulphate attack and therefore determination by testing as to whether sulphate resisting cements have been used is of limited value. Concretes which are being attacked by sulphates are attacked at the exposed face first and in most cases damage caused by sulphate attack, if any, especially in concretes of less than ten years of age, will extend into the porous concrete for only a few centimetres. Buildings and structures under normal circumstances are designed to last for a minimum of 75 years according to the accepted design Standards. Some building guarantee companies only require a design standard of 60 years. However, in either case, the presence of elevated levels of sub-slab sulphates due to the presence of pyrite may reduce the effective design life of your property.