Chapter 37: Sulfates and sulfides in soils and rocks

Sulfur is widespread in rocks and soils and occurs in two principal forms; in an oxidised state as sulfate, principally gypsum (Ca.SO₄.2H₂O); and in a reduced state as sulfide, principally pyrite (FeS₂). Other less stable and possibly more problematic forms occur in some situations. The oxidation of pyrite and other sulfides can occur as a result of geotechnical activities when soils and rocks are disturbed and exposed to air and water. This can lead to the generation of high concentrations of sulfate and low pH in percolating water, which can have a number of adverse consequences, including chemical attack on construction materials such as concrete and steel, precipitation of gypsum leading to heave of foundations and floor slabs, and pollution of watercourses with precipitation of hydrous iron oxides (ochre). Pyrite oxidation can also be triggered in ground stabilisation involving mixing susceptible clay materials or pyrite-bearing aggregates with lime or cement; the resulting high pH values lead to formation of expansive sulfates (ettringite and thaumasite) that cause heave of the stabilised material. Test methods are available to identify and characterise problem materials but must be used with care because of the variability of sulfur compounds. Geotechnical engineers need to recognise the types of material that could lead to problems in civil engineering works and understand how these problems can be avoided or mitigated.