Murray Reid, Transport Research Laboratory. This note was first published in GE’s April 2000 edition.
Sulfur can occur in soils, rocks and fill materials. The most common forms are sulfates such as gypsum (CaSO,.2H,O) and sulfides such as pyrite (FeS,).High concentrations of sulfate in groundwater can lead to attack on construction materials such as metals and concrete. Mere presence of sulfates may not be the only cause – it can also occur because of oxidation of sulfides during the disturbance resulting from construction activity (Figure 1).
Recent problems with the thaumasite form of sulfate attack on concrete bridge piers were ascribed to oxidation of pyrite in the Lias Clay backfill as a result of exposure during construction. This resulted in an increase in the amount of sulfate available to attack the concrete (Thaumasite Expert Group, 1999).