Ettringite - hydrated calcium aluminium sulphate hydroxide - is formed when the cementitous compounds produced by the reaction between hydrated lime (CaOH 2) and aluminates in the clay are attacked by sulphate ions in a wet environment. Ettringite is highly hydrophilic, and as long as free water is available crystals will grow almost exponentially.
Calcium sulphate - gypsum - can also be formed from a direct reaction between sulphates and unhydrated lime (CaO). It is less expansive than ettringite, but can also cause heave.
Iron pyrites or 'fool's gold - FeS2 - readily oxidises in the presence of air to produce ferrous sulphate.
Site investigations can sometimes fail to pick up the presence of sulphide pockets deep in the ground, which will only begin to oxidise when earthworks expose them to the atmosphere. Pyrites were a major factor in the thaumasite form of sulphate attack that hit bridges on the M5 in1998.
(NCE 2 February 1998)