Delayed ettringite formation (DEF) can damage concrete that has experienced a temperature above about 70°C. Claims that slow release of sulfate from the clinker can have a similar effect in concrete not thus heated are unsupported. Chemical and microstructural aspects of DEF are reviewed. Expansion results from formation of ettringite crystals of submicrometre size in the paste, the larger crystals readily observed in cracks and voids being recrystallisation products. The rate and ultimate extent of expansion are influenced by factors of three types: chemistry, which determines how much ettringite can be formed; paste microstructure, which determines the stresses produced by its formation; and concrete or mortar microstructure, which determines the response of the material to those stresses. Alkali present before the end of the heat treatment can increase expansion, but when present subsequently, it decreases expansion by inhibiting ettringite formation. Leaching therefore promotes expansion. © 2001 Elsevier Science Ltd.