Delayed ettringite formation (DEF) is a process which can lead to swelling and cracking of concrete. This paper proposes a chemical model to predict the kinetics and the amount of DEF in concretes subjected to high-temperature curing. The modelling considers several types of phenomena: the thermodynamic equilibria of hydrate crystallisation, the binding of ionic species to hydrated calcium silicates and the mass balance equations, which include the diffusion mechanisms. All the constitutive equations are provided and the thermodynamic constants found from a wide-ranging literature review are given in particular detail. The model has been implemented in a finite element code. The numerical results give the amount of ettringite and monosulphates, and ionic concentration fields in the simulated structure. They are compared with experimentation in which the early-age thermal cycle and long-term alkali release combine to cause DEF.