Modelling the Kinetic Behaviour of Delayed Ettringite Formation in Concrete Prisms

Delayed ettringite formation (DEF) is a deleterious reaction which can result in expansion and cracking of concrete. The reaction occurs in the cement paste and is associated with the recrystallisation of the ettringite at a later age after hardening of the concrete. As ettringite is an expansive phase and as recrystallisation occurs in the hardened state, its formation may lead to cracking of the concrete. Laboratory tests of DEF induced expansion use specific conditions to initiate expansion which include elevating the alkali and sulphate contents (1% Na₂O_e and 4% SO₃) of the cement as well as heat curing the specimens at elevated temperatures (90 ℃) for extended periods (12 h). These conditions coupled with the appropriate calcium aluminate content have yielded significant expansion within a year of heat curing the specimens. The expansion process is characteristic of an underlying reaction which once initiated accelerates before depleting the available reactants. In this paper, simple solid state kinetic models have been applied to modelling the expansion of concrete prisms. The Prout-Tompkins’ auto-acceleration model was found to best fit the expansion data.