Research On Influence Factors Of Delayed Ettringite Formation

Delayed ettrigite formation is a potential damaging effect on concrete.Damage caused by delayed ettrigite formation can be reduced to some degree or avoided by understanding the mechanism of DEF. In this study, the effects of curing temperature and components of binder on delayed ettringite formation have been investigated, and a haypothesis about expansion mechanism of DEF has been proposed.This study refers to the standard of LCPC (Laboratoire central des ponts et chaussées), which is an assessment on resistance of delayed ettringite formation for concrete. Specimens undergo heat treatment including four courses: pre-setting, heating, temperature retaining, cooling after preparation. Self-made combined temperature-control curing system is used to match the standard.After heat treatment, specimens proceed drying-wetting cycle twice, then placed in water which is kept at 20℃. The result presented here show that there would not be delayed ettringite formation reaction in concretes or mortars when the maximum curing temperature is below 70℃. When the curing temperature exceed 70℃and SO₃ is sufficient (gypsum Volume up to 5%), delayed ettringite formation reaction intensifys as the curing temperature and gypsum Volume increase. when the curing temperature reaches 90℃and gypsum volume adds to 7%, concrete expansion ratio achieves 0.194% at day 135 with the phenomenon of water absorbed, quality increasing and compressive strength and dynamic elastic modulus decreased. Mortar specimens show similar results. When SO₃/Al₂O₃(molar) is equal to the concrete which has 7% gypsum, expansion ratio increases further with Alkali content added up and reaches 0.316% at day 135.Cluster-like ettringite cause no obvious expansion and needle crystal is the main reason for expansion which generally exists in crack and air void. This study claims a mechanism of DEF. At the tip of crack, ions deposit on the nuclear of ettringite, and the crystal keeps growing. As the space is limited, the huge crystallization pressure leads crack extend and macroscopical expansion. Besides, with the temperature, gypsum Volume and alkali content added up, the quantity of ettringite increases fast and performs like needle clusters.Expansion caused by delayed ettringite formation can be ameliorated obviously by mixing fly ash, especially when the volume reaches 40%. And ettringite perfoms scattered rather than clustered.When utilizing the standard of LCPC. with the curing temperature increasing, linear expansion and SO₃/ Al₂O₃(molar) ratio, DEF index shows linear relationship and square relationship better separately.