Present state of investigation on Damaging Late Ettringite Formation (DLEF) in mortars and concretes

Since the past 25 to 30 years, preferably heat treated precast concrete members (> 70 °C) manufactured with high early strength Portland Cements with higher sulphate contents, under adverse exposure conditions have sometimes exhibited structural damage in the form of map cracking and loss of strength. These damages are characterized as Damaging Late Ettringite Formation (DLEF) (1). The cause for the increased occurrence of DLEF starting about 1970 is to our opinion the world wide increase of the permissible sulphate contents of the Portland-Cements (2-5). It is important that 1 wt.-% SO₃can form 5 wt.-% of ettringite or 7.7 wt.-% thaumasite. In "German directions" a maximum heat treatment (HT) temperature of 80°C is specified for concrete exposed to dry environmental conditions. For concrete exposed to intermediate or permanent wet conditions, a maximum HT temperature of 60°C is specified. Though the standard does not provide clear directions, if is believed that cements containing pozzolanic admixtures can be subjected to more intensive HT (6). DLEF is caused by a formation. destruction and a later renewed formation of ettringite occuring preferably after HT at ≥ 70°C of pastes, mortars and concretes made with high strength Portland cements. - Very early start of HT showed no significant influence on DLEF. Late or repeated HT resulted in more severe or repeated damaging. - Humidities ≤ 95 % at 20°C resulted up to 780 d of starage in no DLEF.- After treatment with interim some FTC and/or cured at low temperatures provoked more early and severe destruction.