During the past few years delayed ettringite formation (DEF) has probably received more attention, and been involved in more controversy, than any other concrete deterioration mechanism. Even its name has been subject to dispute. Our extensive experience on the investigation of many occurrences of DEF is presented here as a series of questions, with some answers. Where answers have become available, they have explained phenomena that have greatly bothered us and other investigators. Where answers are not available, the questions will provide directions for needed research.

The formation of ettringite in hardened concrete is not only a problem of heat treatment. Ettringite also occurs in no heat-treated concrete, which is exposed only to normal climatic conditions. In some cases the mechanism of damage in concrete pavements correlates with this ettringite formation in the hardened concrete. Structural changes by ettringite formation were caused above all by varying moisture conditions and, as a result, by transportation of moisture and substances within the concrete structure, which also lowers the pH value of the pore solution. The primary ettringite from the paste is microcrystallin at normal pH of 13.5 to 14 in the pore liquid. Thus ettringite may dissolve in the pore liquid and recrystalize at a lower pH in larger spaces, where the capillary transportation is interrupted. This recrystallized ettringite in the air voids was stable up to 60°C. But the mechanism of this ettringite formation is supported and accelerated by higher temperatures (e.g. 60°C) because of the intensive drying. Microstuctural defects like microcracks may be created by alternating temperatures and later on filled and may be widened by ettringite crystals. In concrete pavements no indications were found for recrystallized ettringite itself to be the primary cause of crack formation. The expansion of concrete is reduced by introducing artificial air voids, because there is more available space for accumulation of ettringite. But the combined action of freezing and thawing and de-icing salt after filling the artificially entrained air voids with ettringite crystals may causedamages.

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 some-times 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.-% SO3 can 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 forma-tion 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.

The destruction of concrete attributed to the formation of ettringite was originally seen only as a problem caused by heat treatment. In this field, investigations were done by many scientists. Our long term investigations, started in 1989, into the durability of concrete, showed that damages could also occur in untreated concrete under use conditions. The change in the microstructure of the hardened paste and the degree of damage were assessed by expansion, compressive strength, ultrasonic pulse velocity, loss in E-modulus, variation in mass, depth of carbonation, the crack pattern (net-like crack formation), and microscopical and phase-analytical investigations. After six years the highest degree of damage was observed on heat treated samples with a dense microstructure. It was found also that there was no connection between the degree of damage and the detected ettringite content. It was not possible to determine whether ettringite was the cause of damage, as it is in sulfate attack due to crystallization pressure, or whether ettringite was just deposited and enriched in available spaces in the hardened paste (voids, cracks, gaps and peripheral cracks around aggregates). Probably these primary defects, not affecting the use e.g. of a concrete element, originally caused by the hardening, curing and/or use conditions, will likely increase as a result of the accumulation of ettringite until effective destruction of the concrete.

The occurrence of premature concrete deterioration found in Texas Department of Transportation (TxDOT) precast concrete elements and other types of concrete structures found in Texas has prompted TxDOT to conduct an investigation into the cause of this deterioration. TxDOT’s investigation of 69 prestressed concrete box beams, of which 56 are exhibiting various degrees of deterioration, includes a historical documentation review, chemical analysis and petrographic examination with both optical and scanning electron microprob microscopy. The cement mill that supplied the cement for the 56 beams that are deteriorating conducted an independent investigation. An overview of both investigations is presented. The investigation conducted by TxDOT revealed distress associated with pasted expansion. Ettringite was found throughout the concrete samples in the voids, cracks, gaps around aggregate particles and concentrations or nests exclusively within the cement paste. The presence of alkali-silica reaction (ASR) was also observed but little if any distress could be conclusively attributed to the ASR. The cement manufacturer’s investigation concluded that the main cause of distress was ASR and that delayed ettringite formation occurred as a secondary mechanism most likely resulting from poor construction practices. The varying conclusions of TxDOT’s and the cement manufacturer’s investigations typify the polarization of industry nation wide if not world wide over similar case studies involving this phenomenon described herein as premature concrete deterioration. Accordingly, the conclusion of this paper addresses some of the ramifications of not attaining a resolution to this problem.