Delayed Ettringite Formation (DEF), a form of internal sulfate attack, is a subject of considerable current interest and concern, as well as of controversy. In such an atmosphere, it is not surprising that conflicting data and interpretations were presented, and, as a consequence, some of them cannot be corrected. This volume presents 16 papers of varying viewpoints and interpretation in hopes of stimulating thinking and additional research and possibly help in the process of developing a sounder scientific understanding of the DEF phenomena. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP177

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.

Recent debate on delayed ettringite formation (DEF) as a form of internal sulfate attack - a distress mechanism for ambient- and steam-cured concrete - has motivated this paper, which examines relevant data from the literature and from the authors’ laboratory. DEF can be associated with distress in high temperature concrete, but not for ambient-curing in the absence of external SO3 sources, or an aggregate sulfate source. The two internal SO3 sources cited as potentially responsible -anhydrite in clinker and high SO3 concentrations in the silicate phases -are shown to be absent or fundamentally innocuous.

A series of mortars was studied, cured at 20, 80 or 90°C. The variables studied included sulfate level, alkali additions and slag additions. In parallel with measurements of dimensional changes, detailed study of the microstructural and microchemical changes was made by XRD and by SEM. One of the main findings of this study is that the composition of C-S-H gel around partially hydrated cement grains, analysed one day after heat curing, is significantly different between mortars which subsequently expand when water at 20°C and those mortars which do not expand. The details conserved in of this observation, its implications and expansion are discussed. limitations, and possible mechanisms of

The effect of infilling of air voids by ettringite on resistance of concretes to freezing and thawing was studied. Nine concrete mixtures, made with five cements with or without Class C fly ash, were exposed to freezing-thawing cycles following 110 to 222 days of moist curing. Prior to freezing-thawing, the specimens were examined by a low-vacuum scanning electron microscope (SEM) for the degree of infilling. It was found that the extent of the infilling depends on the length of moist curing as well as the wet/dry treatment. The infilling implies that these air voids are water-accessible. The function of the air-void system to protect concrete from freezing and thawing has been compromised due to the presence of water in some air voids. The infilling seems also to increase effective spacing factor. These might cause concrete to be more vulnerable to freezing-thawing damage.