International Concrete Abstracts Portal

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

This project originated because of premature deterioration of concrete pavements in Wisconsin. The deterioration took the form of a “V” at ,the joints of the pavements. A number of hypotheses had been put forward by various investigators of the damaged concrete. These included filling of the air voids by ettringite, which was thought to reduce the ability of the air void system to protect the concrete against frost damage. The purpose of the work reported here was to recreate the damage mechanism in the laboratory and investigate the sequence of events leading to the deterioration of the concrete. Three cements produced from the same raw materials were used in the project. Two were commercial Type I and Type II cements; the third was made by intergrinding the Type I cement with additional gypsum to increase the amount of available sulfate in the concrete. Concrete prisms 3 x 3 x ll-l/ 4 inches (75 x 75 x 285 mm) were subjected to the conditions specified by ASTM C 666 Procedure A, except that 3% NaCl solutions either with or without added gypsum (to simulate road salt) were used instead of water. The freeze/thaw cycles were interrupted over the weekends, when the specimens were allowed to dry out in laboratory air. The specimens were tested to destruction in most cases. Companion specimens were examined petrographically during the course of the test period in order to establish a sequence of ettringite deposition and damage. Damage was measured by mass loss, length change, and relative dynamic modulus. The findings show that the ettringite deposited in the air voids did not cause cracking, nor did it contribute to the propagation of existing cracks. Rather, it appears to have been opportunistic: cracks due to frost damage created space for ettringite crystals to grow.

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.

The stability domain of stoichiometrically prepared ettringite and monosulfate hydrate in water at 30 and 1OOoC also in the presence of lime, alite, monocarboaluminate hydrate, or each other is reported. The formation conditions of both calcium sulfoaluminate hydrates in pure ordinary portland cement dealing mainly with the hydration behavior of the tricalcium aluminate and the tetracalcium aluminoferrite in the presence of gypsum, also of lime, alite, or each other at 300C is summarised. The type of hydrate formed in sodium -hydroxide, sulfate solution or as a result of alkali doping in the structure of the anhydrous phase is given. The ettringite/monophase formation and duration in the three stages of C3 A and C4 AF hydration (instantaneous reaction, dormant, and acceleration period) are described. The present review focused on the stability of the calcium sulfoaluminate hydrates is a summary of 13 publications.

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.