Paste, mortar, and concrete cured at temperatures above certain limits may exhibit expansion and cracking during subsequent exposure to varying moist conditions. This phenomenon originally became known as delayed ettringite formation, DEF. DEF results in a typical microstructure which is demonstrated with examples from field samples and laboratory-made samples. The microstructure is compared with examples of internal sulfate attack in laboratory samples. These typical features include gaps around the aggregate where the paste shows an almost perfect replica of the individual aggregate surfaces. Expansion of the paste on a scale which is homogeneous relative to the aggregate would lead to such features. The chemistry in DEF is similar to that of sulfate attack. A mechanism involving hydrates of aluminates and possible unhydrated cement clinker particles is discussed.

The mechanical consequences of delayed ettringite formation (DEF) have been monitored for specimens stored under water at room temperature after undergoing different initial curing regimes, including some designed to simulate practical steam curing. Factors investigated were cement composition, water-cement ratio, and aggregate type. The data show a correspondence between the patterns of expansion shown by mortars and concretes, for the range of portland cements examined. Storage periods have now been extended to over 6 years, and very late expansions are revealed for certain concretes, including those containing limestone aggregate. The possibility that an accelerated mortar-prism test can be devised to predict expansive behavior for practical concretes has been considered: it appears that testing for between 6 and 12 months may be necessary before useful predictions can be made from mortar prism expansions. Mechanisms for expansion due to DEF are discussed in the light of the laboratory findings reported.

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.

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 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.