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.

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

Changes in the cement manufacturing process such as the use of higher sulfur fuels have tended to raise clinker sulfate levels and SO3/alkali ratios. As a consequence, interground gypsum additions to cement have dropped because more sulfate is available from the clinker. Also, these clinker sulfates tend to be available as double sulfate salts; calcium langbeinite instead of potassium sulfate. What is the impact of clinkers with high SO, level on concrete performance; mainly on its workability and durability ? The aim of this study is to provide some answers to this question. Cements made from either high SO, clinker or low SO, clinker and gypsum or hemihydrate, but with a given chemical composition, have been simulated by pure phase materials and hydrated up to one hour. Calcium langbeinite is rapidly dissolved. Because of its dissolution rate and ability to form “blocking ettringite”, high calcium langbeinite clinkers should provide improved rheological properties. Moreover, cements made with clinkers containing significant quantities of calcium langbeinite should have a similar workability and durability to a cement made with a low sulfate clinker to which larger quantities of gypsum have been added. The dissolution rate of anhydrite potentially existing in very high SO3/alkali clinker has also been simulated. Experiments indicate that it dissolves and reacts quite quickly so that it should not provide any durability problem if present in cement and concrete.

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 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.