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.

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.

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.

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.