SULFATES IN HIGH SO3/ALKALI CLINKER. THEIR DISSOLUTION KINETICS AND THEIR INFLUENCE ON CONCRETE WORKABILITY AND DURABILITY

摘要

Changes in the cement manufacturing process such as the use of higher sulfur fuels have tended to raise clinker sulfate levels and SO_3/alkali ratios. As one of the consequences, interground gypsum addition to cement have dropped because more sulfate is available from the clinker. Also, clinker sulfates tend to be available as double sulfate salts; calcium langbeinite instead of alkali sulfate. Anhydrite is present in the clinker only when the quantity of clinker sulfate is higher than the amount which can be combined by the alkali ions as alkali sulfates and calcium langbeinite. This is not the case of clinkers from regular industrial production, even clinkers with fairly high sulfate levels and SO_3/alkali ratios. It has recently been alleged that clinker anhydrite would provide non-heat cured concrete with a lesser durability. The objective of this study is to establish the impact of increasing the clinker SO_3 level on concrete workability and durability. It has been shown that calcium langbeinite is rapidly dissolved. Because of its dissolution rate and ability to form "blocking ettringite", high calcium langbeinite clinkers will 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 effect of clinker anhydrite on durability has been examined by following its dissolution rate during hydration and by evaluating its influence on expansion. Experiments indicate that the majority of anhydrite dissolves in a matter of hours and nearly all within a few days and that anhydrite does not produce a non-heat treated concrete with expansion. Therefore, if present in clinker, anhydrite does not affect the concrete durability.