Anhydrite in High Sulfur Trioxide (SO3)/Alkali Clinkers: Dissolution Kinetics and Influence on Concrete Durability

Abstract

Abstract Changes in the cement manufacturing process, such as the use of higher sulfur fuels, have tended to raise clinker sulfate levels and sulfur trioxide (SO3) alkali ratios. As one of the consequences, the nature and quantity of the clinker sulfate phases have been modified. Clinker sulfates tend to be available as double sulfate salts; calcium langbeinite instead of potassium sulfate. Anhydrite is present in the clinker only when the quantity of clinker sulfate is higher than the amount that can be combined by the alkali ions as alkali sulfates and calcium langbeinite. It has been demonstrated recently that in clinkers with sulfate/alkali ratios as high as 3.6, anhydrite is not detectable and only begins to form at levels above this. This is consistent with observations from regular industrial production. Even with fairly high sulfate levels and SO3/alkali ratios, industrial clinkers do not attain high enough SO3/alkali ratios to permit anhydrite formation. Thus, the presence of clinker anhydrite is illusive. It has been alleged recently that clinker anhydrite would provide nonheat-cured concrete with a lesser durability. The objective of this study is to establish what would be the effect of anhydrite on durability in the hypothetical case that this phase was present in clinker. The impact of clinker anhydrite has been examined on a laboratory clinker with a SO3/K2O ratio of 5.2 and a SO3 level of 3.8%. The laboratory clinker anhydrite amount has been evaluated at 1.1%. Two aspects have been studied: • The dissolution rate of clinker anhydrite present in a very high SO3/alkali clinker during hydration, and • The influence of clinker anhydrite on expansion. Experiments indicate that anhydrite dissolves and reacts quite quickly, mainly within a few days, and does not produce concrete with expansion. Therefore, clinker anhydrite, if it did exist, would not contribute to ambient temperature DEF.