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

摘要

Changes in the cement manufacturing process, suchas the use of higher sulfur fuels, have tended to raise clinkersulfate levels and sulfur trioxide (SO3) alkali ratios. As one of theconsequences, the nature and quantity of the clinker sulfate phaseshave been modified. Clinker sulfates tend to be available as doublesulfate salts; calcium langbeinite instead of potassium sulfate.Anhydrite is present inn the clinker only when the quantity ofclinker sulfate is higher than the amount that can be combined bythe alkali ions as alkalis sulfates and calcium langbeinite. It hasbeen demonstrated recently that in clinkers with sulfate/alkaliratios as high as 3.6, anhydrite is not detectable and only begins toform at levels above this. This is consistent with observations fromregular industrial production. Even with fairly high sulfate levelsand SO3/alkali ratios, industrial clinkers do not attain high enoughSO3/alkaliratios to permit anhydrite formation. Thus, the presenceof clinker anhydrite is illusive.It has been alleged recently that clinker anhydrite would providenonheat-cured concrete with a lesser durability. The objective ofthis study is to establish what would be the effect of anhydrite ondurability in the hypothetical case that this phase was present inclinker. The impact of clinker anhydrite has been examined on alaboratory clinker with a SO3/K2O ratio of 5.2 and a SO3 level of3.8%. The laboratory clinker anhydrite amount has been evaluatedat 1.1%. Two aspects have been studied:? The dissolution rate of clinker anhydrite present in a very highSO3/alkali clinker during hydration, and? The influence of clinker anhydrite on expansion.Experiments indicate that anhydrite dissolves and reacts quitequickly, mainly within a few days, and does not produce concretewith expansion. Therefore, clinker anhydrite, if it did exist, wouldnot contribute to ambient temperature DEF.