What are the different forms of sulphate in Portland cement and how can we analyse cement for SO3?
Sulphates in Portland cement can be broadly categorised as: 1. Added sulphates – gypsum, hemihydrates, anhydrite, several synthetic forms of sulphates (typically by-products like flue gas desulphurisation materials). Clinker sulphates include arcanite, aphthitalite, calcium langbeinite and thenardite. Although normally reported as SO3 (% by mass) for consistency, sulphur can be found in any combination of forms. Elemental sulphur is almost never found in Portland cement, except in trace amounts. Added sulphates are blended with clinker during the final grinding of the cement, in amounts needed to control early setting properties as well as shrinkage and strength development. The amount needed varies depending on the chemistry and fineness of the cement, but is typically on the order of 5% by mass. The most common form of sulphate added to Portland cement is gypsum, some of which is intentionally dehydrated by the heat of grinding to form hemihydrates, which are more soluble and therefo
