Anhydrite/hemihydrate-blast furnace slag cementitious composites: Strength development and reactivity

• We studied binders of CaSO4 hemihydrate or waste anhydrite and blast-furnace slag.
• Initial strength is low but further gain was notable especially in anhydrite pastes.
• Reactions of anhydrite were notably slower and less exothermic than hemihydrate.
• Slag and pozzolanas hydration reactions were mainly promoted from 7 to 28 days.
• Fly ash or silica fume were beneficial for curing at 60 °C improving impermeability.

This paper reports on the hydration and properties of composite binders with 20–70% blastfurnace slag and 50–70% calcium sulphate (commercial hemihydrate or waste anhydrite) cured for up to 600 days; other small additions included silica fume or fly ash. Formulations of gypsum-based binders with good mechanical and hydraulic properties were obtained. Isothermal conduction calorimetry and non-evaporable water indicated that rapid initial gypsum formation is responsible for the initial strength: hemihydrate reacts intensively, whereas anhydrite reacted slower but resulted in better mechanical properties. Selective chemical dissolution demonstrated that slag hydration was slow but constant in the first 28 days.