Hemihydrate or waste anhydrite in composite binders with blast-furnace slag: Hydration products, microstructures and dimensional stability

• We studied composite binders of hemihydrate or anhydrite and blast-furnace slag.
• The main hydration products are gypsum, amorphous C–S–H and ettringite.
• Gypsum from anhydrite is less porous than the gypsum matrix from the hemihydrate.
• Excellent dimensional stability is observed when silica fume and fly ash are used.
• C–S–H with Ca/Si composition from 1.08 to 1.78 was intimately mixed with the gypsum.

This paper presents results of an investigation of calcium sulphate-blast furnace slag binders cured for up to 600 days with small additions of fly-ash or silica fume. Hemihydrate and fluorgypsum were used as the calcium sulphate sources. The binders showed good mechanical and hydraulic properties; the water resistance was attributed to the formation of C–S–H with Ca/Si ratio between 1.08 and 1.78 finely intermixed with the gypsum initially formed. Ettringite was formed after the reaction between gypsum and aluminum from the slag and fly ash; however, its formation was hindered in the presence of silica fume, which improved the dimensional stability.