Hydraulic binders of Fluorgypsum–Portland cement and blast furnace slag, stability and mechanical properties

The effect of various additives (Ca(OH)2, K2SO4, Na2SO4, Al2(SO4)3) was evaluated on the hydraulic character and stability of pastes of 50–75% Fluorgypsum, 15–30% Portland cement and 10–20% Blast furnace slag. Characterization included length changes, compressive strength, SEM, DTA and XRD. The combination of Na2SO4 and Al2(SO4)3 favored early strength but caused detrimental expansion and strength losses after 90 days; whereas the use of only K2SO4 was favorable for strength and dimensional stability. The type of additive had a more important effect on stability and strength than the amounts of cement and slag. XRD indicated the presence of anhydrite, gypsum, ettringite, CaCO3 and an unidentified phase, the interaction of these is proposed to explain the behavior of the cements investigated. SEM showed that cement and slag reacted forming C–S–H that enhanced the hydraulic character by engulfing the gypsum crystals.