Gypsum of improved performance using blends with Portland cement and silica fume

The present work describes a study intended to develop a blend of gypsum and Portland cement that would possess the advantages of gypsum (high early strength and enhanced workability) and Portland cement (improved durability in moist conditions), but would be free of the deleterious effect of ettringite, which is formed when gypsum and Portland cement interact. This was achieved by preparing a blend of 75% gypsum with a 25% mixture of Portland cement and silica fume. An optimum silica fume—Portland cement ratio of 0·25-0.66 provided a system with strength levels twice as high as those of pure gypsum. The system also exhibited a high wet/dry strength ratio of ∼60 % after 200 days of water immersion. This improvement was explained by the reduction in ettringite formation and the development of a microstructure in which gypsum crystals were engulfed by CSH.