Influences Of Complement Gypsum On The Performances Of Cement-Slag Binder

Blast furnace slag as mineral admixture in cement has been widely used, but its sulphate content (SO3 of about 1%) is low, and far below the SO3 top limit of 3.5% of Portland cement. When using cement compounding with slag, the total content of SO3 in binder can be reduced, so this affects the hydration performance and the strength of composite binder. Therefore, the influences of complementing gypsum on the performances of cement-slag binder were systematically investigated by cement paste,cement mortar and direct thermometry. Meanwhile, the early hydration characteristics of composite binder was also discussed by the means of X-Ray diffraction(XRD) and scanning electronic microscope(SEM)The experimental results show that complementing gypsum in cement-slag binder slightly affects the soundness, setting time, water demand, and the adaptability of binder with commonly used superplasticizers. The proper dosage of complementing gypsum can obviously activate the early activity of slag,increase the height of temperature peak of hydration of binder. It also can increase the hydration heat at Id and 3d, improve the strength at 3d and 28d.So it can effectively overcome the disadvantage of lower early strength of binder when adding slag separately.But the excessive dosage of complementing gypsum may decrease the velocity of early hydration and strength of binder again. The analyses of XRD and SEM show that the early quantity of Ca(OH)2(CH) and ettringite(AFt) of hardened binder paste can raise substantially, the hydration gel increase, the holes decrease and the structure becomes more dense, compared with the specimen without complementing gypsum.