Abstract
Abstract Natural gypsum can degenerate into hemihydrate during cement clinker grinding which changes the physical and chemical properties of cement hydration, affecting therefore the fresh and hardened properties of cement based materials. Cement systems containing a constant total amount of calcium sulfate (4%) with relative proportions of hemihydrate and natural gypsum were considered. Rheological measurements were executed on an Anton Paar MCR51 rheometer to evaluate the flow properties of cement pastes. Results show that, the yield stress and the plastic viscosity of cement pastes were affected when the degeneration of natural gypsum exceeded 50%. Above this concentration, the yield stress remarkably increased and a variation in plastic viscosity of about 50% was observed. Using TG-DSC techniques, it was shown that, the amount of formed ettringite could not explain these rheological changes. However, centrifugational packing and SEM-SE measurements confirmed that, more than the amount of ettringite precipitated, ettringite morphology plays a major role in controlling the yield stress and plastic viscosity of fresh cement pastes.
Highlights
In ordinary Portland cement manufacturing, clinker is ground together with calcium sulfate as a source of sulfate to prevent premature and detrimental precipitation of some calcium aluminate phases during the early stage of cement hydration
Cement systems with only hemihydrate presented higher Bingham parameters than those with pure natural gypsum. These parameters seemed to change with respect to the replacement of natural gypsum by hemihydrate in cement systems with mix proportions of these two calcium-sulfate bearing materials
We suggest that the decrease in packing density with the increase in hemihydrate fraction within the cement system can be attributed to the elongated shape of ettringite formed
Summary
In ordinary Portland cement manufacturing, clinker is ground together with calcium sulfate as a source of sulfate to prevent premature and detrimental precipitation of some calcium aluminate phases during the early stage of cement hydration. The concentration of sulfate ions within cement suspensions varies depending on the water to cement mass ratio (w/c) along with the amount and solubility of calcium sulfate sources in the system [1]. The amount and type of sulfate calcium sources are generally chosen by the cement manufacturers by considering the C3A and alkali sulfate contents present in the clinker. These can vary between gypsum (CaSO4·2H2O), hemihydrate (CaSO4·0.5H2O), anhydrite (CaSO4) or mixtures of these sulfate sources. The final ground cement product results into two different types of sulfate bearing materials with variant solubility
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