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- Author or Editor: Baohong Guan x
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Abstract
To use flue gas desulfurization (FGD) gypsum and limestone as supplement of cement, conduction calorimetry was applied to investigate the early hydration of ternary binder of calcium aluminate cement (CAC), Portland-limestone cement (PLC), and FGD gypsum, supplemented with the determination of setting times and X-ray diffraction (XRD) analysis. Different exothermal profiles were presented in two groups of pastes, in which one group (group A) sets the mass ratio of FGD gypsum/CAC at 0.25 and the other group (group B) sets the mass ratio of PLC/CAC at 0.25. Besides the two common exothermal peaks in cement hydration, a third exothermal peak appears in the pastes with 5–15% FGD gypsum after gypsum is depleted. It is found that not PLC but FGD gypsum plays the key role in such ternary binder where the reaction of ettringite formation dominates the hydration process. PLC accelerates the hydration of ternary binder, which mainly attributes to the nucleating effect of fine limestone particles and PC clinker. The modified hydration process and mechanism in this case is well visualized by the means of calorimetry and it helps us to optimize such design of ternary cementitious material.
Abstract
The dehydration behaviors of FGD gypsums from three power plants were investigated at N2 atmosphere (autogenous and negligible partial pressure of water, 
) in non-isothermal and isothermal condition. The dehydration of gypsum proceeded through one step, i.e., CaSO4·2H2O → γ-CaSO4 (γ-anhydrite) or two steps, i.e., CaSO4·2H2O → CaSO4·0.5H2O (hemihydrate) → γ-CaSO4 depending on temperature and 
. The discrepancies of three FGD gypsums on dehydration behavior were very likely due to the different crystalline characteristics (size and habit) and impurities, such as fly ash and limestone. Experimental data of non-isothermal analysis have been fitted with two ‘model-free’ kinetic methods and those of isothermal analysis have been fitted with Avrami and linear equation. The apparent empirical activation energies (E
a) suggest that the transition from gypsum to hemihydrate is mainly controlled by nucleation and growth mechanism, while the transition from gypsum to γ-anhydrite is mostly followed by phase boundary mechanism.
Abstract
The use of by-product gypsum is an important alternative in concrete design. In present experiment, conduction calorimetry was applied to investigate the early hydration of calcium aluminate cement (CAC)/flue gas desulfurization (FGD) gypsum paste, supplemented with the determination of setting times and analysis of hydrates by X-ray diffraction (XRD). It was found that different profiles of heat evolution rate were presented depending on the CAC/FGD gypsum ratio. Two distinct exothermic peaks, associating with CAC hydration and ettringite formation respectively, appeared when the FGD gypsum content was less than 20%. Hydrate barrier mechanism was introduced to explain the difference in induction periods of the pastes with or without FGD gypsum. It is concluded that the blending of FGD gypsum accelerates the hydration of CAC for the quick formation of ettringite and generates greater hydration heat from per gram of pure CAC for the high exothermic effect of ettringite formation. The dissolution and diffusion of gypsum plays an important role of reacting controller during the hydrations of the pastes with FGD gypsum. The modified hydration process and mechanism in this case is well visualized by means of calorimetry.
