Effect of autoclave curing on hydration of anhydrite in CFBC fly ash

Abstract

Circulating fluidised bed combustion (CFBC) fly ash contains plenty of anhydrite, so it is important to investigate the hydration behaviour of the anhydrite for the reutilisation of the ash in autoclaved products including autoclaved aerated concrete. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to study the effect of curing conditions on the hydrates and existence form of the anhydrite in CFBC fly ashcement clinker pastes. Results show that autoclave curing can significantly limit hydration activity of the anhydrite in CFBC fly ash, and the hydration ability of the autoclaved anhydrite cannot be recovered, even if the anhydrite is submerged in water for the subsequent curing, because it is completely covered by fine hydrates. This work confirms that autoclave curing can effectively limit the expansion of anhydrite in CFBC fly ash, and suggests that CFBC fly ash is suitable to be used for the preparation of autoclaved products. NotationP pressure Q reaction quotient R gas constant, 8 . 314 J/(mol K) S Ł m standard molar entropy T thermodynamic temperaturẽ G Gibbs free energy changẽ f G Ł m standard molar Gibbs free energy of formatioñ r G Gibbs free energy change of chemical reactioñ r G m molar Gibbs free energy change of chemical reactioñ r G Ł m standard molar Gibbs free energy change of chemical reactioñ H enthalpy changẽ f H Ł m standard molar enthalpy of formatioñ r H enthalpy change of chemical reactioñ r H Ł m standard molar enthalpy change of chemical reactioñ S entropy changẽ r S entropy change of chemical reactioñ r S Ł m standard molar entropy change of chemical reaction Ł as superscript, indicates standard state

Adsorption properties of CFBC ash–cement pastes as compared with PCC fly ash–cement pastes

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