Study On Hydration Of Fluidized Bed Combustion Ashes

Fluidized bed combustion (FBC) technology is advanced because it can meet the demand of environment protection, but the byproducts, namely FBC ashes, are still something new to most people. As a result, the utilization of them and even the further promotion of the technology are seriously hampered. The utilization in building materials is very important for FBC ashes, closely related to the hydration properties of them. The sulphur-fixing products, mainly free lime andâ…¡-CaSO4, and calcined clay minerals in FBC ashes play the different roles in the hydration process, studied respectively in this thesis. The physico-chemical properties relating to hydration of FBC ashes are analyzed. The chemical kinetics method is introduced in to study the pozzolanic hydration properties of the calcined clay minerals in FBC ashes. This paper also deals with the effect of the hydration properties of the calcined clay minerals on the hydraulic properties of FBC ashes. The hydration products from the sulphur-fixing products are studied by XRD, SEM and EDS, and the expansion properties and mechanism are also investigated.Eleven FBC ashes with different content of free lime and SO3 are adopted to make the study representative, by contrast with Pulverized Coal Combustion fly (PF) ashes. The difference between the morphology of FBC ashes and that of PF ashes is controlled by the combustion temperature. The polymerization degree of [SiO4] and [AlO6] of FBC ashes decreases with the CaO content. The polymerization degree of [SiO4] and [AlO6] of FBC ashes with lower CaO content is close to that of PF ashes.All the present studies on the pozzolanic reaction kinetics of pozzolans regard the active components as a whole and model the reaction kinetics by the change of Ca(OH)2 or chemically combined water content. For the first time, the method of the change of active components content is used to investigate the pozzolanic reaction kinetics of FBC ashes and PF ashes. The new method can differentiate the pozzolanic reactivity of active SiO2 and Al2O3, and obtain the useful parameters, such as reaction rate constant and activation energy. The pozzolanic reaction of active SiO2 and Al2O3 of FBC ashes and PF ashes follows apparent first-order kinetics, and the relationship between the pozzolanic reaction rate constant and temperature conforms to the Arrehenius Law. The reaction rate of FBC ashes is greater than that of PF ashes, while it is just the contrary for the energy barrier. Density separation is used to study the mineral phases of FBC ashes. On this basis, this work confirms that the pozzolanic reaction among calcined clay minerals, free lime and anhydrite can enable the hydraulic properties of FBC ashes to be obvious and fast.If contact with air is not restricted, the ettringite formed earlier will decompose at 90 d, its place being taken by gypsum and calcite, and free lime can affect the crystal morphology of ettringite in FBC ashes pastes, which have a great influence on the expansion properties of FBC ashes. The work confirms that the expansion properties of FBC ashes depend mainly on the content and crystal morphology of ettringite.