Study On Hydration Process Of Alkali-activated Fly Ash Microbeads Under Microwave Field

Microwave curing is an effective way to improve the early strength of alkali-activated fly ash-based cementitious materials.But the lack of reasonable microwave curing regime limits the application of microwave curing.Fly ash particles consist of floating beads,carbon particles,silica-aluminium beads and magnetic beads.The composition and structure of four kinds of micro-beads are quite different,the response to microwave field is different accordingly.Therefore,the hydration process under microwave curing conditions is also different.While the hydration behavior of these microspheres under microwave field determine the mechanical properties of alkali-activated fly ash-based cementitious materials directly.For this reason,based on the multi-stage microwave curing regime,fly ash groups rich in various micro-beads(carbon particles,floating beads and magnetic beads)were prepared in incorporation various micro-beads into roasted fly ash,which are obtained by flotation,magnetic separation and roasting.The effects of micro-beads under microwave field on the mechanical properties of alkali-activated fly ash-based cementitious materials were studied.And the hydration products and microstructure of the samples were analyzed by XRD,FTIR and FESEM-EDS.The difference of hydration process of various micro-beads under microwave curing were fine characterized in order to provide theoretical reference for the selection of raw materials and quality control as well as the optimization of microwave curing system while preparation of alkali-activated fly ash precast concrete by microwave curing technology in the future.The results show that the addition of 5wt.% or 10 wt.% floating beads and 5wt.%magnetic beads can improve the compressive strength of alkali-activated fly ash cementitious materials when 4 groups all reach their strength peak,while 5wt.% carbon particles has little effect on the compressive strength.In the later period of microwave curing,the phenomenon of over curing appeared,and the compressive strength of all groups decreased.But the existence of carbon particles could relieve its strength lossing.In the process of mixing hollow floating beads with alkaline solution,the hollow structure of floating beads stored a certain amount of alkali solution under mechanical pressure.During microwave curing,hydration reaction occurs simultaneously inside and outside the floating beads’ wall.Generated a large number of hydration products rapidly,thus shorten the microwave curing time when samples reached peak strength.While the carbon particles and magnetic beads have microwave absorbing properties.First,hydration products are formed on the surface of wave-absorbing micro-beads and their adjacent beads.When wave-absorbing micro-beads are completely encapsulated by the hydration products,the interfacial bonding force between microbeads and their adjacent microbeads is further enhanced,and other microbeads farther away from carbon or magnetic beads continue to produce more hydration products with alkali solution,resulting in a dense structure.In all samples,hydrosodalite and chabazite-Na formed during microwave curing process.With the prolongation of microwave curing stage,there has no change in the type of hydration products.And the addition of carbon particles,floating beads or magnetic beads has not cause the change of hydration products type of samples.In addition,the strength reduction of samples are due to the growth of spherical products generated in groups.These spherical hydration products which grow up to 1micron make the particle size distribution worse and overlap in space,resulting in the crushing of microspheres.Ultimately,samples structure become loose.However,shortening the microwave curing time are beneficial to the later strength development of alkali-activated fly ash-based cementitious materials.