The Effect Of Fly Ash Physical Properties On The Mechanical Properties And Microstructure Of Cement Paste

With the increasing demand for electric power,fly ash as solid waste produced by thermal power plant has had a serious negative impact on the ecological environment and human health.Incorporating fly ash as auxiliary cementitious material into concrete has become a main approach to sustainably utilize resource.Compared with the original fly ash,crushing fly ash shows the more advantage of chemical activity,which contributes to improving the compressive strength of cement paste.However,the composite slurry is different in performance,because the physical properties of fly ash particles prepared by different grinding processes are different in particle size distribution,apparent morphology and other physical properties.Hence,this paper chooses steam jet mill and ball mill to prepare ultra-fine fly ash,their influences of content,particle size and gradation on compressive strength of cement composite cementitious material past are studied by such micro analysis means as mercury intrusion porosimetry,scanning electron microscopy,X-ray diffraction,thermogravimetry and X-ray photoelectron spectroscopy.Meanwhile,this paper combined with the analysis of the physical characteristics of fly ash and composite slurry to reveal its influence mechanism.Its purpose is to provide theoretical guidance and experimental reference for improving the product performance of fly ash,and to increase the economic value and utilization rate of fly ash.The main conclusions are as follows:Under the same D₅₀ particle size,compared to the pulverized product of the ball mill,the pulverized product of the steam jet mill has a smaller D₉₀ particle size and has more fine particles,which is beneficial to improve the hydration degree of fly ash and generate more hydration products.Spherical particles mainly play a filling role in the early stage increasing the compactness of the slurry,reducing the chance of contact with external CO₂,and retaining more Ca（OH）₂ to react with fly ash.I the resulting hydration products will be further refined in the later stage.The pores of the slurry are transformed to improve the compressive strength of the slurry.At the same crushing process,as the particle size of fly ash decreases,the degree of hydration raises,the pores of the slurry decreases,the degree of structural compactness increases,and the compressive strength enhance.When the particle size of the fly ash prepared by the steam jet mill is 3.3μm,the 28d compressive strength of the cement mortar is 45.8 MPa and 44.3 MPa at the 30%and 40%content,respectively,which can reach 425cement Strength standard（42.5 MPa）.Compared with the compounds of the corresponding elements in cement,the compounds containing Ca,Al,and Si elements in fly ash have higher binding energy,indicating that the compounds of fly ash are more stable and the hydration reaction rate is slow.Therefore,as the dosage increases,the degree of hydration of the higher dosage of fly ash-water composite cementing material decreases,resulting in a decrease in the compressive strength of the fly ash-cement slurry.Based on the Fuller,Dinger-Funk model and the principle of fly ash hydration characteristics,the proportion of fly ash is designed.It is found that compactness of the specimens is increased because the particle size distribution of the composite powder is improved when fly ash of different particle sizes is mixed into cement.Analysis of the particle size distribution of the composite powder found that particles<10μm of size mainly play a role in hydration in the slurry;particles with 10～60μm of size have both hydration and filling effcts in the slurry,and the proportion of 10～30μm of size is the most important;And particles>60μm of sizes have low activity and only have a filling effect in the slurry.In conclusion,the compressive strength of the net slurry of fly ash-cement composite cementing material is the best,and its values are 81.29 MPa and 87.07 MPa respectively,under the conditions of 30%the tatal amount of fly ash,15%the equivalent amount of fly ash with a particle size of 9.57μm and 4.67μm.