Study On The Activation Of Fly Ash And Its Mechanism

Fly ash is an ideal mineral addition for high performance concrete.As pozzolanic material,it can get so much attention and favor for improving the workability of fresh concrete,reducing hydration heat,and improving the long-term strength and durability of hardened concrete to some extent.However,due to some shortcomings such as low early strength and so on,the utilization degree of fly ash concrete is low.Therefore,the research on the activation of fly ash is carried out.It is of great significance to solve the problems of low early strength and application of fly ash and to increase the content of fly ash in concrete.On the basis of analyzing and testing the physical and mechanical properties of fly ash,such as chemical composition,morphology and particle size distribution,a series of sulphate activators and alkali-activators for calcium?slacked?lime were selected.The effects of the amount of several activators and compound activators on the hydration products,hydration process,physical and mechanical properties of fly-ash cement system were studied by means of experiments so as to determine the best variety and amount of activators for the activation of fly ash.The effects of activators on the composition,morphology and microstructure of hydration products of fly-ash cement system were studied by means of XRD and SEM,in order to reveal the active mechanism of activator.Finally,the influence of fly ash active activators on the workability and mechanical properties of the concrete with large amount of fly ash pumping mechanism sand was discussed.The experimental results show that the excitation effect of anhydrous sodium sulfate is the most obvious in the sulfate activators,and the optimal amount of anhydrous sodium sulfate is 1.5%.The excitation effect of slacked lime is better than that of calcium lime,and the flexural strength and compressive strength of fly-ash cement mortar can be increased by 94.7%and 70.4%respectively with the addition of10%slaked lime powder for 3 days.The compound activators of 1.5%anhydrous sodium sulfate and 10%slacked lime powder can significantly improve the strength of mortar for 3 days,among which the flexural strength is increased by 121.1%and the compressive strength is increased by 91.4%,and the strength is also improved to a certain extent for 28 days.The compound activators can not only improve the workability of mechanism sand pumping concrete mixing,but also improve the early strength of concrete.The activing mechanism of fly ash was analyzed by means of XRD and SEM.It was found that the compound activators of anhydrous sodium sulfate and slacked lime powder could excite the activity of fly ash efficiently.The mechanism of excitation is that the compound activators can provide more OH^-,Ca²⁺andSO₄^2-,so it can accelerate the reaction rate of active SiO2,Al2O3 and so on in fly ash and increase the quantity of rod-like hydrats ettringite?AFt?.Therefore,it speeded up the hydration rate and the hydration process of fly-ash cement system and the early strength of cement-based composites with high-volume fly ash was improved.