The Effect Of Raw Materials On Alkali-activated Fly-ash/Slag Blended System

Cement is still the largly consumed in the world as building materials.A lot of coal will be used and emitting CO2 when producing of cement,which is a burden for the environment.An effective way to reduce energy consumption and CO2 emissions is imminent.Alkaliactivated fly ash / slag material(AAFS)use the industrial waste as the main raw material and mix with alkaline to produce a gel material.It not only has very good economic and environmental benefits,but also with high strength,anti-freeze,anti-acid and alkali corrosion and other excellent performance.It is a new type of green cementitious materials.However,basing on waste as raw material will bring the fluctuations of engineering performance.Studies have shown that the physical and chemical properties of fly ash and slag have a significant effect on the early performance,strength and durability of alkali-activated materials.However,how these properties affect the mechanism of AAFS are still unclear.So,in this paper,five different types of fly ash and four kinds of slag were selected as the raw material,and their physical and chemical properties were characterized.The effects of these properties of raw materials on the flow,setting time and compression of AAFS were studied.Methods such as XRD,SEM/EDX,isothermal calorimetry,MIP and nitrogen adsorption were used to investigate the reaction characteristics of AAFS.The effect mechanism of the physical and chemical properties of different raw materials on the AAFS was analyzed and studied.Research Conclusions:In the high-calcium fly ash/slag blended system,the high content of Ca O in the high calcium fly ash quickly reacts with the activator to form gels.Therefore,the paste has a short setting time,almost no flow.The gels increase the compressive strength of 7d but hinder the growth of late strength.At room temperature,the chemical activity of low calcium fly ash has little effect on the blended system.With the increase of fly ash loss,the flow of the paste decreases.The larger the bulk density of fly ash and slag,the longer the initial setting time,and the higher the compressive strength of 7d of the system.The larger the specific surface area of fly ash is,the smaller the compressive strength is.In the AAFS with different slags,the setting time and flow are shortened with the increase CaO content in the slag.The compressive strength increases first and then decreases with the increase of the total amount of CaO,SiO2 and Al2O3 or the CaO/(SiO2 + Al2O3).The specific surface area of the slag increases the 7d compressive strength,but the increment of the strength between 7 and 28 days increases first and then decreases.The results of reaction exotherm show that the content of active calcium in the raw material is the main factor affecting the curve shape of the exothermic rate and the cumulative heat of the AAFS.The more active calcium content in the raw material,the more the initial exothermic rate.The higher cumulative heat,the more gel formed and the denser of the microstructure,then the higher compressive strength of AAFS.XRD results show that no new crystals are formed after the reaction in AAFS.SEM / EDX studies show that the reaction products of the blended system are related to the content of calcium.The reaction product of the system with high calcium fly ash is C-S-H(II)gel with three-dimensional network struture,whoes Ca/Si is about 2.The product in blended system with low calcium fly ash is C-S-H gel containing Al or Na with low Ca/Si.However,in different slag systems,the product is C-S-H gel with larger Ca/Si.Aluminosilicate and carbonate may be present in all systems.The microstructures of the AAFS were observed by Scanning electron microscopy(SEM).The results show that the higher content of calcium in the raw materials,the more gel producted.The closer the gel is to the unreactive particles.The results of pore structure analysis show that the effect of different fly ash or slag on the cumulative pore volume of the AAFS depends on the content of active calcium,the content increase,the pore volume decreases,which has little effect on the most probable pore,which is about 4 nm in all samples.The smaller the porosity,the higher the compressive strength of the hardened paste.