Effect Of Physical And Chemical Properties Of Raw Materials On Alkali-activated Fly Ash/Slag Blended System

Alkali-activated materials,prepared through the reaction of solid aluminosilicates and alkaline activators,have been studied extensively.Alkali-activated fly ash/slag materials use industrial by-products or other inexpensive materials such as fly ash and slag as raw materials.Because the raw materials are widespread and the production process is simple,alkali-activated fly ash/slag materials are considered as a promising candidate to ordinary Portland cement.It not only has good environmental economic and environmental benefits,but also has high strength,durability,and other excellent characteristics such as the resistance of high temperature and acid/alkali corrosion.Therefore,the research and application of alkali-activated fly ash/slag materials have great significance.However,the physical and chemical properties of fly ash and slag from different sources are significantly different.Those differences will affect the performance and durability of alkali-activated materials,which limit the wide application of alkali-activated fly ash/slag materials in engineering.In this paper,four kinds of fly ashes and slags from different sources were selected as the raw materials.The combination of fly ash and slag was used to form a binary system.The mixture of water glass and NaOH were used as the activator to study the alkali-activated fly ash/slag blended system.In this paper,we tried to study the influence of different raw materials,modulus ratio of activator,fineness and chemical composition of fly ashes on the reaction process,microstructure and macroscopic properties of the cementitious materials.The chemical composition,phase composition,degree of polymerization,morphology and specific surface area of the raw materials were characterized by XRF,XRD,FTIR,SEM,BET nitrogen adsorption.The research shows that,for low calcium fly ashes,the early reaction is shlow.In the initial stage of the early reaction,the physical activity of fly ash plays a leading role.With the continuous depolymerization-polymerization of the vitreom,the chemical activity gradually plays an important role.Besides,the increase of later strength in high-calcium fly ash system is more significant than that of high-calcium fly ash system.For high-calcium fly ashes,the early reaction is fast.But this material is easy to react with CO₂ in the air to form Ca₂CO₃,which leads to lower increase or even decrease of the compressive strength.Ca²⁺has a substitution effect on Na⁺in the Si-O-Na structure,which reduces the linkability and degree of polymerization of gel.As a result Ca-rich gel phase mainly composed of SiQ³and SiQ²tetrahedral structural units is gradually formed.For different slags,due to the higher content of active ingredient,its physical activity may be a secondary factor of the early reaction of slag compared with chemical activity.The degree of polymerization is related to the content of basic oxide in the slag.Slag with higher content of CaO and MgO has higher activity and lower degree of polymerization.The reactivity of slag show different performances in different alkaline environments.In the high alkaline environment,the dissolution rate of Al increases,which leads to the gradual replacement of Si with Al in the network.As a result,the pore structure mainly composed of SiO₂ gel is changed.The modulus of activator has a great effect on the performance of the alkali-activated material.The OH^-and active SiO₂ in the activator play a key role in the activate of raw materials:low modulus with higher alkali content,can increase the dissolution of Ca²⁺.However,the Si in activitor is reduced,which can not meet the demand of gel production.The setting time is prolonged,the fluidity is reduced,the reaction rate is accelerated,the induction period is shortened,the heat release becomes higher,the degree of dissolution becomes higher,the gel formation becomes less,the compactness is reduced,the carbonization reaction is enhanced,and the compressive strength is lowered.Mechanical grinding has different effects on high-calcium fly ash and low-calcium fly ash.For high-calcium fly ash,chemical activity plays a leading role in the early stage.The refinement of particles does not greatly increase the reactivity and the effect of physical activity under the cover of strong chemical activity.For low calcium fly ash,physical activity is the primary factor of early activity,and the refinement of particles significantly increases the reactivity.The refinement of the particles narrows the pore size distribution of the reaction product,increases the amount of the gel pores,and increases the density of the gel phase.But the water demand sharply increases with too many fine particles.The slurry gradually becomes thicker,the heat release rate is slowed down,the reaction induction period is prolonged,the cumulative heat release is reduced,and the strength is lowered.Al and Si can accelerate the reaction,promote the reaction to polymerization,reduce the fluidity of the system,reduce the total porosity of the gel,shorten the condensation time,increase the proportion of the gel pores,increase the formation of gel products with pore diameter less than 5 nm,refine the average pore size,denser gel structure and inhibits the occurrence of carbonization reaction.