Study On Micromechanical Properties Of Alkali-activated Fly Ash/Slag

Portland cement concrete is widely used in building and civil engineering infrastructure,but its energy consumption and carbon emissions limitations can not be ignored.Compared with Portland cement,alkali-activated material(AAM)has the advantages of low energy consumption,high strength,and good durability,and is considered a potential substitute for ordinary Portland cement.The macroscopic mechanical properties of alkali-activated concrete are closely related to the microscopic mechanical properties of reaction products and the interface transition zone(ITZ)between the products and aggregates.Micro and nano scales are the basis for studying macroscopic mechanical properties,and the influencing factors of concrete performance can be explored by studying the material properties at micro and nano scales.In this study,nanoindentation test combined with backscattered electron images were used to investigate the evolution of micro-mechanical properties and product distribution characteristics of alkali-activated fly ash/slag paste gels under different fly ash/slag contents,and further study the thickness range and formation mechanism of the ITZ in alkali-activated fly ash/slag mortar/concrete interfaces.The results show that:(1)The elastic modulus of the gel products of the alkali-activated fly ash/slag composite system is higher near the raw material particles,mainly ranging from 22 GPa to 40 GPa.InIVthe distant region from the raw material,due to the slow dissolution rate of elements such as Ca,Si,and Al in the precursor,the elastic modulus of the gel products is lower,mainly ranging from 14 GPa to 25 GPa.(2)According to the distance between the raw materials,the elastic modulus of the gel products around the alkali-activated slag paste mainly ranges from 15 GPa to 32 GPa and from 37 GPa to 49 GPa.When the ratio of fly ash to slag is 5:5,the elastic modulus of the gel products mainly ranges from 21 GPa to 43 GPa,when the fly ash content is higher,the reaction degree of the system is reduced due to the low reactivity of fly ash.When the ratio of fly ash to slag is 7:3,the elastic modulus of the gel products mainly ranges from 15 GPa to 23 GPa and from 31 GPa to 41 GPa.Compares with the pure slag system,the Na/Al and Si/Al in the products of the alkali-activated fly ash/slag composite system decrease.(3)With the increase of age,the interaction between fly ash and slag gradually becomes stable,which shows that the elastic modulus of gel products at the age of 120 d mainly ranges from 18 GPa to 34 GPa,Si/Al between 1.5 and 2.5,Ca/Si between 1.0 and 1.5.The participation of Na for charge balancing in gel products increases gradually,and the Na/Si and Na/Al in the later alkali-activated fly ash/slag system enhance gradually.(4)The elastic modulus of gel products in alkali-activated mortar is mainly between 15 and 30 GPa,which is lower than that of the gel products of alkali-activated paste.It can be seen from the scanning electron microscope image that the addition of slag improves the microstructure of alkali-activated mortar,and the structure around the aggregate is denser.With the increase in slag content,the thickness of the interfacial transition zone in the alkali-activated mortar changes slightly,and the elastic modulus increases,but the increase gradually slows down.The width of the interfacial transition zone at 28 d in the alkali-activated mortar under different slag contents is about 20 μm.Due to the influence of the wall effect,the alkaline reaction process around fine aggregate is accelerated,which promotes the formation of gel products.The elastic modulus of the interfacial transitions zone is about 29 GPa,which is 1.12 times that of the paste matrix,and the micro-mechanical properties are improved.(5)The elastic modulus of gel products in alkali-activated concrete ranges from 25 GPa to 35 GPa,which is lower than that of the gel products in the alkali-activated paste.The width of the interfacial transition zone of the alkali-activated concrete at 28 d is about 20 μm,which is lower than the thickness of the interfacial transition zone of the cement concrete with the same water-binder ratio and age(～50 μm).With the increase of slag content,the elastic modulus of the interfacial transition zone and paste matrix of the concrete increase.Due to the interaction of various mechanisms such as the wall effect and micro-bleeding effect,the reaction rate around aggregate decreases,and the elastic modulus of the interfacial transition zone is lower than that of the paste matrix.The elastic modulus of the interfacial transition zone in the alkali-activated concrete is about 31 GPa,which is 77% of the paste matrix and is a weak link.