Preparation And Properties Of Fly Ash-based Geopolymers

Fly ash based geopolymer can use solid waste and reduce carbon emissions effectively,it is a new type of inorganic polymer material showing a three-dimensional network structure developed in recent years,and it has the potential to replace cement as the main cementitious material.In this paper,the effects of silica fume(SF)content,Si/Al,water-binder ratio(W/B),curing temperature,different alkali,sodium silicate(SS)modulus and Na/Al on the mechanical properties and microstructure of fly ash based geopolymers were studied.The main conclusions are as follows:When the incorporation ratio of SF to fly ash(FA)is 1:4,the compressive strength at 28days is 12.1MPa,and when the incorporation ratio is 4:1,the compressive strength at 28 days is 23.89 MPa,the compressive strength of the fly ash based geopolymer increases gradually with the increase of the incorporation amount of SF.When Si/Al is 2,the compressive strength at 28 days of the fly ash based geopolymer net slurry is 27.7 MPa,and when Si/Al is 4,the compressive strength at 28 days of the fly ash based geopolymer net slurry can reach 34.3MPa.With the increase of Si/Al,the compressive strength of the fly ash based geopolymer gradually increases.When Si/Al is 3,the compressive strength of the fly ash based geopolymer decreases with the increase of W/B.When W/B is 0.5,the compressive strength at28 days reaches the maximum of 37.3 MPa.Increasing the curing temperature will increase the compressive strength of the fly ash based geopolymer.When the curing temperature is80?,the maximum strength of the fly ash based geopolymer is 36.6 MPa at 28 days,but the compressive strength at 28 days decreases when the curing temperature is over 80?.The effect of M on the mechanical properties of fly ash based geopolymer is studied.When Si/Al is unchanged,the maximum compressive strength at 28 days is 32.5 MPa as M is2,and the maximum compressive strength at 28 days is 11.8 MPa as M is 10.The compressive strength of fly ash based geopolymer paste at 28 days decreases gradually with the increase of M.When sodium silicate is added into the alkaline activator of the fly ash based geopolymer,the compressive strength of the geopolymer increases at 28 days and the internal structure becomes more uniform.When the concentration of Na₂O remains unchanged,the compressive strength of the fly ash based geopolymer gradually increases with the increase of M_S,and the maximum is 41.4 MPa.When Si/Al is constant,the compressive strength of fly ash based geopolymer decreases gradually with the increase of Na/Al.When Na/Al is 0.3,the compressive strength at 28 days reaches the maximum of 35 MPa.When Si/Al is 2,the compressive strength decreases obviously with the increase of Na/Al,and when Si/Al is 4,the decreasing trend is no longer obvious.KOH can replace Na OH as the alkaline activator of fly ash based geopolymer.The strength of fly ash based geopolymer mortar prepared under the same conditions is 20 MPa at 28 days,which is 37%lower than that of fly ash based geopolymer activated by Na OH,and the activation effect is inferior to that of Na OH.It founds that fly ash based geopolymer structures mainly exist in the new generation of zeolite facies(Chabazite-Na),the amorphous N-A-S-H gel phase and hematite and quartz phase in the raw material of fly ash,incomplete hydration of FA particles and SF particles is common in the structure,and the densification is not complete.The appropriate water-binder ratio(W/B=0.5)and curing temperature(80?)promote the particle activation of the precursor more efficient,and make the internal structure more compact.Fourier transform infrared(FTIR)results show that the existence of N-A-S-H gel and a small amount of structural water in the structure.H-O-H stretching vibration and bending vibration peaks is found in the fly ash based geopolymer,the change of the structural monomer and the depolymerization or aggregation of T-O-Si(T=Al and Si)cause the change of the compressive strength of fly ash based geopolymer.