Study On The Structure And Properties Of Fly Ash-Metakaolin Composite Based Geopolymer

Geopolymer,a new kind of inorganic polymer cementitious material,it had excellent mechanical properties,fire resistance,acid resistance and three-dimensionally network structure that analogues of zeolites.At present,geopolymer as the immobilization material was concerned on scholars due to the ability of stabilization/solidification the toxic and harmful ions such as heavy metals and radioactive wastes.In addition to the requirements of mechanical performance of solidified waste,it is more important is its long term retention for ion,which is derived from solid-dissolving,adsorption and physically encapsulated.The immobilization capacity was closely link to the phase composition and the density of solidified matrix material.In this paper,geopolymer samples were prepared by fly ash,metakaolin,Na OH and water-glass as the raw materials,the phase composition,microstructure,pore-structure and ion leaching rate were analyzed by XRD,SEM,MIP and AAS testing techniques,respectively.The effects of material composition?such as composition of alkali activator,mixture ratio of powder and particle size?and technology conditions?such as low temperature curing system,hydrothermal synthesis temperature and molding pressure?on the structure and mechanical properties of geopolymers were systematic studied.Geopolymer materials were used to immobilization simulated radionuclide Cs,and the leaching mechanism of Cs ion in the solidified of geopolymer was explored.The main results are as follows:?1?By studying the effects of material composition on the pore structure and compressive strength of geopolymer,the results show that the water cement ratio is one of the key factor affecting the pore structure of geopolymer.When the water cement ratio is constant,the change including the alkali activator solid content and modulus,blend ratio of metakaolin/fly ash in power and low temperature curing condition,has no obvious effect on total porosity of geopolymer.Moreover,the refinement of fly ash particle size is an effective measure to reduce the porosity of gels.Sample FF 6MF2MK5C1 owns excellent density and mechanical properties,its porosity and compressive strength is 11.04% and 62.6MPa,respectively.The order from large to small of the effec ts of each parameters on compressive strength is blend ratio of metakaolin/fly ash,composition of alkali activator and the refinement of fly ash particles.When the fly ash was substituted partly by metakaolin,there are small quality interfacial defects between unreacted fly ash particles and the gel phase.Meanwhile a large amount of continuous gel network structure are formed,so that the compressive strength is enhanced.The correlation between compressive strength and total porosity in accordance with exponential function,the series MHS and FMC geopolymer meet mathematical equation of Y=844.27·exp^-8.06X?R2=0.958?and Y=84.07·exp^-2.64X?R2=0.96?,respectively.?2?The effect of preparation technics on pore structure of the geopolymer is investigated,it is found that the hydrothermal condition has a significant influence on the pore structure.With the increase of hydrothermal temperature,the amorphous gel phase change gradually to Na P1 zeolite phase,and then to analcime phase.The total porosity is increased in this process.The total porosity of analcime-riched geopolymer reachs 40.12%.However,the low temperature curing system and the compression molding have no obvious effect on the total porosity of geopolymer.The high strength?>100MPa?geop olymer can be prepared by molding pressure of 200 MPa.The compressive strength of sample PS1 will increase from 58 MPa to 106 MPa compared to ordinary molding,as well that of sample doped with resin will elevate from several MPa to 110 MPa.It is meaningful that providing a new method to immobilization radioactive resin.?3?By studying the immobilization of simulation radionuclide Cs in geopolymer,it indicates that compared to plugging ability of high dense geopolymer solidified waste to radionuclide,analcime and Na P1-zeolite has the stronger adsorption to Cs⁺,improves better retention capability of radionuclide.Although analcime-riched geopolymer has high porosity?40.12%?,the cumulative leaching fraction of Cs⁺ is 9.9×10^-5cm/d at 42 d.Decreasing the porosity of greater than 50 nm in geopolymer,improves obviously the physically encapsulated effect to simulated nuclide.The relationship of leaching fraction of Cs with leaching time satisfies the first-order reaction model: CLF=Q₀(1-exp^?-kt?),leaching mechanism of nuclide cesium is the infiltration of the leaching solution into the solidified and the exchange of the nuclide Cs from the surface of the solidified to the leaching solution at the same time.