Study On Structural Evolution And Solidification Of Simulated Nuclide Of Phosphoric Acid-based Geopolymer

Phosphate based geopolymer?Al₂O₃·nSiO₂-mH₃PO₄,HPG?is a three-dimensional silicate network of inorganic silicates composed of[SiO₄],[AlO₄]and[PO₄]tetrahedra.HPG could not only achieve rapid curing in high temperature and acidic environment with a low heat release,but also had high compressive strength and can maintain long-term structural stability.In the process of treating acidic waste liquid,it could avoid complicated pretreatment procedures.The development and utilization of nuclear energy has alleviated the energy crisis appropriately,but has brought a series of environmental security threats.In the treatment process of nuclear waste,low and intermediate level radioactive wastes was usually solidification by cement.However,in some special occasions,such as nuclear leakage accidents,the use of cement solidification method requires complex pretreatment procedures,furthermore,the accidents were often accompanied by higher environmental temperature and acidic curing conditions,the high heat release of cement curing will cause further damage to those occasions.This experiment intended to prepare HPG as a rapid and efficient emergency disposal material.This study used high activated silicon aluminate powders and metakaolin as raw materials to prepared HPG with different Si/P,Al/P,water/cement ratio and curing temperature.The compressive strength,setting time,chemical bonds,microstructure,the evolution and formation mechanism of structure of HPG were characterized by FT-IR,XRD,FSEM-EDX and other NBS test method respectively.The phase structural stability of metakaolin/phosphoric acid based geopolymer?HPMG?at high temperature was studied.And the solidified ability of HPMG to simulated nuclide Cs⁺and Sr²⁺was preliminarily explored.The main research results of leaching performance and the assigned state are as follows:?1?In experiment,high activated silica-aluminate powders with different components was synthesized by sol-gel method.Using phosphoric acid as the activator,HPG was prepared by changing water-cement ratio,curing temperature,SiO₂/Al₂O₃ and H₃PO₄/Al₂O₃ molar ratio.When the water cement ratio was 0.60,curing temperature was 45?,and the molar ratio of H₃PO₄/Al₂O₃ was 1,HPG has dense structure and largest compressive strength to 26.8MPa;Al-O was an important reactive group and increase the ratio of Al-O in raw material appropriately could improve the reaction degree of HPG.In the whole reaction process,-Al-O-Si-bond was broken to form a large number of-Al-O-P-connected tetrahedrons[SiO₄]to form a gel phase structure with-Si-O-Al-O-P-.If the molar ratio of H₃PO₄/Al₂O₃more than 1.0,the reaction would tend to generate a gel phase structure with-P-O-Al-O-P-.At the same time,tetrahedrons[SiO₄]combine with each other to generate cryogenic calcite.?2?Using phosphoric acid as the activator,Metakaolin Phosphoric Acid-based Geopolymer?HPMG?was prepared by changing water-cement ratio,curing temperature and P/Al molar ratio.Changing the water cement ratio?0.6⁰.8?,and the curing temperature?30⁶0??could adjust HPMG's initial and final setting time?1.8h⁷d?in larger scale adapted to achieve the appropriate working parameters to obtain the solidification rate controlled HPMG.With the increase of dosage of phosphoric acid,the compressive strength of HPMG first increased and then decreased.When the molar ratio of H₃PO_4/Al₂O₃ was 1.8,curing temperature was 50?and water cement ratio was 0.65,HPMG achieved maximum compressive strength of 98.8 MPa.At the 1400?high temperature treatment in the process,HPMG generated the cristobalite,aluminum phosphate,mullite phase,such as the mass loss rate lower than 25%,and mainly comes from the loss of free water and bound water.?3?Using HPMG as the solidified body to solidify the simulate nuclides Cs⁺and Sr²⁺.The study showed that MHPG had a dense structure,the pore sizes were all nano-harmless pore,and the compressive strength of the simulated nuclide solidified bodies was 85⁹8%of HPMG.Physical solidation was the main curing method of simulated nuclides Cs⁺and Sr²⁺.The leaching rates of simulated nuclides Cs⁺and Sr²⁺at room temperature were3.7×10^-5cm/d and 5.0×10^-6cm/d respectively,both lower than the national limits.Cs⁺was uniformly distributed in the matrix,while Sr²⁺tended to be distributed in the interlaminar matrix with incomplete reaction.