Study On Preparation Of Phosphoric Acid-based Geopolymer And Immobilization Of Simulation Medium Level Alpha Liquid Waste

Medium level alpha original liquid waste is generally acidic.To facilitate storage,NaOH is usually used to neutralize and alkalize the waste liquid,which is then condensed into a high-salt waste.This also increases the difficulty for the later curing treatment.Phosphoric acid-based geopolymer that is an inorganic polymer prepared from silica-aluminum materials was stimulated by phosphoric acid or phosphate solution.And its internal three-dimensional network structure can effectively contain and solidify toxic heavy metal ions.It has excellent mechanical properties,low porosity and good stability at high temperature.Phosphoric acid-based geopolymers are capable of rapid hardening in acidic environments.Therefore,the curing treatment of intermediate medium level alpha liquid waste by phosphoric acid-based geopolymer may become a new way.In this paper,the phosphoric acid-based geopolymer was prepared with metakaolin as the base material and phosphoric acid solution as the activator under the condition of 50?.And the composition of hydrates,structure and properties of the phosphoric acid-based geopolymer solidification matrix materials and the phosphoric acid-based geopolymer solidified body were studied.It provides some theoretical and technical support for the application of phosphoric acid-based geopolymer in the field of solidification of radioactive waste.The results are as follows:?1?The influence of H₃PO₄/Al₂O₃ molar ratio and water cement ratio on the structure composition and performance of phosphoric acid-based geopolymer solidified substrate was studied by compressive strength,sample pore structure,analysis of hydration heat and SEM-EDS and so on.The results show that with the increase of the molar ratio of H₃PO₄/Al₂O₃?1.4².0?,the compressive strength of the solidification matrix materials first increases and then decreases,and the porosity first decreases and then increases.By changing the water/solids ratio?0.4⁰.7?,the solidification matrix materials has different hydration total heat and hydration heat flow.When the molar ratio of H₃PO₄/Al₂O₃ is 1.8 and the water/solids ratio is 0.5,the solidification matrix materials of phosphoric acid-based geopolymer has the optimal mechanical properties and high hydration heat release.After 28d,its compressive strength reached 78.1 MPa,porosity was as low as 0.84%,and the heat release was about 304 J/g.Phosphoric acid-based geopolymer are amorphous substances.Its basic structural units are-O-Al-O-P-O-Si-O-and-Si-O-Al-O-P-O-.It can remain stable at1200?,and the mass loss is about 24.8%.?2?The stability of the solidification matrix materials of phosphoric acid-based geopolymer with at different erosion time in distilled water,simulated underground water and 3 wt.%of Na₂SO₄ solution was studied by compressive strength,the pore structure and ion concentration and so on.The results show that the solidification matrix materials of phosphoric acid-based geopolymer has better stability in aqueous solution.And the corrosion resistance of groundwater is better than sulfate.The compressive strength of the solidification matrix materials is reducing with different degrees after being immersed in distilled water,simulated underground water and 3 wt.%of Na₂SO₄ solution.3 wt.%of Na₂SO₄ solution shows the greatest attenuation,but the compressive strength above 65 MPa could be maintained after 28 d.With the extension of soaking time,the porosity and pore size of solidification matrix materials increased slightly.After 28 d,the porosity of samples eroded by distilled water,simulated underground water and 3 wt.%of Na₂SO₄ solution was0.99%,1.36%and 1.61%respectively.The three solutions were acidic,and the concentration of Si ions in these solutions was basically the same,about 5 mg/L.Under the influence of SO₄^2-,the concentration of Al ions in 3 wt.%of Na₂SO₄ solution was the highest,reaching22.7 mg/L at 42 d.?3?By means of XRD,SEM-EDS,ICP and other tests of compressive strength,the influence of content of simulation medium level alpha liquid on the structural,stability and leaching performance of the phosphoric acid-based geopolymer solidified body was studied.The results show that the simulated nuclides are uniformly distributed in the solidified body.With the increase of simulation medium level alpha liquid,the X-ray diffraction peak of sodium nitrate in the solidified body gradually increased.And the increasing of the pore size and porosity of the solidified body leads to decrease of the sample density and compressive strength.The most reasonable salt package capacity of the solidified body is 30 wt.%,beyond which the structural composition of the solidified body changes greatly.The simulated underground water erosion results in the loss of the compressive strength of the solidified body by increasing the porosity of the phosphoric acid-based geopolymer solidified body.And the larger the amount of waste liquid package in the solidified body,the more obvious the erosion effect.After 5 freeze-thaw cycles,the strength loss of the solidified body with salt bale capacity of 35 wt.%was the largest,which was 15.08%,lower than the national standard.So the solidified body has good freeze-thaw resistance.The higher the leaching temperature,the lower the 42 d leaching rate and cumulative leaching fraction of the simulated nuclide Nd³⁺and Ce⁴⁺in the solidified body,and the more difficult it is to detect Nd and Ce in the later stage of leaching.At the same temperature,the leaching rates and cumulative leaching fractions of Nd³⁺are lower than Ce⁴⁺.At 25?,the 42 d leaching rates of the simulated nuclides Nd³⁺and Ce⁴⁺are 0.23×10^-7 cm/d and 0.89×10^-5 cm/d,respectively.Under the conditions of 25?,40?,70?and 90?,the cumulative leaching fractions of Nd³⁺and Ce⁴⁺are respectively in the order of 10^-5 and 10^-3,meeting the requirements of national standard.