Study On The Ceramization Mechanism Of Magnesium Phosphate Cement-based Solidified Body

Nuclear energy,as one of the most promising new energy sources for mankind in the future,has played an important role in the development of human society.But while nuclear can benefit us,it also brings us a more difficult environmental problem,namely radioactive pollution.Among them,the most dangerous is the highly radioactive material,which has great radioactivity and will cause serious damage to humans and the environment if not properly disposed of.Especially,the high-level waste liquid has certain fluidity,so it is relatively more difficult to control.There are many researches on the solidification of high-level liquid waste,such as glass solidification,artificial rock solidification and glass ceramic solidification.However,due to the complex process and long solidification time,these solidification methods cannot be quickly and safely handled in response to nuclear emergency accidents.While the solidification process of cement solidification is simple and fast.Therefore,cement solidification is beneficial to the rapid disposal of nuclear emergency accidents.Magnesium phosphate cement（MKPC）has attracted the attention of scholars and achieved good results in solidifying heavy metals and low-to-medium-level radioactive waste due to its fast-hardening agglomeration,high early strength,compactness,and high temperature resistance.In this paper,MKPC is used as the solidified substrate,and the solidified form is sintered at high temperature under the condition of emergency curing to form a stable and dense ceramic-like structure.Through the study of the ceramization process,the effects of material ratio and sintering system on the rapid solidification of waste liquid and the ceramization process of the solidified form are revealed,and the optimal ratio and sintering system of the ceramization of the solidified form are studied,and the X-ray diffraction（XRD）,scanning electron microscopy and energy spectrum（SEM-EDS）,Nitrogen adsorption（BET）and other modern analytical testing methods to analyze the material phase,micro-morphology and micro-area composition,pore structure,etc.,and discuss the ceramic mechanism.The research results show that when magnesium phosphate cement substrates with different mass ratios of magnesium oxide and potassium dihydrogen phosphate（M/P ratio）are mixed with 30%simulated high-level waste liquid,as the M/P ratio increases,The compressive strength of the solidified form is a decreasing trend.When the M/P ratio is 1,the compressive strength is the highest,and the compressive strength in 3 hours reaches 12.29 MPa,which can meet the strength requirements of the cement solidified form during emergency curing.By sintering the solidified form at different sintering temperatures and holding times,the preliminary ceramicization found that under the same M/P ratio and holding time,the volume density of the solidified form became larger as the sintering temperature increased.The density growth can reach 35.4%;when the M/P ratio and the sintering temperature are the same,the bulk density also increases with the holding time,the maximum density is2.855g/cm³,and the relative growth rate is 30%.In order to reduce the liquidus temperature,the strength and sintering properties of cement solidified form were studied by using glass frit prepared with ordinary glass and adding a certain amount of iron oxide.Based on the M/P ratio of 1 as the basic formula,the strength of the cement cured body mixed with 10wt%,20wt%,40wt%glass frit is compared with the sample without glass frit,and the strength in 3 hours increases from 12.29MPa to15.03MPa,indicating that the glass frit has certain benefits for the strength development of the cement solidified form,and is more conducive to the formation of a densely structured cement solidified form.The density of the ceramic solidified form obtained after sintering under the same conditions is higher than that of the cement solidified body,and the apparent morphology also shows that its structure is denser.On the basis of glass frit,5wt%and 10wt%of iron oxide was added,and it was found that its strength still increased,and at 1400℃,holding 135min,the density can reach3.093 g/cm³,the apparent morphology also shows that the structure density is much higher than other samples.At the same time,through the analysis of the simulated nuclide leaching rate of the sample,the leaching rate of Sr²⁺was found to be 2.1473×10^-7 g/m²/d,and the leaching rate of Cs⁺was 2.0118×10^-6 g/m²/d,which were the lowest values among all samples,indicating that ceramization is indeed conducive to the improvement of curing performance.The reason for the improved curing performance is that the ceramic solidified form with forsterite（Mg₂SiO₄）and magnesium potassium phosphate（MgKPO₄）as the crystal phase is formed in the ceramic solidified form.Its crystal structure stability is better than the main hydration product K-struvite（MgKPO₄·6H₂O）of the magnesium phosphate cement solidified form,and its internal structure is also more dense,so it effectively improves the curing performance of the solidified form.