Solidification Of Simulated High-concentration Borate Solution With Modified Calcium Sulfoaluminate Cement

With rapid development of the nuclear power industry,the disposal of nuclear waste liquid has become increasingly prominent.Cement solidification is normally used in the disposal of low and medium-level radioactive nuclear waste liquids due to its efficient waste liquid solidification capability and simple process operation and equipment requirements.Boron acid is commonly used as a moderator for nuclear reaction rate in China's pressurized water reactor nuclear power plants.Nuclear waste liquid often contains a large amount of borate,and borate is commonly used as a retarder in cement to delay the hydration process and seriously affect the cement waste form.Therefore,using reasonable technical means and characterization methods to reveal the hydration behavior of cement in boron solution,the technical formula of cement high-efficiency curing high-concentration boron core waste liquid is proposed to meet the performance index requirements of cement waste form.The topic of calcium sulfoaluminate cement as cement-based material was studied in this paper,as well the hydration process of sulfoaluminate cement in the borate solution.The formation of layered amorphous ulexite which grew up on the surface of cement particles was characterized,and the microstructure of the amorphous form of ulexite was observed.Calcium hydroxide was used to adjust the hydration environment,for the inhibition of the formation of ulexite and accelerating the hydration rate of calcium sulfoaluminate cement in boron-containing simulated liquid.The 28 d compressive strength of cement waste form promised as high as 25 MPa.The paper adjusted the durability of the solidified form by different addition agent.The strength loss after immersion in water for 42 days was 9.36%,and the strength loss after 5 freeze-thaw cycles was 4.72%.The relationship between boron-containing simulated liquid and addition agent on the hydration process and hydration products of modified calcium sulfoaluminate cement was charaterized by XRD,FTIR,TG-DTG,SEM,EPMA and microcalorimetry.The mechanical properties and durability of the cured body were evaluated by compressive strength,impact resistance,loss of freeze-thaw resistance,and loss of immersion strength.The conclusions were listed as following:1)The sulfoaluminate cement cannot be properly hydrated in the boron-containing simulated liquid,and amorphous ulexite as a layer formed on the surface of particles and stopped the dissolvement of inner clinker,leading to a severely delaying the hydration reaction of the cement.2)In the process of solidification of boron-containing simulated liquid by sulfoaluminate cement,calcium hydroxide is introduced as a product regulating medium to control the formation conditions of hydrated products,inhibit the formation of ulexite,and form polyborate with flaky structure characteristics.Calcium can effectively increase the hydration rate of cement.In addition,boron enters the ettringite structure with a borate molecular structure,and increases the molecular structure layer spacing of the hydrated product ettringite.3)Sodium carbonate induces calcium hydroxide which is used in the modified medium to preferentially react to form calcium carbonate.Calcium carbonate can fill the pores and improve the anti-soaking performance of the cement waste form.However,the consumption of calcium hydroxide leads to the failure of the modified sulfoaluminate cement to be normally hydrated in the high concentration boron-containing simulated liquid,the degree of hydration of the solidified cement waste form is low and the mechanical properties are lowered.4)C-LDHs absorb water molecules to transform into LDHs structure.LDHs structure can absorb borate ions and reduce the retardation of borate to sulfoaluminate cement hydration.The high content of C-LDHs in the process of absorbing water will reduce the dissolution rate of calcium hydroxide,thus delaying the hydration rate of the high concentration boron-containing simulated liquid of modified sulfoaluminate cement.The optimum dosage of C-LDHs is 1% of the cement quality.5)Sodium metasilicate reacts with calcium hydroxide for modification in cement slurry to form calcium silicate and sodium hydroxide.Due to the tight structure of calcium silicate and hydration product,its freeze-thaw resistance is improved,and the best sodium metasilicate.The amount of the mixture is 2% of the mass of the cement.