Study On Hydration Process Regulation And Microstructure Evolution Mechanism Of Calcium Aluminate Cement

Refractory castables are widely used in high-temperature industries,such as iron and steel metallurgy,petrochemical industry,cement and glass production.Calcium aluminate cement（CAC）is a commonly used binder in refractory castable due to its excellent high temperature resistance,high early strength and suitable setting and hardening speed.With the promotion of micro powder and ultra micro powder and the use of efficient admixture,low cement and ultra-low cement bonded castables develop rapidly.However,CAC can interact with micro powder and admixture to form a complex binding system dominated by hydrates in castable.Moreover,the metastable phase transformation,dehydration and high temperature evolution of hydrates have further effects on the stability of structure and properties of the castable during baking and high temperature application.Therefore,it is of great significance to optimize the composition of low cement and ultra-low cement bonded castables,regulate the hydration process of CAC,and improve the stability of structure and performance of castables in the process of curing,baking and high temperature using.In order to solve the above problems,the hydration behavior of CAC under different conditions was firstly studied,the dynamics of CAC in different hydration stages was analyzed,and the action mechanism of micro-powder and admixture on hydration process of CAC was discussed.Secondly,the effects of different drying methods on hydrates composition and pore structure of hardened CAC pastes were compared.Finally,the effects of temperature,chemical composition and raw material on the structural evolution of CAC bonded castable matrix during heating were studied.The main conclusions are as follows:The hydration process of CAC is first controlled by nucleation and crystal growth process,and when the hydration degree reaches 0.15,the control process of hydration changes to phase boundary reaction process.When the hydration degree is above 0.48,the hydration process is controlled by diffusion process.CA is the main reactant participated in the hydration reaction during 24 h curing,and the hydrates include AH₃-gel,C₂AH₈,CAH₁₀and C₃AH₆,as well as a small amount of AH₃-gibbsite.The hydration of CAC can be controlled by adjusting the water to solid ratio or particle size distribution of CAC.When the water to solid ratio increases from 1.2 to 2,the hydration of CAC is accelerated and then slows down.When the particle size of CAC decreases and the distribution becomes wide,the formation of hydration products is inhibited and the hydration process of CAC is hindered.The hydration degree of CAC can be improved by introducing alumina,magnesia and silica powder.The addition of alumina powder promotes the dissolution of CAC,accelerates the nucleation of hydrates,and improves the elastic modulus of CAC hardened paste at room temperature.The smaller particle size of alumina powder is,the more obvious acceleration effect is.The addition of fine magnesia powder with low activity reduced the hydration rate of CAC,while the addition of magnesia powder with high activity significantly accelerates the hydration of CAC and the formation of CAH₁₀.Adding silica powder accelerates the formation of hydrates of CAC and improves the crystallization degree of AH₃-gibbsite and C₃AH₆.The larger the specific surface area is,the more obvious the acceleration effect is.It is found that the addition of dispersant accelerates the dissolution of CAC,contributes to the nucleation of hydrates,and accelerates the hydration process of CAC.But the hydration degree of CAC decreases.After adding citric acid,the hydration reaction of CAC is hindered by the formation of unstable complex and stable solvation film.Due to the formation of insoluble Li Al（OH）₄,the concentration of calcium ions and aluminate ions in slurry is changed with the addition of lithium carbonate.At the same time,the hydration reaction rate of CAC is accelerated,and the formation of stable hydration products is promoted.Freeze drying is carried out at a low temperature,which inhibits the transformation of CAH₁₀and C₂AH₈in CAC hardened paste.There are still many gel pores in the hardened paste after drying.The drying rate of microwave is very fast.Due to selective heating for components with different absorption capacity,part of C₂AH₈in CAC hardened slurry is converted into stable hydrates during microwave drying.The pore size is mainly in the range of 20～2000 nm.As the existence of water blocking effect during conventional drying,the drying time prolongs.The metastable phases in CAC hardened paste are completely transformed into stable ones,and the pores are coarsened after drying.Therefore,the unconventional drying method can effectively control the transformation degree of metastable hydrates,regulate the pore structure of the dried castables.By controlling heating temperature,Ca O/SiO₂,Ca O/MgO and magnesia source type,the phase composition of CAC bonded castable matrix can be designed at high temperature.The transformation of C₁₂A₇→CA→CA₂→CA₆occurs at 900～1400℃.The crystallization degree of CA₆is improved by increasing the treated temperature.When the temperature is higher than 1200℃,the mechanical properties of the matrix are significantly improved.Cristobalite and anorthite form after CAC bonded Al₂O₃-SiO₂castable matrix treated at 1200℃.When the temperature increases to1400℃,mullite forms.With the decrease of Ca O/SiO₂,the phase containing Si has a transformation trend of C₂AS→CAS₂→A₃S₂,which is benefit to improve the properties of castable.MA forms when CAC bonded Al₂O₃-MgO castable matrix is treated at 1200℃,while flake CA₆forms after being treated at 1400℃.It is beneficial to the growth of flake CA₆by increasing temperature.When the molar ratio of Ca O/MgO is 1:1,CA₆reacts with MA to form C₂M₂A₁₄at 1650℃.With the increase of the treated temperature,the morphology of the crystal gradually changes from hexagonal sheet to equiaxed shape.When the Ca O/MgO mole ratio is 1:2,both C₂M₂A₁₄and CM₂A₈existes in the matrix treated at 1750℃,and the magnesia-rich spinel is not conducive to the formation of CM₂A₈.