Study On Synthesis Of In-situ Carbon-containing Calcium Aluminate Cement And Its Applications In Refractory Castables

In order to improve the water wettability/dispersion of carbon materials,and improve the corrosion resistance of refractory castables,the in-situ carbon-containing calcium aluminate cement(CCAC)was synthesised through combustion synthesis,and carbon-bed sintering method,respectively;and the properities of the castables bonded by CCAC were studied.The main results of the present paper are as follows:(1)Calcium aluminate cement with phase composition close to that of Secar71was prepared by combustion synthesis with CaO₂,Al,and Al₂O₃ as raw materials,and used it as binders for refractory castables.It was found that when the CaO/Al₂O₃(C/A)molar ratios reduced from 1.1 to 0.74,the content of CaO·2Al₂O₃(CA₂)in the products increased,whereas the contents of CaO·Al₂O₃(CA)and 12CaO·7Al₂O₃(C₁₂A₇)decreased;when the C/A was 0.8,the phase composition of the combustion product was close to that of Secar71.During the process for cumbution synthesis of calcium aluminate cement,O₂ from decomposition of CaO₂ reacted with Al forming Al₂O₃,which released a large amount of heat,making the combustion wave self-propagating;as the reaction temperature rising,the CaO-Al₂O₃ liquid was formed by co-melting of CaO and Al₂O₃;when the reaction temperature reached its maximum,the content of CaO-Al₂O₃ liquid was further increased.Finally,calcium aluminates(CA,CA₂,C₁₂A₇)with good crystallinity would be gradually precipitated from the CaO-Al₂O₃liquid with decreaing the reaction temperauture.After being dried at 110°C,and fired at 1100°C,respectively,the CCS and CMOR values of CS71 bonded castables were higher than that of the castables bonded by Secar71.The CCS and CMOR of both castables after being treated at 1500°C exhibited no obvious differences.The refractory castables bonded with CS71 demonstrate improved volume stability.(2)Fixing C/A molar ratio at 1.12,the effect of molar substitutions of CaCO₃ for CaO₂(20%,30%,40%,50%)on the phase composition and micromorphology of calcium aluminate fabricated through combustion synthesis method were studied.The results indicated that the relative content of CA decreased,while that of C₁₂A₇increased with the increase of molar substitutions of CaCO₃.Interestingly,CA nanofibers with about 200 nm in diameter were observed in the combustion products,when the molar substitutions of CaCO₃was 20%,30%,40%,respectively;and the more nanofibers could be found in the products,as the content of CaCO₃ in raw material ratios increased.The typical round droplet on head of the CA nanofiber indicated that the growth process of the nanofibers was governed by vapor-liquid-solid(VLS)mechanism.(3)The in-situ carbon-containing calcium aluminate cement with phase composition close to that of Secar71 was prepared by combustion synthesis with CaCO₂,CaO₂,Al and Al₂O₃ as raw materials,fixing C/A molar ratio at 0.8.The results showed that when the molar substitution of Ca CO₃ for CaO₂ was 25%,the phase composition of the product was close to that of Secar71.Its Raman spectra contains three distinct bands(D,G,2D)of the carbon materials,indicating that the in-situ carbons existed in the product;and its carbon content was approximately 0.28%.The in-situ carbon materials in the products were dispersed in calcium aluminates.Further increase the molar substitution of CaCO₃to 50%,the content of C₁₂A₇ in the products obviously increased,whereas the contents of CA and CA₂ decreased.(4)The in-situ carbon-containing calcium aluminate cement(CCAC)whose phase composition was close to that of Secar71 was synthesized through carbon-bed sintering method with calcium citrate tetrahydrate(C₁₂H₁₀Ca₃O₁₄·4H₂O)and Al₂O₃ as raw materials.After being sintered at 1500? for 4h,the phase and chemical compositions of the product approached to those of Secar71.Its Raman spectra contains distinct D,G,2D bands of the carbon materials,indicating that the in-situ carbon materials were generated in the product;and the carbon content of the product was 1.45%.The in-situ carbon materials in the product were uniformly dispersed and embedded in calcium aluminate.During the process for synthesis of CCAC through the carbon-bed sintering method,the in-situ carbon materials in the cement were generated through carbonization of C₁₂H₁₀Ca₃O₁₄·4H₂O at about 500?,and their graphitization was increased with the increase of temperature.When the sintering temperature rises to1000?,CaO and the in-situ carbon materials from the pyrolysis of calcium citrate tetrahydrate were surrounded by Al₂O₃.As the sintering temperature increase to1200?,the CaO would react with Al₂O₃ forming C₁₂A₇,and the in-situ carbon materials were surrounded by C₁₂A₇ and residue Al₂O₃.With further increase the sintering temperature,C₁₂A₇ would be melted,and transformed into small irregular particles of CA and CA₂ through reacting with Al₂O₃ particles.The in-situ carbons were uniformly distributed and embedded in calcium aluminate particles.(5)The water dispersivity and anti-oxidation of CCAC,and carbon back/Secar71(S71CB)composite powders,as well as the properties of Al₂O₃-MgO refractory castables bonded by Secar71,S71CB,CCAC,respectively,were comparatively investigated.The carbon materials in CCAC were wrapped by calcium alumiante,which resulted in that the water dispersivity of CCAC was more excellent than that of S71CB.Calcium aluminate hindered diffusion of O₂into the carbon materials,leading to that the oxidation ratio of CCAC after being sintered at 1000?for 2h was only11.8%,much lower than that of S71CB.The carbons incorporated into the Al₂O₃-Mg O refractory castables through CCAC were uniformly distributed in the refractory castables.Compared with that of the castables bonded by Secar71,S71CB,respectively,the corrosion area percentages of the Al₂O₃-Mg O castables with CCAC as binders were decreased by 22.8%and 18.5%,respectively.Furthermore,the refractory castables bonded with CCAC had well physical properties,hot modulus of rupture.