Effect Of Gelling Agent On The Change Of Chemical Bond Of Colloidal Silica Bonded Castables During The Curing Process

The low demoulding strength of colloidal silica-bonded castables has narrowed the range of application.There are many researches on the physical properties of room temperature and high temperature performance of colloidal silica bonded castables,while rare researches investigate the influence of gelling agent on the change of chemical bonds between Si O2 particles of colloidal silica.Therefore,this work aims to study the relationship between demoulding strength and the change of chemical bonds of the particles of colloidal silica with and without gelling agent.The influence of different gelling agents(calcium aluminate cement(CAC),magnesium hydroxide(Mg(OH)2)or magnesia(Mg O))on the change of chemical bonds of colloidal silica(solid content of 30%)was investigated during the dehydration condensation reaction.The chemical bonds of the colloidal silica were investigated by Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)to systematically characterize the relationship between the chemical bonds and demoulding strength of colloidal silica bonded castables.The results show that the conductivity of colloidal silica decreased with the addition of gelling agent(CAC,Mg(OH)2 or Mg O),then gradually stabilized.Some Si O2 particles in colloidal silica were bonded each other with the increase of curing temperature.With the addition of the gelling agent,more Si O2 particles in colloidal silica were connected with each other.Moreover,the higher the curing temperature(60°C),the more obviously the morphological change.FTIR spectral analysis results show that the band at 1115 cm-1 was assigned to the Si-O-Si asymmetrical stretching vibration,while the band at 799 cm-1 on behalf of the symmetrical stretching vibration,the band at 474 cm-1 belonged to the bending vibration of Si-O-Si and the band at 976 cm-1 is the Si-OH vibration.New peaks of Si-O-Al(876 cm-1)and Si-O-Ca(858 cm-1)bonds were observed in the FTIR spectra with the addition of CAC.While the new weak vibration peak of Si-O-Mg(864 cm-1)binding could be found in the FTIR spectra with the addition of Mg(OH)2 or Mg O.XPS spectra analysis showed that Ca 2p(~346 e V),Ca 2s(~439 e V)and Al 2p(~74 e V)could be observed in the XPS spectra with the addition of CAC(added 0.5% and cured 24 h at 60°C).It was confirmed that the Al3+ and Ca2+ ions dissolved by the hydration of CAC involved in the condensation reaction of colloidal silica.Mg 1s(~1302 e V),Mg 2s(~87 e V)and Mg 2p(~48 e V)were observed in the XPS spectra of colloidal silica with the addition of Mg(OH)2(or Mg O)(added 0.5% and cured 24 h at 60°C).The higher the curing temperature,the more obvious the characteristic peaks of Mg 1s,Mg 2s and Mg 2p,which confirmed that the Mg2+ ions released from the hydrolysis of Mg(OH)2(or hydration of Mg O)were involved in the condensation reaction,and the higher the curing temperature,the better the coagulation.The results of the corresponding O 1s spectra were again confirmed the new chemical bonds of Si-O-Al and Si-O-Ca were formed with the addition of CAC.And the new chemical bond of Si-O-Mg was formed with the addition of Mg(OH)2(or Mg O).The demoulding strength of colloidal silica bonded castables increased with the addition of gelling agent(CAC,Mg(OH)2 or Mg O).The increased demoulding strength coincides with the formation of new chemical bonds of Si-O-M(M: Al,Ca,Mg)in the colloidal silica.These new chemical bonds of Si-O-M(M: Al,Ca,Mg)will connect colloidal silica particles together,and form a three-dimensional network structure,which will bond castable particles into a whole,increasing the demoulding strength of castables.It can be deduced that the increase of demoulding strength of colloidal silica bonded castables is closely to the recombination of chemical bonds of colloidal silica.