Formation Of Magnesium-Silicate-Hydrate And Its Influence On The Properties Of Magnesia Castables

With the development of cleanliness steel metallurgy,tundish,which is the connection between the refining of steel and the making of solid casting billets,plays increasingly important role in steel quality ensurance.The devices such as wall,weir and dam installed in the tundish can control the flow of steel liquid,promote the inclusions floating up and being captured,and eventually improve the steel quality.Magnesia refractory castables,prior to other kinds of refractories,have been widely applied in such devices due to their high refractoriness,good basic slag resistance and positive effect on the quality of steel.Recently,microsilica is the main binder of magnesia castables and its hydraulic bonding phase is consisted of brucite and magnesium-silicate-hydrate?M-S-H?.However,the formation of brucite easily leads to instability of structure of materials and the damage of performance.Compared with brucite,M-S-H documents better bonding properties and slow dehydration behavior,which endows the higher mechanical strength and explosion resistance of castables.However,the formation rate and content of M-S-H in common magnesia castables are rare limited.Although,increasing the amount of microsilica is a way to promote the M-S-H,it also increases the content of liquid phase which has detrimental effect on the thermal mechanical properties and slag resistance.In the field of concrete,sodium hexametaphosphate has been commonly used to promote the formation of M-S-H;however,it will augment the phosphorous content of steel liquid if such phosphorous containing castables were applied in tundish.In the present work,the main research work includes:the factors that affect the formation of brucite and magnesium silicate hydrate?M-S-H?in the MgO-SiO₂ slurries;followed by the influence of sodium silicate additive and brucite on the microstructure and mechanical properties of magnesia castables;meanwhile,pure M-S-H was chemically synthesized and the relationship between its dehydration behavior and temperature;then the effect of pre-formed M-S-H on mechanical properties of magnesia castables;last but not the least,influence of composition of magnesia castables on the slag resistance performance with different slag.Based on the studies,the conclusions are listed as follows:?1?Enhanced formation of M-S-H in MgO-SiO₂-H₂O system was obtained by change of magnesium source?silicate source?curing temperature and pH value.Caustic magnesia,nano silica and increased temperature increased the dissolution of raw materials;the increase of ions concentration accelerated the M-S-H formation.Besides,addition of sodium silicate increased the pH value of system,in that connection not only the silicate ions but also the magnesium ion concentration were multiplied and the M-S-H formation was promoted.The morphology of M-S-H was related to the formation rate of M-S-H.The rapid generation of M-S-H was displayed with flower-like cluster shape;otherwise,it showed the amorphous stacked shape.?2?Application of the brucite powder improved the workability and strength of castables.The in-situ formation of M-S-H phase in magnesia castables was enhanced by adding caustic magnesia or sodium silicate,which improved the mechanical properties and explosion resistance.However,with increase of the amount of caustic magnesia or sodium silicate,the flowability of castables decreased.Use of brucite powder with small size and high reactive in castables increased the early strength and explosion resistance.It was attributed to the filling effect and the formation of M-S-H by reactions between the brucite and microsilica in the matrix.Furthermore,brucite enhanced the forsterite formation and increased the cold modulus of rupture and hot modulus of rupture about10-20%after firing.The sound comprehensive mechanical properties of castables were obtained by 1wt%addition of brucite.?3?M-S-H was belonged to layered silicate and presented memory effect;mechanical properties and explosion resistance were improved by introduction the pre-heated M-S-H powder.The pure M-S-H gel phase,containing loosely bonded water and structure water,exhibited the memory effect with a calcination temperature of200^oC or 400^oC,as the rehydrated M-S-H could be reconstructed to a similar structure of initial M-S-H.However,the absence of memory effect took place when M-S-H was calcined at 800^oC.Subsequently,addition of the pure M-S-H which was pre-heated at low temperature powder as binder,in partial replacement of microsilica,improved the early strength of castables ascribed to the memory effect.Furthermore,the pre-heated M-S-H owned higher reactivity,which improved sintering and resulted in 13-26%increase in the hot modulus of rupture.?4?Promotion of the in-situ M-S-H formation in magnesia castables reduced the microsilica content and ensures the strength at the same time.Enhancement of the in-situ formation of M-S-H in castables via decrease the content of microsilica was significant for improvement of slag resistance.When hydratable alumina was added into the MgO-SiO₂-H₂O cementitious system bonded magnesia castables,on one hand the strength was increased as well as apparent porosity was decreased;on the other hand the liquid phase was also reduced and it was uniformly distributed.Therefore,the slag resistance of magnesia castables was modified.In addition,magnesia castables also showed better resistance to the low basicity slag?CaO/SiO₂=2.5?than high basicity slag?CaO/SiO₂=9.2?.