Study On Gelation Behavior Of Al₂O₃-MgO Composite System

In order to avoid the adverse effect of calcium oxide in cement on the high temperature performance of castables,low cement,low cement and cementless refractory castables have been widely used.Hydratable alumina can be hydrated at normal temperature.The hydration products boehmite and bayerite form a network structure to provide strength for the castable,and the hydration products can form stable?-Al₂O₃ after high temperature treatment.Therefore,it can be hydrated.Hydratable alumina can be used as an alternative binder for calcium aluminate cement.When hydratable alumina is used in castables,the hydration of hydrated alumina is a simple hydration behavior.The presence of reactive magnesia will affect the hydration behavior of hydratable alumina.Reactive magnesia will accelerate the hydration of hydratable alumina and form hydration products such as hydrotalcite,brucite,boehmite and bayerite.Brucite,boehmite,and bayerite can improve the green strength of the castable.Hydrotalcite-like can form a microporous network during the drying of the castable to prevent the castable from bursting and falling off during drying.And when it is used in castables,it can form a self-binding system,which significantly improves the high temperature performance of the castables.However,reactive magnesia will accelerate the hydration of the hydratable alumina,resulting in poor fluidity of the castable,the rapid hardening,large water demand,and easy blowout and so on.Aiming at these problems mentioned above,taking hydratable alumina as research object,the effect of additives on the hydration behavior of hydratable alumina was studied,and the hydration kinetics of hydratable alumina introduced with additives was investigated.Taking the composite system of magnesium oxide and hydratable alumina as the research object,the hydration behavior of the composite system of magnesium oxide and hydratable alumina was studied.And the effects of additives,types of magnesium oxide and magnesium oxide content on the hydration behavior of the composite system were also discussed.The effect of dispersant on the rheological properties of the composite system slurry and the normal temperature performance of the corundum castables combined with magnesium oxide and hydratable alumina composite system were studied.The conclusions are as following:?1?The hydration rate of hydratable alumina is fast.The hydration products are mainly a large number of layered bayerite and a small amount of boehmite.The hydration quality increase rate of hydratable alumina slurry at 25?for 24 h reaches 10.05%.The accumulated heat release per unit of hydration for 72 h reached 84.82 J·g^-1.The introduction of citric acid,aluminum lactate,and aluminum sulfate can reduce the hydration rate of hydratable alumina,delay the hydration process of hydratable alumina,and reduce the hydration quality increase rate of hydratable alumina to 6.64%,4.75%,and 7.25%,and reduce the cumulative heat release per unit hydration of hydratable alumina slurry for 72 h to 57 J·g^-1,57 J·g^-1,and 72 J·g^-1.The hydration products are mainly boehmite.?2?Long molecular chain citrate complexation of citric acid,lactic acid complexation of aluminum lactate and flocculation adsorption of aluminum ions,and flocculation and adsorption of aluminum sulfate.They reduced the contact area of magnesium oxide and hydratable alumina composite particles with water,When MgO:Al₂O₃=50:50?wt.%?,the relative increase of the hydration quality of the active magnesium oxide and hydratable alumina composite gel system was increased to 1.75%,1.88%,and 3.12%.In the sample with aluminum lactate and MgO:Al₂O₃=50:50?wt.%?,the hydration quality increase rate of the composite gelling system of reactive magnesia and hydratable alumina is 1.88%,the hydration quality increase rate of the composite cement system of light-burned magnesia and hydratable alumina is 4.51%,hydration mass increase rate of fused magnesia and hydratable alumina composite gelling system is 0.88%.Light-burned magnesia has a larger specific surface area and higher activity.Therefore,the rate of increase in hydration quality of light-burned magnesia and hydratable alumina composite gel system is relatively large.?3?Sodium hexametaphosphate and sodium tripolyphosphate can significantly reduce the thixotropy of magnesium oxide and hydratable alumina composite gelling system slurry,and improve the fluidity of the slurry.Among them,the initial viscosity of the lightly burned magnesia and hydratable alumina composite gel system with the introduction of sodium hexametaphosphate decreased more significantly.From about 25000 Pa?s to 38 Pa?s?with citric acid?,27 Pa?s?with aluminum lactate?and 425 Pa?s?with aluminum sulfate?,The slurry introduced with citric acid and aluminum lactate exhibited characteristics similar to Newtonian fluid,and the fluidity of the slurry was significantly improved.?4?The introduction of different admixtures and different types of magnesia and hydratable alumina composite gelling system in combination with corundum castables at400?,500?,and 600?test temperatures showed good explosion-proof performance.When the test temperature reaches 700?,the introduction of aluminum lactate and citric acid of different kinds of magnesia and hydratable alumina composite gelling system combined with corundum castables showed good explosion-proof performance.However,the samples introduced with aluminum sulfate all burst.Therefore,compared to the type of magnesium oxide,the effect of the additive on the explosion-proof performance of the sample is more obvious.