Rheological Performance Of Calcium Aluminate Cement Containing MgAl₂O₄ Spinel And Its Effect On Properties Of Corundum Castables

The MgAl₂O₄ spinel contained calcium aluminate cement?CMA?was synthesized by raw materials of industrial alumina,magnesium hydroxide;light fired magnesite,and ground calcium carbonate.The phase composition,microstructures,hydration and rheological behavior were studied by techniques of XRD,SEM,rheometer and BET surface area measurement.At the same time,the influence of chemical composition of CMA cement on thermal shock resistance of corundum-based castables was also studied.?1?The main phases of cement synthesized at 1400°C were MgAl₂O₄?MA?and CaAl₂O₄?CA?,and a small amount of CaAl₄O₇?CA₂?were also found.The CA phases show a tabular morphology,while the MA phases'shape is in the form of granular,and the particle size of MA is about 0.5?m.The MA particles are in clusters and distributed around the CA particles.As the synthesized temperature is raised to 1500°C,the diffraction peaks of MA phase on XRD pattern are slightly enhanced,and the SEM investigation indicates the MA phases are in the form of octahedral,and the particle form size is about 1.5?m.It is found that the particles of MA are homogeneously distributed among CA particles.?2?With the increase of synthesized temperature,the CA particles begin to grow up which lead to the reduction of hydration activity.Therefore,the setting time of CMA cement was lengthened.On the other hand,the lower content of the MA resulted in a shorter setting time.?3?Different synthesized temperatures and chemical composition of CMA cements are in accordance with Bingham fluid model.There is a yield stress in CMA slurry.When the yield value is reached,the slurry will change to be a Newtonian fluid.Both the viscosity and yield stress of CMA slurry are higher when CA particle size is smaller or the content of CA is higher.The strain-sweep curves indicate that the rheological behavior of four kinds of CMA slurries is similar.The storage modulus?G'?is rapidly increased with the increase of time and tends to be stable finally.At the same time,higher CA content or tinier particle size of CA will cause a strong flocculation structure,and both yield point and flow point will be increased.?4?It was found that the setting time of CMA cement is significantly influenced with the addition of dispersants?ADW1 and ADS3?.Specifically,a better dispersion of slurry leads to a higher hydration rate.The results show that the more ADW1 addition,the higher yield stress and plastic viscosity will be.In particular,only adding ADW1can accelerate setting,the yield stress and plastic viscosity of which are 150 Pa and 3Pa·s.?5?The cold modulus of rupture?CMOR?of CMA cement bonded corundum-based castable sample?CMA75?is largest after heating treatment at 1500°C.It is found that the mechanical properties at room temperature results in a slightly growth with the increase of MA cement in CMA.Thermal shock test results show that higher MA content in castables results in a better thermal shock resistance.The sample of CMA75 had the best thermal shock resistance;its residual ratio of modulus of rupture is 33% after thermal shock tests.