Effect Of Micro-sized Precursors On In-situ Reaction,Matrix Structure And Properties Of Castable

To investigate the influence of carbonate/hydroxide precursors on the properties,especially the volume stability and thermal shock resistance of refractory castables bonded with calcium aluminate cements（CACs）,micro-sized basic magnesium carbonate（uf-BMC）/aluminum hydroxide（uf-Al（OH）₃）was added into CAC-bonded high alumina-based castables.This work examined the influence of the highly reactive sub-micro-sized（nano-sized）MgO/Al₂O₃ from the decomposition of the precursors on the in situ formation of micro-sized or nano-sized magnesium aluminate（MA）spinel,calcium aluminates（CA₂/CA₆）and the properties of the castables,especially the volume stability and thermal shock resistance of the castables.Also investigated were the affect of the increased amount（uf-BMC）/aluminum hydroxide（uf-Al（OH）₃）on in situ formation and morphology evolution of MA/CA₂/CA₆,pore size and properties of lightweight castables.（1）The precursor uf-Al（OH）₃ addition affects the in situ formation of CA₂/CA₆ and microstructure of castable matrices.The results indicate that uf-Al（OH）₃ favors the in situ formation of CA₂/CA₆.In situ CA₂ is found in the sample after firing at 1100 ℃.The favored CA₂ formation enhanced sintering effect in the castable.At temperature over 1400 ℃,CA₆ is formed with small crystal size,and the interconnection of formation CA₆ is hindered.As the reactive Al₂O₃ in micro-size formed from uf-Al（OH）₃ decomposition uniformly distributed in the matrices,the density of matrices is lowered and the interconnection of CA₆ crystals is reduced,thereby improving the volume stability.Simultaneously,the newly-formed reactive Al₂O₃ can easily react with calcium aluminate phases from the added cement,which hinders the gro.wth of in-situ CA₂/CA₆ and weakens the density of the matrices composed of CA₆ framework leads to higher thermal shock resistance.（2）It is found that the addition of uf-BMC suppresses the in situ formation of CA₂/CA₆.The initial formation temperature of MA is favored by the added uf-BMC,as reactive MgO in sub-micron size was formed from uf-BMC decomposition.The addition of uf-BMC promotes the in situ formation of MA spinel,lowers the formation temperature of MA and reduces liquid sintering temperature,forming fine in situ MA crystals and weakens the interconnection of CA₆.Sub-micro-sized MgO from decomposition of uf-BMC further lowers the in situ MA formation in the matrices,forming MA at the interface of CA₂/CA₆,which behaves a physical barrier to the growth of CA₂/CA₆.As a result,the crystal sizes of CA₂/CA₆,with the precursor are finer than those without the precursor and decrease the interconnection of CA₆.Consequently,the thermal shock resistance of castables is improved.（3）It is found that the added micro-sized precursors improved both volume stability and thermal shock resistance of alumina-based castables.With the increase of the uf-BMC/uf-Al（OH）₃ contents,the permanent linear change,thermal expansion coefficient and thermal expansion ratio decrease with temperatures,indicating the volume stability improvement of castables.At same time,the residual cold modulus of rupture of the castables after thermal shocking tests increases,showing the improved thermal shock resistance of the castables.This is because the decomposition of precursors provides space to encounter balance the volume expansions associated with in situ MA/CA₂/CA₆ formation and the decreased crystal size of MA/CA₂/CA₆ and lowered CA₆ interconnection hinder the crack propagation.Compared with castables containing uf-Al（OH）₃,the thermal shock resistance of the castables with uf-BMC is better because the formed MA hinders the crystal growth of CA₂/CA₆.（4）The influence of uf-MgCO₃ and uf-Al（OH）₃ addition on the properties of lightweight alumina-based castables is investigated with emphasis on in situ MA/CA₂/CA₆ formation and morphology of castable matrices.The in situ formed MA with addition of uf-MgCO₃ hinders the in situ formation CA₂/CA₆,lowers the liquid sintering of CA₂.Besides,the formed micro-sized MA among CA₆ grains changed the growth behavior of plate-like CA₆,forming MA+CA₆ crystal particles distributed in the matrix pores.In comparison,the added uf-Al（OH）₃ favors the sintering effect associated with CA₂ formation,decreases the grain size of CA₂,and changes the morphology of plate-like CA₆ formed at 1500 ℃.The CA₆ crystal fibers are distributed in the matrix pores,which changes the plate-like structure of CA₆.It can be concluded that the increased addition of uf-MgCO₃ and uf-Al（OH）₃ decreases the density of castable matrices,favors the formation of micron pores inside the matrices,lowers the thermal conductivity and improves the insulation properties of lightweight castables.Furthermore,the change in MA/CA₂/CA₆ reaction process leads to the change in crystal morphology of plate-like CA₆.（5）The influence of added uf-MgCO₃ and uf-Al（OH）₃ on the changes in porosity in lightweight alumina-based castables was explored.Castables containing with the same amount uf-MgCO₃ have higher porosity level than that with uf-Al（OH）₃.As magnesium aluminate spinel is mainly generated by the diffusion of Mg into alumina grains whereas CA₂ and CA₆ form by the inward diffusion of Ca species into Al₂O₃ formed from Al（OH）₃ decomposition,pores are generated differently with addition of uf-Al（OH）₃ and uf-MgCO₃.As a result,uf-MgCO₃ favors the porosity generation more than uf-Al（OH）₃ in calcium aluminate cement-bonded castables,which is conducive to decreasing the density of the matrix and improving the thermal insulation performance.This work has the following main contributions.Addition of micro-sized precursors into matrix of the corundum-based castables produced reactive MgO/Al₂O₃ uniformly in the matrix,which changed the in situ MA/CA₂/CA₆ formation process and reduces dense structure of the matrices.The change in the amount of the precursors affects the density of interlocked CA₆ in the matrix,which favors the volume stability and thermal shock resistance of the castables.The increased addition of the precursors in the dense matrix of the lightweight castable considerably changes the matrix microstructure and modifies the morphology of CA₆ crystals,forming the matrix composed of high melting temperature phases of MA and CA₆ with micro pores.Thus,the thermal conductivity of the castables is decreased and this result possibly shows new paths for the high-temperature properties of lightweight castables.