Problems and Current Research Status of Al2O3-MgO-CaO Composite Castables
High-density Al₂O₃-MgO-CaO castables face limitations in thermal shock resistance when applied to ladle walls. During cyclic operations including steel receiving, refining, casting and maintenance, the materials endure periodic thermal stress shocks caused by temperature fluctuations, such as abrupt temperature surge when the preheated ladle contacts molten steel. Parallel cracks tend to form on the working surface or at the boundary of infiltrated layers, eventually leading to spalling damage. To promote practical application, domestic and foreign scholars have conducted extensive in-depth studies on corundum-spinel castables of this system. Researches cover aggregate selection, dosage and incorporation method of spinel, as well as the influence of pure calcium aluminate cement addition on material properties, yielding substantial research findings.
Selection of Aggregates
Corundum serves as the main raw material of Al₂O₃-MgO-CaO based castables. Variations in preparation techniques and raw material types lead to differences in chemical composition and crystal structure among corundum materials, bringing disparities in service performance, thus rational selection is required in practical application.
Researches indicate that corundum-spinel castables with tabular corundum aggregate deliver superior conventional physical properties and high-temperature service performance compared with other corundum aggregates.
A performance comparison was conducted on castables adopting tabular corundum, white corundum and brown corundum respectively. Brown corundum based castables boast optimal flowability, yet abundant impurities migrate and accumulate from grain boundaries to particle edges with rising temperature, triggering particle fragmentation and decline in high-temperature strength. Samples using tabular corundum exhibit outstanding thermal shock resistance.
Given that existing corundum-spinel castables already possess satisfactory strength, tabular corundum is adopted as aggregate to enhance thermal shock stability.
Effect of Spinel Incorporation
Corundum-spinel castables are fabricated by introducing spinel as the second phase into high-purity corundum castables. Multiphase toughening effectively improves the thermal shock resistance of the castables.
Alumina-rich spinel is widely adopted in Al₂O₃-MgO-CaO based castables represented by corundum-spinel products due to its superior merits:
1. During high-temperature heating, alumina and spinel with disparate thermal expansion coefficients produce microcracks from expansion mismatch. These cracks relieve thermal stress and elevate fracture surface energy, thereby enhancing spalling resistance.
2. Precipitated alumina from alumina-rich spinel reacts with surrounding pure calcium aluminate cement to form CA6. The generated CA6 closely bonds spinel and corundum phases, boosting high-temperature strength.
3. Alumina-rich spinel forms limited solid solutions with FeO, MnO and other components in molten steel slag. Excessive alumina also reacts with slag to raise slag viscosity, restrain slag penetration and improve spalling resistance.
Researches were carried out on the influence of spinel particle size and dosage. Fine spinel powder contributes to higher high-temperature strength and slag penetration resistance yet weakens spalling resistance. Granular spinel impairs high-temperature strength but improves spalling and slag corrosion resistance.
The optimal spinel addition range is 10% to 25%. Excessive spinel reduces relative alumina content, lowers solid solubility, loosens material structure and deteriorates high-temperature strength.
Functions of Pure Calcium Aluminate Cement
Pure calcium aluminate cement serves as a conventional binder for Al₂O₃-MgO-CaO based castables. It hydrates at low temperature to form cementitious bonding and provide mechanical strength for castables. At high temperature, calcium hexaluminate is generated to improve high-temperature strength and spalling resistance.
Researches prove that various calcium aluminate phases produced by the reaction between CaO and Al₂O₃ in the cement significantly affect the high-temperature properties of corundum-spinel castables. Increasing cement addition can effectively enhance residual flexural strength and strength retention ratio after thermal shock, and optimize thermal shock stability.
It is found by adjusting the mass ratio of CaO to Al₂O₃ in the matrix that the high-temperature physical properties and thermal shock resistance reach the peak value when the ratio stands at 0.09. Corundum in the matrix fully reacts with cement and transforms into massive hexagonal flaky calcium hexaluminate, building an interlocking skeleton structure.