The development of high-temperature industrial technologies, such as steelmaking and cement production, relies critically on refractory materials. Conventional systems like magnesia, alumina-silica, and magnesia-alumina spinel, despite their maturity, exhibit inherent limitations, i.e., poor thermal shock resistance, insufficient stability under reducing atmospheres, or high costs. Furthermore, environmental concerns regarding toxic Cr⁶⁺ from magnesia-chromium bricks have an urgent demand for eco-friendly, high-performance alternatives.

In this context, the CaO-MgO-Al₂O₃(CMA) ternary system, specifically the compounds CaMg₂Al₁₆O₂₇ (CM₂A₈) and Ca₂Mg₂Al₂₈O₄₆ (C₂M₂A₁₄), has attracted considerable attention. These materials possess a unique hybrid crystal structure composed of alternating magnetoplumbite (CaAl₁₂O₁₉, CA₆) and spinel (MgAl₂O₄, MA) layers stacked along the c-axis. This structure ingeniously combines the high melting point and chemical stability of CA₆ with the favorable thermomechanical properties of MA, having CMA as a promising candidate for next-generation refractories.

However, the same layered structure that confers these advantages also causes an intense anisotropic grain growth during conventional solid-phase sintering. Grains develop plate-like or rhombohedral morphologies and prematurely interlock, forming a rigid porous skeleton that entraps pores and severely impedes densification. Consequently, CMA materials typically exhibit a high apparent porosity (i.e., >20%) when processed by conventional methods, creating a fundamental contradiction between performance potential and processability.

To overcome this densification challenge, researchers have explored three main technological pathways, i.e., 1) Hot Pressing Sintering: Application of uniaxial pressure during sintering effectively inhibits an anisotropic grain growth, yielding near-fully dense (>97% purity) CMA ceramics with apparent porosity of <0.2%. The properties reported include bulk density (i.e., 3.66-3.71 g/cm³), flexural strength (i.e., 248-262 MPa), and fracture toughness (i.e., 2.17-2.23 MPa·m^1/2). However, high costs and low productivity limit this method to fundamental research; 2) Additive Engineering: Introduction of trace additives modifies grain boundary energy or diffusion kinetics, promoting a morphological transformation from plate-like to equiaxed grains. For instance, 3% MnO addition reduces apparent porosity from 32.5% to 8.4% at 1700 ℃, while 1% Fe₂O₃ achieves 12.5% porosity at 1780 ℃. However, foreign ions may compromise a long-term high-temperature performance, requiring careful optimization; and 3) Electric Melting Composite Method: Melting raw materials at > 2000 ℃ followed by rapid cooling produces CMA-based multiphase materials with a bulk density up to 3.50 g·cm^-3. Despite relatively dense structures, this approach suffers from high energy consumption and compositional homogeneity challenges.

Based on the cost and complexity of achieving full densification, a paradigm shift is proposed, rather than solely pursuing densification. The inherent characteristics of CMA should be used for specific applications.

For high-end dense products (e.g., metallurgical crucibles), optimized additive engineering combined with advanced forming techniques remain the most viable approach for achieving low porosity and high strength.

For monolithic refractories (i.e., castables, gunning mixes), the plate-like morphology that hinders densification becomes advantageous. Some studies demonstrate that introducing CMA aggregates enables plate-like grains to form interlocking structures within the matrix, significantly enhancing toughness and thermal shock resistance through crack deflection and bridging mechanisms. Crucially, plate-like CMA crystals can grow into adjacent corundum aggregates, creating mechanical interlocking that substantially strengthens aggregate-matrix bonding.

For lightweight and functional applications, the decomposition behavior of CMA can be exploited. Introducing C₂M₂A₁₄ into magnesia or spinel-based systems generates uniform micropores via the Kirkendall effect during firing, achieving lightweighting, while improving thermal shock resistance through pore-induced crack blunting and thermal stress buffering.

Regarding corrosion resistance, CMA exhibits a highly selective performance governed by specific interface reaction mechanisms. Against high-basicity LF refining slag, CM₂A₈ forms a continuous dense CaAl₂O₄ protective layer through Ca²⁺exchange, while dissolved Al³⁺ and Mg²⁺ increase a slag viscosity for creating a dual barrier. Against low-basicity RH slag, such protective layers fail to form, leading to an accelerated penetration. This underscores that CMA applications must precisely match service environments.