Design of High-Performance Cement-Free Alumina-Spinel Castables via Lactate-Assisted Brucite Suppression

Conventional alumina-spinel castables are typically formulated utilising calcium aluminate cement, a strategy which not only substantially elevates carbon dioxide emissions but also promotes the formation of brucite (Mg(OH) 2 ). The presence of brucite is recognised as a principal contributor to microcrack development during the drying phase. Although the addition of microsilica has been shown to suppress brucite formation, such modifications are accompanied by a marked reduction in the refractoriness of the resultant material. The present study advances an innovative, entirely cement-free approach, employing aluminium lactate as a dual-functional additive. Aluminium lactate serves both to inhibit the crystallisation of brucite and to impart sufficient green strength to the castable matrix. X-ray diffraction analysis confirms the complete absence of crystalline brucite in systems modified with lactate, with diffraction patterns instead revealing the formation of a nanocrystalline or amorphous gel-like phase, indicating a substantive alteration of the hydration pathway. The cement-free formulation (IS-20MA) achieves a green (24-hour curing) cold crushing strength of 5 MPa and a cold modulus of rupture of 2 MPa in the absence of calcium aluminate cement, enabling fully cement-free processing with adequate early-age mechanical integrity. Following firing at 1600 °C, the cement-free composition displays exceptional thermo-mechanical properties (cold crushing strength: 184 MPa; refractoriness >1600 °C) and a uniform, silica-free microstructure. Resonant frequency damping analysis elucidates distinctive sintering behaviour, highlighting the role of transient liquid phases in calcium aluminate cement-containing systems. Dynamic corrosion testing indicates similar levels of initial chemical dissolution compared with conventional castables, while revealing a markedly improved resistance to crack propagation under thermal-shock conditions, attributable to the simplified and homogeneous microstructure of the cement-free system. A comparative cost and carbon-footprint assessment shows that the aluminium-lactate-based castable exhibits a modest increase in both metrics relative to CAC-bonded formulations, while significantly outperforming pre-formed spinel systems. Collectively, these findings establish aluminium lactate as an effective enabling agent for cement-free, spinel-forming castables, providing a viable pathway towards high-performance refractories with improved thermo-mechanical reliability and competitive sustainability relative to pre-formed spinel alternatives.