Although high-performance basic refractory materials play an essential role in state-of-the-art steel-making, magnesia castables have not been widely used until now. A major challenge is that brucite formation (Mg(OH)₂) during MgO hydration causes volume expansion and subsequent cracking—a phenomenon commonly called “slaking”. To overcome problems related to slaking, a new specialty product, SioxX-Mag, has recently been developed for cement-free MgO castables based on MgO-SiO₂-H₂O bond. Specimens from industrial-scale-sized blocks after dry-out were characterised by SEM and XRD techniques. Using SioxX-Mag in combination with microsilica, cracking caused by brucite formation is suppressed. The results demonstrate that the dosage of microsilica plays essential roles in the crack formation in large samples. Furthermore, the compatibility of SioxX-Mag with different types of basic aggregates, medium to high-grade dead-burned magnesia and olivine, has also been evaluated. High-performance, self-flowing cement-free MgO castables have been developed.