Reactive magnesium oxide (r-MgO) is often treated as a single material class, yet its performance is governed by a chain of coupled decisions extending from magnesium source and precursor chemistry to heat and mass transfer, calcination severity, atmosphere, particle size, purification, and post-calcination aging. This structured critical narrative review reframes reactive MgO production as a process–structure–reactivity engineering problem rather than a catalog of synthesis methods. A fixed core evidence corpus of 103 publications from 2020 through August 2026 was coded by production route, evidence scale, reactivity endpoint, and environmental/economic dimension; three pre-2020 foundational sources were used only for terminology and mechanistic context and were excluded from corpus counts. The review compares mineral-derived, brucite-derived, dolomitic, seawater/brine, waste-derived, sulfate-derived, and specialty precursors; conventional, flash, fluidized-bed, steam-assisted, solar/electrified, precipitation, carbonation, electrochemical, and hybrid routes; and the analytical methods used to describe reactivity. No single metric—calcination temperature, BET surface area, hydration rate, or CO₂ uptake—defines reactive MgO across applications. Instead, reactivity emerges from the preservation or destruction of accessible mesoporosity, surface defects, dissolution sites, crystallite-scale disorder, and pore connectivity, provided that sufficient precursor conversion and chemical purity are achieved. Fast thermal routes can reduce sintering exposure, whereas brine and residue routes can decouple purity from the original mineralogy but introduce reagent demand, washing, mother-liquor management, and scale-up penalties. Environmental advantage is likewise route-specific: avoiding magnesite decarbonation can reduce process CO₂, but upstream alkalis, electricity, solids handling, and unrealized carbonation can reverse apparent benefits. The industrially optimum product is therefore not the MgO with the highest nominal reactivity, but the product whose application-specific reactivity window is achieved at acceptable purity, energy demand, carbon footprint, cost, throughput, and consistency. The review concludes with a research agenda centered on standardized reporting, direct route-to-route experiments, impurity-tolerance maps, continuous pilot validation, and integrated mass–energy–carbon–cost assessment.
