Cooling-rate-dependent structural evolution of Portland cement clinker: Phase fixation, performance implications, and cooling optimization

Cooling is the final thermal stage of Portland cement clinker production, but it has often been treated mainly as a process step for heat removal, heat recovery, and production continuity rather than as a kinetic stage governing clinker structure. This review conceptualizes clinker cooling as a phase-fixation process and shows that cooling effects cannot be fully described by a single averaged cooling rate, because different clinker phases and structural features evolve within distinct phase-critical temperature windows. Based on this temperature-window perspective, the concept of an effective cooling regime is introduced, in which clinker structure is governed by the residence time of particles within these windows. Focusing primarily on ordinary Portland cement clinker, this review critically synthesizes cooling-controlled effects on phase-specific structural evolution, including silicate polymorphism, foreign-ion-assisted polymorph stabilization, interstitial-phase crystallization, glass retention, crystal growth, and free-oxide distribution. Properly controlled rapid cooling generally favors metastable and defect-rich silicate structures, β-C₂S retention, higher glass retention, and finer dispersion of potentially deleterious free oxides, whereas slow cooling promotes structural ordering, crystal coarsening, more complete crystallization, and transformation toward more stable but often less reactive states. By synthesizing representative phase-critical temperature windows for major clinker phases/features, this review provides a temperature-window-based framework for interpreting cooling-controlled phase fixation, distinguishing phase-specific cooling-sensitive intervals rather than treating cooling as a single high-temperature stage. These windows are linked with structure-mediated performance responses, including hydration, strength development, soundness, durability-related behavior, and grindability. From an industrial perspective, model-assisted reconstruction of local cooling profiles is highlighted as a route for translating cooler operating variables into particle temperature–time histories and phase-critical thermal exposure. This structure–property perspective shifts cooling optimization from maximizing cooling intensity to controlling phase-specific thermal exposure within a balanced process window, and provides a basis for extending cooling control to low-carbon and alternative clinker systems. Insufficient consideration of cooling-controlled structural evolution may lead to inconsistent clinker reactivity, unstable strength development, increased grinding-energy demand, and higher soundness risk. For example, periclase coarsening or clustering may increase delayed-expansion risk because hydration of MgO to Mg(OH)₂ is associated with a theoretical molar solid-volume increase of approximately 118%, thereby affecting cement quality consistency in construction applications.