<p>Traditional cementitious materials, primarily based on Portland cement, destabilize in hot, corrosive geo-fluids, impeding the advancement of geothermal systems as a scalable energy solution. Yet, identifying geo-stable alternatives that meet multifaceted performance demands throughout the inherently non-equilibrium structural evolution over geothermal well lifetimes remains a fundamental challenge. Here, we present a cure-to-service cement design principle that orchestrates the co-evolution of mechanical properties, phase development and permeability from initial cure through prolonged service, confining metastability by decoupling early-stage Al/Si release to enforce a controlled reaction toward stable aluminosilicate forms. In this design, coarse-grained silica embeds within the nascent boehmite matrix to mitigate permeability, wherein the alkali activator acts as a kinetic lever steering phase selection along the non-equilibrium crystallization pathway. In-situ synchrotron XRD and geochemical modeling show preferential Al dissolution and rapid boehmite formation, while delayed Si availability suppresses premature intermediate formation. Under exposure to supercritical H₂O, analcime serves as a Na⁺ reservoir whose gradual dissolution feeds Na-rich phyllosilicates (e.g., paragonite). The nascent composite gains strength and toughness while maintaining low permeability as metastable species convert to stable phases through staged crystallization. This work defines a design principle of aluminosilicate-based cementitious systems for extreme subsurface applications.</p>