An Endogenous pH‐Clock‐Programmed and Thermally Executable Three‐State Interface in Biomass Fiber/Silicate Composites

ABSTRACT The interfacial transition zone (ITZ) between biomass fibers and alkali‐cured silicate matrices governs load transfer, moisture resistance, and fire‐barrier performance, yet it forms passively during curing and remains the weakest link under thermal exposure. Here, we report an endogenous pH‐clock‐programmed, thermally executable three‐state interface that turns the passive ITZ into a timed, functionally staged interphase. The hydration‐derived pH–time trajectory defines two sequential interfacial windows: in State‐1, near‐neutral RF@CDs (ζ = +10.6 mV) electrostatically localize silicate precursors at the fiber surface; in State‐2, alkaline charge reversal (ζ ≈ −25.7 mV) enables KH560‐mediated cross‐phase covalent locking, compresses the ITZ from 33.9 to 13.5 µm, and retains a partially reconstructed Cu‐MOF reservoir. In State‐3, heating converts this retained reservoir into copper‐oxide‐containing species and reorganizes the deoxygenated, aromatized char into a char/inorganic shielding layer. The resulting composite reaches 23.5 MPa compressive and 14.1 MPa flexural strength, with 8.7% water uptake and 0.19% mass loss after 168 h immersion; cone calorimetry records a 35.6% lower peak heat‐release rate and 99.4% lower total smoke production. This work establishes hydration‐derived State‐1 and State‐2 windows, followed by thermal execution in State‐3, as a sequence‐level design axis for biomass fiber/silicate composite interphases, complementing component‐level optimization with interfacial control.