We demonstrate an unexpected decay-recovery behavior in the time-dependent ${}^1\mathrm{H}$ NMR relaxation times of water confined within a hydrating porous material. Our observations are rationalized by considering the combined effects of decreasing material pore size and evolving interfacial chemistry, which facilitate a transition between surface-limited and diffusion-limited relaxation regimes. Such behavior necessitates the realization of temporally evolving surface relaxivity, highlighting potential caveats in the classical interpretation of NMR relaxation data obtained from complex porous systems.

• Received 14 October 2022
• Accepted 21 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.126204