<p>The bonding performance between healing materials and natural rock plays a critical role in the mechanical stability of sandstone cultural relics. This study investigates the interfacial microstructure and bonding behavior of microbially induced magnesia carbonation (MIMC) applied to sandstone. High-resolution micro-CT imaging reveals the presence of a previously unreported porous transition zone (PTZ) at the MIMC-sandstone interface, characterized by elevated porosity (~ 3%) and enlarged pore sizes (~ 298&#xa0;μm) relative to the bulk MIMC material. Localized splitting tests indicate that this PTZ governs the initiation of interfacial failure. Based on the quantified pore structure, a microstructure-informed predictive model was developed that incorporates both porosity and pore size, demonstrating strong correlation (<i>R</i>² = 0.86) with the measured bonding strength. The proposed framework provides new mechanistic insights into biomineralized interface behavior and offers a robust tool for evaluating and optimizing MIMC-based rock-healing applications.</p>