Autogenous healing of fiber reinforced magnesia-based composites (FRMC) with different matrix crack widths

Controlling the crack width in cementitious composites has been proven as a feasible approach to improve the autogenous healing efficiency. This study investigated effects of cracks width on the healing efficiency of tensile performances in fiber-reinforced magnesia-based composites (FRMC) at both micro- and composite-scales. A preloading-healing-reloading protocol was applied to single-fiber pull-out specimens and notched FRMC specimens with controlled crack widths ranging from 0.1 mm to 0.5 mm. The healing process was induced through wetting-air (W-A) cycles, and the effectiveness was evaluated via mechanical testing, SEM-EDS observations, and micromechanics-based modeling. On the micro-scale, the fiber-matrix interface bond not only recovered but was significantly enhanced due to the transformation of brucite into hydrated magnesium carbonates (HMCs), facilitated by CO₂ and water infiltration through interface microcracks. On the composite scale, all pre-cracked FRMC specimens demonstrated improved post-healing fiber-bridging strength (22 %–62 %), with the highest recovery observed at a crack width of 0.2 mm. Microstructural characterization and micromechanics-based modeling results revealed the blocking effect of healing products that quickly formed within the fine crack mouth, resulting in non-uniform healing of fiber-matrix interface bond along the crack depth. These findings highlight that an optimal crack width exists for maximizing tensile recovery, challenging the conventional notion that finer cracks always result in superior healing.