Unravelling chemical-microstructural pathways of deformation in MgO-fly ash cementitious systems

MgO expansive agents (MEA) and fly ash (FA) are widely combined to mitigate shrinkage in concrete, yet their interaction mechanisms remain unclear. This study clarifies how FA regulates MEA-induced expansion through microstructural evolution and pore solution chemistry, with swelling and crystallization pressures identified as the driving forces. At early stages, FA lowered pH, elevated Mg 2+ concentration, accelerating periclase hydration. Higher mesoporosity enlarged dissolution interface, promoted formation of finer brucite with stronger water adsorption capacity, increasing swelling pressure. At later stages, pozzolanic reaction of FA reduced portlandite formation, diminishing spatial confinement near MEA and alleviating crystallization pressure. Suppressed portlandite barriers and enhanced Mg 2+ mobility promoted brucite precipitation into surrounding voids, refining pore structure and improving dimensional stability. This work extends understanding of MEA-induced deformation to a coupled chemical–microstructural level and shows that FA regulates expansion driving forces through ionic and microstructural interactions, establishing a framework for achieving full-stage shrinkage compensation.