Synergistic effect of MgO expansive agent and fly ash on the volume stability of low-heat Portland cement-based materials

Abstract

This study investigates the synergistic effect of fly ash (FA) and MgO expansive agent (MEA) on the volume stability of low-heat Portland cement (LHC). Drying shrinkage, autogenous shrinkage, chemical shrinkage, unitary mass loss, internal relative humidity, isothermal calorimetry, XRD, TGA, MIP, and SEM-EDS were employed to comprehensively evaluate the effects of FA and MEA on shrinkage behavior, hydration, and microstructure. Results show that LHC exhibits superior volume stability compared to ordinary Portland cement (OPC). The combined use of FA and MEA reduces 7-day chemical shrinkage by 40.91 % relative to the LHC reference, while maintaining low drying shrinkage despite substantial water loss, and enhancing both micro-expansion and moisture retention under sealed conditions. LHC undergoes a mild and sustained hydration process, facilitating the formation of a denser microstructure. FA refines the pore structure and promotes the formation of C-(A)-S-H, while MEA generates Mg(OH)₂ crystals that fill pores and compensate for shrinkage via micro-expansion. Their synergy not only delays early hydration and continuously promotes hydration product formation, but also further refines the pore structure and facilitates the formation of a dense and interwoven microstructural network composed of C-S-H, Mg(OH)₂, and C-(A)-S-H in localized regions. This refined microstructure effectively mitigates shrinkage-induced stress, enhancing the long-term volume stability and crack resistance of LHC.