Mechanistic understanding of hydration process and microstructure of Portland cement altered by in-situ polymerization

Monomer in-situ polymerization (MiSP) offers significant potential for enhancing the engineering properties of cement-based composites, yet the underlying mechanisms of concurrent cement hydration and polymerization remain poorly understood. Here, we employ calorimetry, low-field nuclear magnetic resonance (LF-NMR), cryogenic scanning electron microscopy (cryo-SEM) and thermalgravimetric analysis to investigate Portland cement hydration modified by sodium acrylate in-situ polymerization (SA-iSP). The results show that SA-iSP reduces the peak heat release by up to 87.8% and prolongs the induction time from 247 min to 2590 min at an 8% monomer dosage, while initial dissolution remains unaffected. SA-iSP selectively suppresses Portlandite, while decreases C–S–H and ettringite. LF-NMR identifies a transition period within induction where gel pores form despite retarded hydration; immobile water decreases from 45% to 15% at 180 h, and capillary-to-gel pore ratio shifts from 1:1 to 8:2. Cryo-SEM reveals a deformable polymer film coating the cement particles, creating confined spaces for nano-hydrate growth before film rupture triggers accelerated hydration. These findings provide mechanistic insights into coupled hydration-polymerization processes, establishing a foundation for designing advanced cement composites through controlled organic–inorganic interactions.