Synergistic effects of metakaolin and limestone on early hydration kinetics, strength development, and microstructure of low water-to-binder ratio cement composites

Metakaolin (MK) and limestone (LS) have attracted attention as a cement replacement for enhancing mechanical and durability properties. The hydration and strength development of MK-LS-cement composites with low water/biner ratio remained unexplored. This study investigated early hydration and strength of binders with MK, limestone (LS), and their combination at a low water/binder ratio of 0.16. Samples containing 13.4 wt% MK and 6.6 wt% LS (C+LSMK) showed comparable hydration heat and 3-day strength to the reference, while 20 wt% LS (C+LS) yielded the lowest hydration heat and early mechanical strengths. XRD indicated greater hemicarboaluminate and monocarboaluminate formation in C + LSMK, supported by TG/DTG evidence of portlandite consumption. BJH analysis revealed higher capillary pore volume in C + LSMK. Hydration kinetic modelling confirmed that MK enhanced hydration through filler effects and pozzolanic reactions. These findings provided new insights into the synergistic effects of MK and LS in low w/b cement systems, advancing the design of high-performance, low-carbon cementitious materials.