Synergistic effects of limestone powder and metakaolin on early hydration and pore structure evolution of reactive powder concrete: A low-field NMR study

Artikel i vetenskaplig tidskrift, 2026

The early hydration behavior and pore structure evolution of low water-to-binder ratio (w/b = 0.16) reactive powder concrete (RPC) incorporating metakaolin (MK) and limestone powder (LS) were primarily characterized using ¹H low-field nuclear magnetic resonance (LF-NMR), which enables non-destructive and highly sensitive detection of water distribution and pore structure evolution even under conditions of limited free water and rapid hydration typical of dense RPC systems, complemented by isothermal calorimetry and X-ray diffraction(XRD)to support the analysis of hydration and pore structure evolution. The incorporation of LS or MK individually resulted in reduced early-age strength, while the LS-MK composite (10%:10%) exhibited a 3-day compressive strength comparable to that of the control sample and achieved higher strength than either single blend. Calorimetry results showed that LS delayed hydration due to dilution, whereas the LS-MK system shortened the induction period and increased cumulative heat release within 46 h. LF-NMR revealed that, compared with the reference RPC, the LS-MK composite reduced total porosity by 5.7% at 24 h and 7.2% at 72 h, with a 19.2% decrease in capillary pores (>10 nm) and a corresponding increase in gel pores (≤10 nm), indicating a refined pore structure. XRD confirmed the formation of monocarboaluminate and hemicarboaluminate at 1 d, suggesting that CO3²⁻ participated in aluminate reactions with dissolved Al³⁺, stabilizing hydration products and suppressing excessive AFt formation. The synergistic mechanism between LS and MK involves the filler and nucleation effects of LS, the pozzolanic reactivity of MK, and the coupled formation of carboaluminate phases, which collectively enhance microstructural densification and early strength, providing a reference for the low-carbon design of high-performance RPC systems.