Strength and hydration of metakaolin-containing ultra-high-performance paste

The influence of metakaolin (MK) addition on the mechanical and hydration performance of ultra-high-performance (UHP) paste was investigated. UHP mortars were prepared to evaluate the compressive strength when MK was used as a cement replacement at levels of 0, 5, 10, 15 and 20%. The hydration heat evolutions of pastes blended with MK were examined. The chemically bound water and MK reaction degree were determined to assess the hydration reaction degrees of the pastes. Differential thermal analysis, X-ray diffraction, mercury intrusion porosimetry and scanning electron microscopy were carried out to investigate the phase composition and microstructure of pastes. The results showed that the mechanical performance of UHP mortar at an early age is improved by the addition of MK due to its filling effect and pozzolanic reaction. The reaction rate of MK in the pastes significantly decreased at a later age and at a low water-to-binder ratio, but the incorporation of MK still had an improving effect on the mechanical strength of UHP mortars at a later age. The incorporation of MK reduced the content of calcium hydroxide crystals and increased the content of calcium silicate hydrate and calcium aluminium silicate hydrate phases in the pastes. The hardened MK-dosed paste samples showed improved microstructure with lower porosity and a refined pore structure.