Effect of metakaolin and magnesium oxide on flexural strength of ultra-high performance concrete

Abstract

The effect of metakaolin (MK) and magnesium oxide (MgO) on the flexural strength of ultra-high performance concrete (UHPC) was investigated. The 0–15 wt% ordinary Portland cement in UHPC was replaced with MK and MgO (MM) with a weight ratio of 3:2. The fluidity and strength of UHPC, the pullout performance of a single steel fibre in UHPC matrix and the microstructure (X-ray diffraction, thermogravimetry, 29Si and 27Al nuclear magnetic resonance, microhardness, scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry) of UHPC were studied. Results show that the flexural strength of UHPC initially increases and then decreases with an increase in MM replacement ratios. UHPC with 10% MM at the age of 60 days have the highest flexural strength, which is 33.8% higher than that of UHPC without MM. When MM replacement ratios increases from 0 to 10%, MM promotes the formation of C-(A-)S–H and LDHs. The addition of MgO increases the pH value, resulting in more Al leaching from MK, which favours Al uptake by C-(A-)S–H. MM also improves the densification of UHPC matrix and the bond properties between fibres and UHPC matrix. However, excessive MM (＞10%) exhibits a stronger clinker dilution effect, and some of the cement particles are covered by MM. Therefore, the flexural strength of UHPC decreases.