Effect of curing regime on ultra-high performance concrete containing metakaolin and magnesium oxide

The effects of different curing regimes on the performance of ultra-high performance concrete containing metakaolin and magnesium oxide (MM-UHPC) were investigated. Four curing regimes were included, namely standard curing (SC), hot water curing (HC), steam curing (MC), and autoclave curing (AC). Additionally, the hydration products, pore structure, microhardness, interfacial transition zone (ITZ) properties, and single fiber pullout properties of MM-UHPC were characterized to reveal the influencing mechanism. The results showed that compared to SC, the thermal curing (HC, MC, and AC) facilitated the incorporation of Al elements from metakaolin into hydrated calcium silicate (C-S-H), increasing the average chain length of C-S-H and promoting the crosslinking of long straight silicate chains. The thermal curing also promoted the secondary hydration reaction of calcium hydroxide [Ca(OH) 2 ] enriched in the ITZ, improving the ITZ properties, increasing the microhardness value of the ITZ, and enhancing the friction force between steel fibers and matrix. Furthermore, the thermal curing reduced the porosity of matrix and refined the pore structure of matrix. These factors resulted in an increase in 120 days flexural strength of MM-UHPC by 14.9 %, 10.5 %, and 20.9 % for HC, MC, and AC, respectively, compared to SC. Similarly, the compressive strength increased by 12.0 %, 11.4 %, and 24.9 %, respectively. The 120 days flexural and compressive strength of MM-UHPC are in the following order: AC > HC > MC > SC. • The performance of MM-UHPC was investigated at various curing regimes. • Thermal curing enhances ITZ properties by promoting secondary hydration. • Thermal curing decreases matrix porosity and improves pore structure. • The 120-day strength of MM-UHPC ranks as follows: AC > HC > MC > SC.