The effects of calcium content of fly ash on hydration and microstructure of ultra-high performance concrete (UHPC)

In this study, the effects of low-calcium fly ash (LFA) and high-calcium fly ash (HFA) on C–S–H structure, hydration products, hydration heat, and compressive strength of UHPC paste/concrete were investigated and compared via XPS, MIP, FT-IR, 29Si NMR, XRD, SEM, and the thermal analysis method. The results showed that fly ash reduced the rate of exothermic hydration and early strength, but it increased the compressive strength and reduced the porosity in the middle and late ages, and denied the microstructure of UHPC. By contrast, HFA exhibited the higher band fluctuation of Si–O and Al–O and lower Ca2p-Si2p energy separation values, which indicated that more amorphous silicon oxide and alumina participated in the pozzolanic reaction. This also demonstrated that HFA played an important role in enhancing the polymerization degree and increasing a higher level of Al-substitution for Si in C–S–H than LFA. HFA enhanced the pore volume with diameters less than 10 nm from 28 to 90 days due to pozzolanic reaction and filling effect, whereas the LFA mainly played a filling role and a lower pozzolanic reaction during this period. Durability results showed that fly ash reduced the drying shrinkage and chloride impermeability coefficient of UHPC, and the effect of HFA was better than that of LFA.