Effects of the hydration reactivity of ultrafine magnesium oxide on cement-based materials

The size of magnesium oxide may greatly affect its behaviour in cement-based materials (CBMs), and this has not been fully investigated. The reactivity of ultrafine magnesium oxide (UFM) of size 40 nm to 20 μm was assessed, and its effects on the compressive strength and volume stability of cement mortar were investigated. The results showed that the hydration of UFM followed the first-order reaction mode in the first 3 d and then slowed down due to the alteration of the reaction to the diffusion-controlled mode. UFM contributed to an increase in compressive strength and a decrease in shrinkage of CBMs at the very early age, but hindered the hydration of cement at later ages, coarsened the microstructure, and decreased the later-age shrinkage to a much smaller extent than normal light-burnt magnesium oxide. The relatively high hydration reactivity of UFM may contribute to the formation of a more compact gel structure around cement particles at the very early age, which may hinder the reaction of cement at later ages, thus leading to the slowed property gain of CBMs at later ages. The findings from this study may help in the selection of magnesium oxide types for achieving a desired CBM with certain properties.