Influence of ultra-fine slag and silica fume on properties of high-strength concrete

The effects of ultra-fine slag (UFS) and silica fume (SF) on the compressive strength and durability-related properties of high-strength concrete were compared, and the mechanism of these effects was analysed. Portland cement was replaced with UFS or SF in amounts of 10% and 15% by mass in cement with a water/binder ratio of 0.25. The cement paste containing UFS shows better fluidity and dispersibility than that containing SF. The UFS concrete shows faster strength development at early ages (3 and 7 d). However, UFS concrete has a lower strength after 28 d than SF concrete. The resistance of the UFS concrete to chloride ion penetration at ages of 28 and 60 d was slightly lower than that of SF concrete, but UFS concrete could reach the same chloride ion permeability grade as SF concrete. The water absorption of UFS concrete at 60 d is relatively higher than that of SF concrete during the entire absorption process. Significantly, concrete with 15% UFS replacement can achieve the same water absorption effect as concrete with 10% SF. The incorporation of UFS in the cement pastes induced quicker early-age cracking than was observed in SF-blended cement pastes.