Influence of cement type and mixing pressure on air content, rheology and mechanical properties of UHPC

In this study six Belgian cement types are chosen to make ultra-high performance mortar. The cement types were selected, based on their low C3A-content and moderate Blaine-fineness. All the mixtures were made according to the same mix design and mixing procedure. The effect of the different cement types on the workability, rheology and compressive strength is examined. The microstructure has been investigated by means of an air void analyzer which give information on the air cavities. A mercury intrusion porosimeter is providing information on the capillary pores. The microstructure is influenced by the chemical composition of the cement, and by the workability of the mixture. It is shown that cement with a higher product of C3A and Blaine-fineness, a higher alkali content and a lower content of SO3 give the lowest performance. Due to the rather high air content of the ultra-high performance mortar, compared with self-compacting mortar, the impact of a reduced air pressure during mixing is also examined. Effects of this vacuum mixing on the microstructure and thereby the workability and compressive strength are investigated.

► The chemical composition of cement influences the workability and the rheology of UHPC.
► A less workable UHPC contains more air bubbles.
► A reduced mixing pressure decreases the air cavities.
► A reduced mixing pressure has little effect on the capillary pores.
► A strength increase for UHPC with a suitable workability, is determined in case of vacuum mixing.