Effect of slag cement on the properties of ultra-high performance concrete

Abstract

This study investigates the effects of replacing a portion of Portland cement with slag cement (0%, 25%, 50% and 65% by mass) on the properties of plain ultra-high performance concrete (UHPC) mixes. It is observed that slag cement substantially enhances the flowability of the UHPC mixes and reduces the superplasticizer dosage. Mechanical properties are evaluated for mixes with similar air void characteristics, indicating that early age strength is reduced while there is more strength gain in the later age. This early age strength suppression is consistent with semi-adiabatic hydration heat evolution. The presence of slag cement reduces the maximum temperature rise. Linear deformation measurement on duplicate sealed specimens reveals four distinct stages for the shrinkage development and slag cement increases the shrinkage strain in the steady state, associated with the pozzolanic reaction. Simultaneous measurement of moisture uptake, mass loss and relative dynamic modulus of elasticity (RDM) on UHPC mixes for F-T durability shows that the cumulative moisture uptake and mass loss, when normalized with respect to the paste content, are almost negligible compared with a regular concrete mix. This reconciles the capillary suction dominated surface scaling mechanism. No internal bulk cracking is detected due to the dense matrix restricting the ingress of external moisture and the amount of freezable pore water.