Mechanical properties, carbon footprint and cost of ultra-high performance concrete containing ground granulated blast furnace slag

Abstract

This paper deals with the possibility of formulating low-carbon UHPC by replacing cement with ground granulated blast furnace slag (GGBS). Slags of two fineness levels (420 m2/kg (SL1) and 700 m2/kg (SL2), respectively) are incorporated as volume replacements for cement at 30% and 50%. The possibility of completely replacing silica fume (SF) with superfine slag (SL2) in UHPC is also investigated.The results show that reducing the SF allows for the production of UHPC with satisfactory mechanical properties. The incorporation of 30% of GGBS (SL1 or SL2) accelerates the hydration process due to the reduced SP content and the nucleation site effect, which improves long-term compressive strength. In the presence of a high level of ordinary slag (50% of SL1), the dilution effect dominates and a decrease in compressive strength can be observed at all ages. Due to the physical effect of superfine slag, 30% of SL2 yields a high 28-day compressive strength (151.2 vs. 136.5 MPa for the SL2-based reference mix) despite the absence of SF particles. The relatively low bound water content, as determined by thermogravimetric analysis (TGA), indicates the presence of a large amount of unreacted cement, which contributes to the improved mechanical performance of UHPC mixes. A 28-day splitting tensile strength greater than 7 MPa is measured across all UHPC produced, with a maximum value of 9.2 MPa for concrete containing 30% of SL2 and without SF. A 50% SL2 UHPC with a very low carbon dioxide index (approx. 4.15 kg/m3/MPa) and a compressive strength of 139 MPa, similar to the SF-based reference mix, have indeed been achieved. However, this mixture is found to be the most expensive (at 496 €/m3), due to the high cost of superfine slag, in comparison with cement.