Composite cements: synergistic effects of particle packing and pozzolanicity

The production of cement is a carbon dioxide-intensive process. Replacing ordinary Portland cement (OPC) with industrial by-products can bring down the carbon footprint associated with cement production. Various industrial residues are currently used as alternative cementitious materials in this regard. However, developing a low carbon dioxide composite cement with different pozzolans alters the packing density, which influences its properties. Although studies have been conducted on the use of fly ash and slag at lower cement replacement levels, studies on the packing density and strength of ternary and quaternary composite cements with higher replacement levels are limited. In this study, fly ash, blast-furnace slag, ultra-fine fly ash and ultra-fine slag are used as a partial replacement for cement in various proportions. Out of the 51 mixtures tested in the study, 11 combinations were selected, based on the maximum packing density, for further investigations on fresh and hardened properties to arrive at the best trade-off between cement reduction and desired properties. The early-age strength is influenced by the packing density of composite cements, whereas the later-age strength is found to be highly governed by the amount of OPC and the pozzolanic potential of the industrial by-products.