Synergetic optimizing particle size distributions of aggregate and cementitious materials: Toward lower chloride diffusivity of concrete

Particle size distributions (PSDs) of aggregate and cementitious material dominate the microstructure and properties developments of concrete. Optimizing the proportion of aggregate can mitigate chloride ingress pathways, leading to a significant increase in surface area and augmented generation of the interfacial transition zone (ITZ). Overemphasis on aggregate proportion can compromise workability of fresh concrete due to limited cement paste lubrication. Although gap-graded distribution benefits cementitious material hydration, accurately classifying cementitious components for broad implementation is challenging. This study designed the aggregate proportion and PSD based on a proposed diluted packing region with low estimated chloride diffusivity. The gapgraded distribution at the cementitious material scale was conveniently achieved by incorporating fine ground blast furnace slag and coarse fly ash into commercial Portland cement. Consequently, dual-scale designed concretes, taking into account PSDs at both aggregate and cementitious material scales, were proposed. These concretes exhibited chloride diffusion coefficients ranging in (1.26-1.46)x10- 12 m2/s, with aggregate proportions spanning 52 % to 74 %. Their compressive strength was comparable with Portland cement concrete, even with 60 % supplementary cementitious materials (SCMs). The improved properties of the dual-scale designed concrete can be attributed to the high aggregate proportion with rational PSD, which does not result in a significant rise in the ITZ proportion. Efficient hydration of the cementitious material is beneficial to microstructure densification and chloride binding capacity improvement, helps to increase the chloride diffusion distance and hinder the penetration of chloride. This study provides insights into designing chloride-resistant concrete with less Portland cement reliance.