Influence mechanisms of porous aggregate morphology, maximum size and optimized gradation on ultra-high performance concrete with ferrochrome slag

Ferrochrome slag (FCS) is one of the main by-products generated from the smelting of ferrochrome alloy. Its utilization as aggregate can reduce the mining of natural aggregate and cost of ultra-high performance concrete (UHPC). The morphology of aggregate, maximum size (D_max) and particle gradation are key factors that affect the properties of concrete. Herein, the morphology of FCS compared to river sand was quantitatively characterized. Aggregate gradation was optimized according to the MAA model. Influence mechanisms of aggregate morphology, D_max, and optimized gradation on the properties of UHPC were clarified. The results indicated that large-size FCS above 2.36 mm had higher circularity and roughness, which was beneficial for enhancing the interface bonding and restraining shrinkage. Grading optimization improved the mechanical properties of UHPC (up to 14.1 % at 7 days), interface hardness by 8.6 % and reduced the autogenous shrinkage by 6.0 %. This shrinkage was further reduced by 12.0 % at larger sand-binder ratio of 1.4 due to the enhanced restraint capacity of compactly stacked aggregates. Plastic viscosity of fresh mixture increased with the decrease of D_max, which resulted in a poor workability. Moreover, small-size FCS below 1.18 mm had less roughness and more needle-like particles, which was detrimental to the mechanical properties. The finer particles in FCS also accelerated the hydration process and led to a larger autogenous shrinkage. Thereby, it is not appropriate to adopt more small-sized porous aggregates in UHPC manufacturing. This study provides a theoretical basis for the mixing design and property improvement of UHPC with porous aggregate.