The role of carbonated steel slag on mechanical performance of ultra-high performance concrete containing coarse aggregates

Carbonated fine steel slag (CSS) can be used as supplementary cementitious materials (SCM) to enhance the sustainability of ultra-high performance concrete (UHPC). However, the role of CSS on the mechanical performance of UHPC containing coarse aggregates (UHPC-CA) has not been well understood. Here, the influence of cement substitution by CSS on the hydration products, microstructure, mesoscale fracture characteristics and macroscopic strength of UHPC-CA is investigated. Results reveal that the C-S-H content in UHPC increases with the increase of CSS substitution up to 15%. Replacement of cement by CSS results in a more porous interfacial transition zone (ITZ), consequently, a lower mechanical contribution from CA. Both the compressive and the flexural strength decrease with the increase of CSS substitution, and the negative effect is rather limited till 30% CSS substitution. Moreover, due to a more porous ITZ at a higher cement substitution, the ratio of flexural to compressive strength decreases with the increase of CSS substitution.