Pre-treatment of BOF steel slag aggregates and effect on the mechanical and microstructure properties of alkali-activated mortars

This paper investigated the potential use of basic oxygen furnace steel slag (BOF SS) as a single aggregate in alkali-activated mortars (AAMs) for building applications. It assesses the influence of wet-curing and carbonation pre-treatments on the mechanical, microstructural properties (computerized microtomography and scanning electron microscopy), as well as on the susceptibility to volumetric expansion (autoclave accelerated test). Untreated BOF SS, natural aggregate, and a Portland cement (PC) matrix were used for comparison. The results show that the wet-curing pre-treatment is effective, as it ensures that the AAM mortar with BOF SS aggregate is a sound building material after accelerated expansion testing. The wet curing of BOF SS aggregate provides a denser microstructure, less porous interface transition zone (ITZ), and the absence of microcracks in the mortars, which ensures equivalent mechanical behavior to those mortars made with natural aggregate. In contrary, the carbonated BOF SS aggregate negatively affected the early age strength development of AAMs mortars due to the conversion of the activator into sodium carbonate in AAMs. AAMs and PC mortars with carbonated BOF SS also fail in accelerated expansion tests, like those containing untreated BOF SS. [Display omitted] •The influence of BOF SS aggregates pre-treatments on AAM properties was assessed.•Early strength of AAM was affected when carbonated BOF SS aggregate was used.•Mortars with untreated and carbonated BOF SS fail in accelerated expansion tests.•Wet-curing is effective and highly advised; it ensures a sound building material.•Wet-cured BOF SS in AAM improved the cohesion of the mortar and improved the ITZ.