Effects of Alkali Activation and CO2 Curing on the Hydraulic Reactivity and Carbon Storage Capacity of BOF Slag in View of Its Use in Concrete

Abstract

This work investigates the sequential application of alkali activation and CO2 curing to BOF steel slag as a technique for improving its hydraulic reactivity for use in concrete, while also exploiting its potential as a carbon sink. Activation with either a sodium hydroxide/sodium-silicate or a sodium hydroxide/sodium carbonate solution was first evaluated in a preliminary calorimetric study for selecting the solution compositions leading to the formation of early stage hydration products. The pastes produced with the selected solutions were then cured either in a humidity chamber or in a carbonation chamber (at 20 or 50 °C) for up to 28 days, in order to assess long term reaction products. Mineralogical and thermal analysis showed the formation of a C–S–H like phase, specifically in the samples activated by the sodium hydroxide/sodium-silicate solution, whereas significant occurrence of gaylussite was noticed in the samples activated with the sodium hydroxide/sodium carbonate solution. A maximum CO2 uptake of 6% by wt, due to calcium carbonate formation, was observed in the latter samples, whereas a 5% value was achieved in the former ones. The compressive strength of the mortars prepared with sodium hydroxide/sodium silicate and cured in the carbonation chamber at 50 °C was above 2 MPa, while it was lower for the other samples, particularly those activated with sodium hydroxide/sodium carbonate. Alkali activation employing sodium hydroxide/sodium silicate solutions followed by CO2 curing at relatively high temperature (i.e. 50 °C) resulted a promising treatment for BOF slag valorization in the manufacturing of concrete for non structural applications.