Alkali activated composites – An innovative concept using iron and steel slag as both precursor and aggregate

Abstract The present paper addresses the feasibility of using an iron and steel slag aggregate (ISS), resulting from electric arc furnace, as a recycled raw material in the production of a new concept of mortar composite, based on alkaline activation, designated as ‘alkali activated composite’ (AACO). The original ISS aggregate was used both as a precursor and an aggregate, either as received (aggregate fraction) or after mechanical activation (precursor fraction). In the first stage, to determine its potential as a precursor, it was mixed with fly ash (FA), using weight ratios of 100/00, 75/25, 50/50, 25/75 and 00/100. The most effective results were obtained with the blend 50/50, which was subsequently used to study the performance of the aggregate fraction. The second stage was comprised of tests on the mortar composites, combining some of the previously tested pastes with the ISS or with a normalised sand (for reference). A 8 M sodium hydroxide solution was used as the activator for all pastes and mortars tested. The curing developed throughout 2 or 28 days, at 22 °C, after an initial 20 h period at 85 °C. The reaction products were characterised using X-Ray Diffraction and microscopy (scanning electron microscopy, for the pastes, and back-scattering electron microscopy, for the mortars). Results show that, indeed, the ISS can be applied as a precursor and/or as an aggregate in the production of alkaline composites.