Recyclability potential of alkali-activated Ca-Fe-rich slag mortars

The impact of aggregate type on the recyclability of mortars was investigated. Two types of mortar specimens were made through alkali-activation of Ca-Fe-rich slags, generated by the steel industry, using coarse slag particles... [ view full abstract ]

The impact of aggregate type on the recyclability of mortars was investigated. Two types of mortar specimens were made through alkali-activation of Ca-Fe-rich slags, generated by the steel industry, using coarse slag particles as fine aggregate in one case, and quartz sand in the other. The selected Na-silicate activation solution dosage was established from an empirical response-surface model built for the 28-day compressive strength behavior of paste specimens. Both mortar types were produced ensuring equivalent volumetric proportioning of binder to aggregate, binder fineness and particle-size distribution. Study of the binding matrix formed included backscattered electron (BSE) imaging and energy dispersive spectroscopy (EDS) mapping; the workability of the fresh mixtures was assessed using flow table test, and the flexural and compressive strength of 40x40x160mm³ specimens was established at 7 and 28 days. The tested specimens were re-milled to a defined particle size distributions, including always a finely milled fraction that acted as a binder. Recycled mortars were created without any addition of fresh material, and using different weak alkaline solutions and water for re-activation. The properties of the fresh reactivated mixes and the mechanical performance of the recycled mortars were then compared to their parent mixtures. It was confirmed that the recycling of slag-based mortar is possible without the use of any additional fresh binder, and its properties are impacted by the final milled material characteristics (fineness and PSD) and the re-activation solution used.