Synthesis and characterization of cemented paste backfill: Reuse of multiple solid wastes

Abstract

The alkali-activated composite cementitious materials composed of fly ash (FA) and ground granulated blast-furnace slag (GGBS) can effectively overcome the performance deficiency of alkali activated single precursors. In order to make full use of the synergistic cementing effect among multi-source solid wastes, cemented paste backfills (CPB) were prepared with iron tailings as the aggregate, FA and GGBS composed as the binder, soda residue (SR) and calcium carbide slag (CS) composed as the activator. The effects of varying FA/GGBS and SR/CS proportions on the workability, mechanical, and microstructural properties of SR–CS–activated FA-GGBS CPB were analyzed. The results indicate that the fluidity of the freshly-mixed backfilling slurry increases first and then decreases with the increase in FA content; whereas it increases with the decrease in the SR content. The unconfined compressive strength (UCS) of CPB decreases with the increase in FA content; it first increases and then decreases as SR content decreases. When the mass ratio of GGBS, FA, SR, and CS was 5:3:1:1, the 28d UCS of CPB was 2.66 MPa, while the fluidity of the freshly-mixed slurry was 158 mm. The microscopic analysis shows that the hydration products are mainly C–S–H gel, C-A-S-H, Friedel's salt (FS), Hemicarbonate (Hc), and Hydrotalcite (Ht). The amount of C–S–H gel produced increases gradually with the curing time and decreases continuously with the increased FA content. More dissolved free aluminum ions promote the conversion from C–S–H to C-A-S-H, while the formation of C-A-S-H inhibit the formation of FS.