Geopolymer as a key material to utilize basic oxygen furnace slag (BOFS) as an aggregate

Basic oxygen furnace slag (BOFS) is the hard waste created when molten iron is treated in a basic oxygen furnace during the steelmaking process. Despite its good hardness and strength properties, the free lime (f-CaO) and free magnesia (f-MgO) in BOFS limit the utilization of BOFS as an aggregate used in construction applications due to the expansion characteristics of their hydration products. However, the interaction between f-CaO and f-MgO and free silicon (f-Si) that come from sodium silicate (Na2SiO3) in geopolymer concrete/mortar mixture can create stable substances like wollastonite (CaSiO3) and enstatite (MgSiO3), and the expansion issue in BOFS may be reduced. In this research, therefore, a total of 5 geopolymer mortar (GPM) mixtures were designed: 4 geopolymer mortar mixtures that comprise a partial substitution of river sand (RS) with BOFS aggregate (25 %, 50 %, 75 %, and 100 %) and one control mixture (100 % RS) with fixed parameters, which contain Na2SiO3 ratio of 1.5 with 10 M NaOH and the combined binder contents with 40 % ground granulated blast furnace slag (GGBFS) and 60 % ASTM Class F fly ash (FA). Then, the fresh and hardened properties of GPM mixtures were evaluated. Test results presented that increasing BOFS aggregate content up to 75 % substitution of RS increases the GPM mixture’s relative flowability. However, the air content and setting time of the GPM mixture decreased with increasing BOFS aggregate content. For compressive and flexural strengths, the GPM mixtures containing BOFS aggregate had comparative strength or higher strength than that containing 100 % RS. The GPM mixture with higher compressive strength showed higher shrinkage than other mixtures. Finally, regardless of water and 1 M NaOH solution submersions, all GPM mixtures have less than 0.1 % expansion value except for the mixture [25 %RS-75 %BOFS] submersed in NaOH solution.