Mechanical properties of fly ash based geopolymer concrete using only steel slag as aggregate

Geopolymer concrete (GPC) is a sustainable and eco-friendly construction material in which industrial byproducts containing Si and Al such as fly ash and slag are polymerized using an alkaline activating solution to create hardened binder or inorganic polymer cement. This study evaluated GPC mixtures using fly ash as a geopolymer binder and granular slag materials in place of natural coarse and fine aggregates. Three GPC mixtures were designed to achieve specified compressive strengths (f’c) of 25, 30, 35 MPa by varying (1) mass of fly ash between 482 to 507 kg/m3 of GPC, (2) the ratio (by mass) of alkali activating solution to fly ash (AAS/FA) between 0.4 and 0.48, and (3) the ratio (by mass) of sodium silicate (Na2SiO3) to sodium hydroxide solution (NaOH) (or SS/SH) between 2 and 3. The mixtures were prepared for testing in hardened conditions. The results showed that GPC specimens (air-cured) yielded good compressive strength (between 34.80 and 44.85 MPa), flexural strength (ranging from 4.50 to 5.91 MPa), elastic modulus (between 30.99 and 35.50 GPa). These properties are likely a result of the high strength of slag aggregates and a chemical reaction between steel slag aggregates and the geopolymer binder, improving the interfacial transition zone and GPC microstructure. This study suggests that slag aggregates can be used in place of natural aggregate materials to improve GPC’s mechanical properties with a conventional curing process for construction applications.