Influence of fineness modulus of fine aggregates on mechanical and thermal resistance properties of alkali-activated slag composites

Even though aggregates make up a larger part of the components of alkali-activated composites (AACs), research on the influence of this parameter is limited. This study aims to investigate the influence of the fineness modulus (FM) of fine aggregates on the mechanical and thermal resistance properties of AACs. The FM of fine aggregates in AACs were varied at 2.4, 2.8, and 3.2, keeping all other parameters constant. As a precursor, ground granulated blast furnace slag (GGBFS) was activated with an alkaline medium solution consisting of sodium silicate (Na₂SiO₃) and a 12 M concentration of NaOH in a 2:1 ratio. The flexural and compressive strength and the workability of AACs with varying FM were examined using ASTM C1437-07, C348, and C109 standard testing procedures, respectively. Furthermore, the composites were subjected to high temperatures of 200, 400, and 600 °C to determine residual density, compressive strength, and ultrasonic pulse velocity (UPV). An electron scanning microscope (SEM) with X-ray diffraction (XRD) was used to examine the microscopic morphology of the AACs. The results of this study demonstrated that the composite with 3.2 FM displayed maximum fluidity due to the small specific surface area of its particle size. On the other hand, the composite with 2.8 FM resulted in maximum compressive and flexural strength as well as thermal resistance due to its packing density and the adequacy of activators around the mixes, plus voids filling within the composite. The results from this study will provide important technical input for the standardization of GGBFS-based AAC mix design.