Flexural and Shear Performance of GGBS-Silica Fume Blended Geopolymer Concrete

This study provides an experimental characterization of the flexural and direct shear behavior of ambient-cured ground granulated blast-furnace slag (GGBS)–silica fume (SF) blended geopolymer concrete. It targets a gap in material-level flexural and shear data for this class of binder. A total of ten mix designs spanning five GGBS-SF blending ratios (100:0 to 0:100) and two alkaline activator solution (AAS) ratios (1.5 and 2.0) were investigated. Four-point bending tests under closed-loop deflection control and modified JSCE direct shear tests were conducted on plain prismatic specimens at 28 days. Complete load-deformation responses were captured, including post-peak softening. These were used to evaluate flexural strength (f r ), shear strength (f v ), initial and secant flexural stiffness (K i and K s ), peak-to-cracking deformation ratio (µ δ ), pre-peak flexural toughness (T), brittleness index (BI), and normalized pre-peak shear work (Gs). GGBS-rich systems achieved the highest absolute peak f r and f v of 3.59 and 4.52 MPa, respectively, under AAS = 2.0. These systems also exhibited the most balanced post-peak fracture response under higher AAS ratio (higher silica availability), in contrast to SF-rich systems, which became increasingly brittle as SF content increased under AAS = 2.0. Within Group A, G50S50 showed the most balanced fracture behavior, with the lowest brittleness index in its group. K s correlated exponentially with splitting tensile strength (f sp ), with R² = 0.93. This is a dataset-specific association and does not establish a causal relationship. Preliminary, dataset-specific empirical correlations for f r and f v were developed and benchmarked against codified provisions. Microscopy analyses within Group A indicated that the Ca/Na ratio tracked the transition between C–A–S–H- and N–A–S–H-dominated microstructures and the associated mechanical performance hierarchy.