Influence of Ground Granulated Blast Furnace Slag on Strength, Durability and Microstructure of M30 Grade Concrete

Ground granulated blast furnace slag (GGBS), a latent hydraulic binder produced as a by-product of iron manufacturing, has been identified as one of the most effective supplementary cementitious materials for improving the long-term strength and durability of Portland cement concrete. This study presents a systematic experimental investigation into the effect of GGBS as a partial replacement of ordinary Portland cement (OPC) at five replacement levels — 0%, 20%, 30%, 40%, and 50% by weight of binder — on M30 grade concrete. Properties evaluated include fresh concrete workability (slump and compacting factor), compressive strength at 28, 56, and 90 days, flexural and split tensile strength at 28 days, water absorption, rapid chloride permeability (RCPT per ASTM C1202), sorptivity, and scanning electron microscopy (SEM) microstructural analysis at 28 days. Results demonstrate that 30% GGBS replacement achieves the optimal balance of mechanical and durability performance, with 28-day compressive strength of 25.9 MPa, 90-day strength of 34.1 MPa (16.4% above control), flexural strength of 4.5 MPa, RCPT charge of 820 Coulombs (41.8% below control), and water absorption of 3.5% (27.1% below control). Higher replacement levels (40–50%) show progressive strength reduction at 28 days attributable to the slower hydraulic activation kinetics of GGBS relative to OPC, but maintain competitive long-term performance due to continued latent hydraulic and pozzolanic activity. SEM analysis at 28 days confirms denser C-S-H gel formation and refined pore structure at the interfacial transition zone in GGBS-modified mixes. The 30% GGBS mix achieves an estimated 24% reduction in embodied CO₂ relative to the OPC control, demonstrating significant environmental co-benefit alongside structural performance improvement