M30 Grade Concrete Incorporating Fly Ash and Ground Granulated Blast Furnace Slag as Supplementary Cementitious Materials

The increasing demand for high-performance concrete (HPC) in Indian infrastructure projects, combined with mounting regulatory pressure to reduce the carbon footprint of construction materials, has intensified research into supplementary cementitious materials (SCMs) as partial replacements for Ordinary Portland Cement (OPC). Fly Ash (FA), a by-product of coal-fired thermal power generation — of which India produces approximately 220 million tonnes annually — and Ground Granulated Blast Furnace Slag (GGBS), generated from blast furnace iron production at rates exceeding 10 million tonnes per year in India, are both established pozzolanic and latent hydraulic SCMs whose individual effects on concrete properties are extensively documented. However, ternary blends combining FA and GGBS at optimised proportions under M30 grade conditions specific to Karnataka's tropical climate and locally sourced aggregates remain underexplored.This study investigates fresh, hardened mechanical, and durability properties of M30 grade concrete incorporating FA (10%, 20%, 30% cement replacement by weight), GGBS (10%, 20% replacement), and a ternary blend (10%FA + 10%GGBS) across seven mix designs. Properties evaluated include workability (slump, compacting factor), compressive strength at 28, 56, and 90 days, flexural and split tensile strength, water absorption, chloride permeability by Rapid Chloride Permeability Test (RCPT per ASTM C1202), and SEM/EDX microstructural analysis at 28 days. Load-deflection response of reinforced concrete beams (150×200×1200 mm) and Mercury Intrusion Porosimetry (MIP) pore structure evolution at ages 3-90 days provide additional structural performance and microstructural development data.The ternary blend achieves 90-day compressive strength of 40.8 MPa (35.5% above control), chloride permeability of 640 C (51.5% reduction versus control), and CO₂ emissions of 318 kg/m³ (22% reduction). SEM analysis confirms dense interfacial transition zones and reduced calcium hydroxide crystallinity in SCM-modified specimens. The M30+20%GGBS mix achieves 90-day compressive strength of 37.6 MPa with water absorption of 2.6%, confirming GGBS's superior densification capability. The ternary blend is recommended as the optimal formulation balancing structural performance, durability, and environmental sustainability for M30 grade reinforced concrete in tropical Karnataka construction conditions.