Abstract: The construction industry's growing emphasis on sustainable practices and cost-effective materials has led to increased research into supplementary cementitious materials. This study investigates the performance characteristics of M35 grade concrete incorporating fly ash as a partial replacement for ordinary Portland cement. The research aims to evaluate the mechanical properties, durability aspects, and economic viability of fly ash modified concrete mixtures while addressing environmental concerns associated with cement production and fly ash disposal.

Methodology
The experimental program involved the systematic replacement of cement with Class F fly ash at varying percentages (10%, 15%, 20%, 25%, and 30% by weight). Standard concrete mix design procedures were followed to achieve the target characteristic compressive strength of 35 N/mm² at 28 days. Fresh concrete properties including workability, setting time, and bleeding characteristics were evaluated alongside hardened concrete properties such as compressive strength, split tensile strength, flexural strength, and elastic modulus. Long-term durability parameters including permeability, chloride resistance, and carbonation depth were assessed to determine the service life implications of fly ash incorporation.

Key Findings:
The incorporation of fly ash in M35 concrete improved strength, durability, and economy. Optimum replacement at 15–20% enhanced compressive strength beyond 28 days, with 90-day gains of 8–12% over control mixes. Pozzolanic action refined pore structure, lowering permeability and increasing chloride resistance. Workability improved with reduced bleeding, though setting time slightly increased at higher replacements. Durability tests showed 25–30% lower chloride diffusion coefficients in fly ash mixes, ensuring better performance in aggressive environments. Reduced heat of hydration minimized thermal cracking risks in mass concreting. Economic evaluation indicated 8–15% cost savings, making fly ash concrete both sustainable and cost-effective.

Conclusions and Implications:
The study confirms that 15–20% fly ash replacement in M35 concrete optimizes performance and sustainability. Long-term strength, durability, and reduced environmental impact make it a viable option for structural use. Improved durability and cost-effectiveness are especially beneficial for infrastructure projects. Standardizing fly ash quality and mix design guidelines is recommended for broader adoption. The research highlights the role of industrial by-products in sustainable construction. Fly ash concrete offers lower permeability, reduced heat of hydration, and economic advantages. Future studies should focus on high-volume fly ash systems and blending with other SCMs to maximize performance. This supports sustainable concrete technology advancement.

Keywords:
Fly ash, M35 concrete, supplementary cementitious materials, partial cement replacement, pozzolanic reaction, strength, durability, workability, permeability, sustainable concrete, mechanical properties, carbonation, thermal cracking, cost-effectiveness, sustainability, construction, infrastructure.

Downloads: | DOI: 10.17148/IARJSET.2025.12921