Study of Macro Mechanical Properties of High Strength Concrete (M70) by Partial Replacement of Cement with Fly Ash and Rice Husk Ash and Fine Aggregate with Steel Slag

Abstract

High-strength concrete (HSC) is increasingly preferred in modern construction due to its superior mechanical performance and improved service life. In the present study, an experimental investigation was carried out on M70 grade high-strength concrete incorporating industrial and agricultural by-products as partial replacements for conventional materials. Cement was partially substituted with Fly Ash (10–30%) and, subsequently, the optimum FA level was maintained while Rice Husk Ash (RHA) was introduced at 5%, 7.5% and 10%. Further, using this optimized binary blend, natural fine aggregate was replaced with Steel Slag (SS) at 10%, 20%, 30% and 40% to examine its influence on overall strength characteristics. The mix design for M70 concrete was prepared as per IS 10262:2019 and tested for compressive, split tensile and flexural strengths at 7 and 28 days. The findings clearly indicate that the mechanical behaviour of high-strength concrete can be significantly improved when FA, RHA and SS are used in controlled proportions. Among all mixes, the blend containing 10% FA, 7.5% RHA and 30% SS exhibited the highest strength values, producing a 28-day compressive strength of 89.2 MPa, split tensile strength of 5.40 MPa and flexural strength of 6.70 MPa, outperforming the conventional mix. Based on the obtained results, the optimized ternary mix demonstrates superior macro-mechanical properties and presents a sustainable alternative to conventional high-strength concrete.

Introduction

The study focuses on developing sustainable M70 high-strength concrete (HSC) by partially replacing conventional materials with industrial and agricultural waste. Fly Ash (FA) (10–30%) and Rice Husk Ash (RHA) (5–10%) were used as partial cement replacements to leverage their pozzolanic reactivity and improve matrix densification, workability, and long-term strength. Additionally, Steel Slag (SS) (10–40%) was used as a partial substitute for natural fine aggregates, enhancing aggregate interlock, hardness, and mechanical performance.

The research evaluates the combined effect of FA, RHA, and SS on compressive, split tensile, and flexural strengths at early and later ages. Literature shows that optimal FA–RHA blends improve strength, durability, and microstructural properties, while steel slag effectively replaces river sand without compromising structural performance. Materials used include OPC 53-grade cement, locally sourced sand, coarse aggregates, FA, RHA, SS, a polycarboxylate-based superplasticizer, and potable water.

Experimental mixes were designed following IS 10262:2019, first identifying 10% FA as optimal, then incorporating RHA at 5–10%, with FA10 + RHA7.5 giving the best results. Subsequently, SS was added at varying levels (10–40%) to assess its effect on mechanical properties. The study demonstrates that sustainable waste-derived materials can be effectively integrated into high-strength concrete, reducing environmental impact while maintaining or enhancing structural performance.

Conclusion

This study investigated the influence of Fly Ash (FA), Rice Husk Ash (RHA), and Steel Slag (SS) as partial replacement materials on the macro-mechanical properties of M70 high-strength concrete. Based on the experimental results and analysis, the following conclusions were drawn: 1) Partial substitution of cement with FA improved mechanical performance when used in controlled amounts. The 10% FA mix slightly exceeded the reference concrete in 28-day compressive strength and maintained comparable tensile and flexural values. Higher FA levels (20–30%) resulted in reduced strength due to slower pozzolanic reaction and lower cementitious content. 2) Incorporating RHA into the optimized FA10 mix significantly enhanced the concrete’s mechanical properties. The 7.5% RHA replacement consistently delivered the highest compressive (87.4 MPa), split tensile (5.27 MPa), and flexural (6.58 MPa) strengths. Improvements were mainly attributed to RHA’s fine particle size and high amorphous silica content, which refined pore structure and strengthened the interfacial transition zone. However, excessive RHA (10%) increased water demand and adversely affected strength. 3) Replacement of fine aggregate with steel slag further improved the mechanical performance of the FA10+RHA7.5 binder system. The 30% SS mix achieved the maximum compressive strength of 89.2 MPa, along with notable gains in split tensile (5.40 MPa) and flexural (6.70 MPa) capacities. The improved performance was attributed to slag’s angularity, higher density, and enhanced packing characteristics. Beyond 30%, slag addition negatively affected the matrix continuity and reduced strength. 4) The combination of 10% Fly Ash + 7.5% Rice Husk Ash + 30% Steel Slag emerged as the best-performing blend across all mechanical tests. This ternary-aggregate system demonstrated a balanced and synergistic improvement in strength properties, outperforming both the reference mix and mixes containing only FA or RHA. 5) The results confirm that industrial and agricultural by-products can effectively replace traditional concrete ingredients without compromising structural performance. The use of FA, RHA, and SS contributes to reduced cement consumption, minimized natural sand usage, and improved waste utilization—supporting sustainable high-strength concrete production.

References

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Copyright

Copyright © 2025 K. Anudeep, T. Chandra Sekhara Reddy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.