Effect of Rice Husk Ash and Fly Ash on Mechanical Properties of Cement Concrete

Abstract

Concrete is one of the most widely used construction materials globally, and cement remains its primary binding agent. Yet cement production is one of the more energy-hungry industrial processes we have, and it releases a significant amount of carbon dioxide into the atmosphere as a byproduct. As concerns about environmental sustainability have grown, researchers and engineers have been looking for ways to reduce how much cement goes into concrete without sacrificing the performance that structural applications demand. Two materials that have attracted considerable interest in this regard are Rice Husk Ash (RHA), an agricultural byproduct generated by burning rice husk at controlled temperatures, and Fly Ash (FA), an industrial residue collected from coal-fired power plants. Both are rich in silica and alumina, which gives them pozzolanic properties — meaning they can react with the calcium hydroxide released during cement hydration to form additional calcium silicate hydrate gel, the same compound responsible for concrete\'s strength and density. This study investigates the combined effect of RHA and FA as partial replacements for cement in M30 grade concrete. Six replacement levels were examined: 4%, 8%, 12%, 16%, 20%, and 24% by weight of cement, with RHA and FA used together in each mix at specific proportional splits. Standard cube specimens measuring 150 mm x 150 mm x 150 mm were cast and cured for 7 and 28 days. Fresh concrete properties were evaluated through the slump cone test, while hardened concrete properties were assessed through compressive strength testing, water absorption testing, and unit weight measurements. The results showed that workability decreased gradually as the replacement percentage increased, primarily because RHA has a high surface area and absorbs more water. However, with the addition of a superplasticizer, satisfactory workability was maintained across all mixes. On the strength front, all mixes up to 20% replacement successfully met the M30 target compressive strength of 30 N/mm2 at 28 days. The 4% replacement mix recorded the highest 7-day strength, while the 16% and 20% mixes showed the best overall performance at 28 days, achieving average strengths of 31.2 N/mm2 and 31.6 N/mm2 respectively. The 24% replacement mix fell below the required strength threshold. Water absorption results mirrored this trend, with the 20% mix recording the lowest absorption at both testing ages — 1.64% at 7 days and 1.13% at 28 days — indicating a denser, more durable concrete matrix. Unit weight also peaked at the 16% and 20% replacement levels, further confirming improved compaction and microstructural density.

Introduction

This study investigates the use of Rice Husk Ash (RHA) and Fly Ash (FA) as partial replacements for cement in M30 grade concrete to create a more sustainable and environmentally friendly construction material.

Concrete relies heavily on cement, whose production contributes significantly to global carbon dioxide emissions (about 7–8% worldwide). At the same time, India generates large quantities of agricultural and industrial wastes such as rice husk ash and fly ash. The research explores whether these waste materials can effectively replace a portion of cement while maintaining or improving concrete performance.

Objectives

The study aims to:

• Evaluate the effects of RHA and FA on concrete strength, workability, water absorption, and unit weight.
• Identify the optimum replacement level for best overall performance.
• Compare blended concrete mixes with conventional concrete.
• Assess environmental and economic benefits of reducing cement usage.

Key Findings from Literature

• Rice Husk Ash (RHA) is highly rich in reactive silica and improves strength and durability by producing additional calcium silicate hydrate (C-S-H) gel.
• RHA performs best at replacement levels of 5–15%; higher amounts can reduce workability due to increased water demand.
• Fly Ash (FA) improves workability because of its spherical particles and contributes to long-term strength through pozzolanic reactions.
• Optimal FA replacement is generally 20–30%.
• Combining RHA and FA creates a synergistic effect: RHA enhances early-age strength while FA improves workability and long-term performance.
• Blended replacements between 15–30% often provide the best balance of strength, durability, and sustainability.

Methodology

• M30 concrete was designed according to Indian Standards.
• Six blended mixes (4%, 8%, 12%, 16%, 20%, and 24% cement replacement) were prepared along with a control mix.
• RHA was produced from rice husks through controlled burning and grinding.
• Fly ash was sourced from a ready-mix concrete plant.
• A superplasticizer was added to maintain workability.
• Concrete specimens were cast, cured, and tested for fresh and hardened properties.

Materials Used

• OPC 53-grade cement
• River sand (fine aggregate)
• Crushed stone aggregates
• Fly ash (Class F)
• Rice husk ash
• Superplasticizer
• Potable water

Significance

The study addresses two major challenges simultaneously:

1. Reducing cement consumption and carbon emissions in concrete production.
2. Utilizing agricultural and industrial waste materials that would otherwise create disposal and environmental problems.

Conclusion

This experimental investigation into the combined use of Rice Husk Ash and Fly Ash as partial replacements for cement in M30 grade concrete leads to a set of conclusions that are both technically robust and practically useful. Workability declined gradually with increasing RHA and FA content, primarily due to the high surface area and water absorption of RHA particles. However, with superplasticizer support at 2% of cement weight, all mixes remained workable and produced true slumps suitable for conventional placement and compaction. Compressive strength at 7 days was slightly lower in the blended mixes than in the control, reflecting the slow-reacting nature of pozzolanic materials at early ages. By 28 days, however, all mixes up to 20% replacement met and exceeded the M30 target strength of 30 N/mm2, with the 16% and 20% mixes performing particularly well at 31.2 N/mm2 and 31.6 N/mm2 respectively. The 24% replacement mix failed the 28-day strength criterion at 26.4 N/mm2 and should not be used in applications where M30 performance at 28 days is required. Water absorption and unit weight results both supported the strength findings. The 16% and 20% replacement mixes showed the lowest water absorption — indicating reduced porosity and better durability — and the highest unit weight, indicating improved compaction and denser microstructure. These outcomes reflect the combined pozzolanic reaction and particle packing effect of RHA and FA operating together at their most effective combined dosage. Based on the totality of the experimental evidence, the optimum replacement level for combined RHA and FA in M30 grade concrete is 16% to 20% by weight of cement. Concrete produced within this range satisfies structural strength requirements, offers measurably better durability indicators than conventional concrete, and delivers meaningful environmental and economic benefits through reduced cement consumption and productive use of agricultural and industrial waste materials. This range is recommended for practical construction applications where both performance and sustainability are priorities.

References

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Copyright

Copyright © 2026 Hiten Pisekar, Om Pawar , Hritik Jadhav, Rohan Khandagale, Asst. Prof. Aasif Habeebi. 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.