A Study on the Performance of Expansive Soil Treated with Copper Slag and Geopolymer for Pavement Subgrade

Abstract

Expansive soils, known for their high plasticity and pronounced shrink–swell behavior, pose a major threat to the stability and performance of pavement subgrades in regions with seasonal moisture fluctuations. Conventional stabilizers such as lime and cement, while effective, contribute significantly to carbon emissions and energy consumption. In response, this study explores a sustainable and high-performance alternative by employing copper slag, an industrial by-product, in combination with geopolymer binders synthesized from sodium hydroxide (NaOH) and sodium silicate (Na?SiO?) solutions. The research aims to evaluate the mechanical, compaction, and durability characteristics of expansive soil stabilized using varying proportions of copper slag and geopolymer for potential application in flexible pavement subgrades. A systematic laboratory investigation was conducted wherein copper slag was incorporated at 5%, 10%, 15%, and 20%, and the geopolymer activator was varied between 0.5% and 2.0% by weight of soil. Standard geotechnical tests, including Atterberg limits, compaction, Unconfined Compressive Strength (UCS), and California Bearing Ratio (CBR), were performed in accordance with IS: 2720 specifications. The inclusion of copper slag led to a marked reduction in plasticity and an increase in maximum dry density (from 1.51 g/cc to 1.75 g/cc), while the geopolymer binder contributed to chemical bonding and improved soil matrix integrity. The optimum combination of 15% copper slag and 1.5% geopolymer exhibited significant improvements, with the CBR value increasing from 1.59% to 8.72% and the UCS value showed almost a twofold improvement over untreated soil. The results indicate that the synergistic interaction between copper slag and geopolymer enhances both the mechanical interlocking and chemical cementation within the soil structure, leading to superior strength, stiffness, and moisture resistance. The stabilized soil satisfies the minimum CBR requirement (?8%) for subgrade material as per IRC:37–2018, confirming its suitability for use in flexible pavement foundations. This study thus establishes a sustainable, low-carbon, and durable alternative for expansive soil stabilization, promoting the beneficial reuse of industrial waste and supporting the principles of green geotechnical engineering and circular economy.

Introduction

Expansive soils, rich in montmorillonite clay, are common in semi-arid and tropical regions of India and pose serious challenges for infrastructure due to their high swelling and shrinkage potential, low bearing capacity, and poor drainage. Without treatment, these soils cause heaving, cracking, and differential settlement in pavements, embankments, and light foundations.

Conventional Stabilization Issues:
Traditional stabilizers like lime, cement, and fly ash improve soil strength and reduce plasticity but raise environmental concerns due to high carbon emissions and resource depletion. This has led to the exploration of eco-friendly alternatives.

Copper Slag (CS) and Geopolymer (AAG) as Sustainable Stabilizers:

• Copper Slag: A non-plastic industrial by-product rich in silica and iron oxide, CS enhances soil densification, interparticle friction, and pozzolanic reactivity.

• Geopolymer: Alkaline-activated alumino-silicate binder that forms a cementitious Si–O–Al network, improving cohesion, compressive strength, water resistance, and durability.

• Synergy: CS provides particle densification and pozzolanic activity, while geopolymer creates strong binding and long-term stability, resulting in improved California Bearing Ratio (CBR) and compaction characteristics.

Experimental Approach:
The study tested expansive clay from Andhra Pradesh, treated with varying proportions of CS (5–20%) and geopolymer (0.5–2%). Baseline soil properties were determined, and improvements in plasticity, compaction, CBR, stiffness, and resilience were evaluated.

Key Literature Findings:

• CS alone increases maximum dry density (MDD) and reduces optimum moisture content (OMC), enhancing load-bearing capacity.

• Geopolymer improves unconfined compressive strength (UCS) and reduces plasticity and swell potential.

• Combined CS–geopolymer treatment significantly improves compaction, strength, and dimensional stability of high-plasticity clay, while promoting sustainable industrial waste utilization.

Conclusion

1) It is observed from the laboratory test results that the Differential Free Swell Index (DFS) of expansive soil reduced by 45.26% with the addition of 15% Copper Slag, and it has been further reduced by 71.15% with the inclusion of 1.5% Geopolymer binder compared to untreated soil. 2) It is observed from the laboratory test results that the Liquid Limit (WL) of expansive soil decreased by 13.89% with the addition of 15% Copper Slag, and it further decreased by 24.55% with the addition of 1.5% Geopolymer. 3) It is observed from the laboratory test results that the Plastic Limit (WP) increased by 1.56% on addition of 15% Copper Slag, and further increased by 20.67% with the inclusion of 1.5% Geopolymer binder. 4) It is noticed that the Plasticity Index (IP) decreased by 26.07% on addition of 15% Copper Slag, and it has been further decreased by 73.28% with the addition of 1.5% Geopolymer compared to untreated soil. 5) It is observed from the laboratory test results that the Specific Gravity (G) of the expansive soil increased by 9.36% on addition of 15% Copper Slag, and it further increased by 1.56% with the inclusion of 1.5% Geopolymer binder, indicating improved particle packing and density. 6) It is observed from the laboratory test results that the Optimum Moisture Content (OMC) decreased by 35.04% on addition of 15% Copper Slag, and it has been further decreased by 2.79% with the addition of 1.5% Geopolymer binder. 7) It is observed from the laboratory test results that the Maximum Dry Density (MDD) increased by 2.65% with the addition of 15% Copper Slag, and it further increased by 12.90% with the inclusion of 1.5% Geopolymer binder, indicating densification and improved compactability. 8) It is observed that the CBR (Soaked) value increased by 205.66% with the addition of 15% Copper Slag, and it further increased by 80.41% with the addition of 1.5% Geopolymer, fulfilling the IRC:37–2018 requirements for pavement subgrade applications. 9) It is observed that the Cohesion (C) decreased by 37.38% on addition of 15% Copper Slag, and further decreased by 12.17% with the inclusion of 1.5% Geopolymer binder, while the angle of internal friction (?) increased by 9.52% and 36.90%, respectively, signifying enhanced shear resistance and stability. The combined use of 15% Copper Slag and 1.5% Geopolymer significantly improved the engineering characteristics of expansive soil by reducing its plasticity and swelling potential while enhancing its density, bearing capacity, and shear strength. The stabilized soil satisfies the strength requirements for pavement subgrade and represents a sustainable, eco-friendly, and cost-effective alternative to conventional stabilizers such as cement and lime. Thus, the combination of 15% Copper Slag and 1.5% Geopolymer provides a sustainable and technically viable solution for stabilizing expansive soils for subgrade applications.

References

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Copyright

Copyright © 2025 Dr. D. Koteswara Rao, Palika Anantha Veera Durga Prasad. 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.