Synergistic Effects of Ternary Additives on Expansive Clay

Abstract

Expansive clays are widely recognized as problematic soils due to their tendency to undergo significant volume changes with variations in moisture content. Such behaviour often leads to ground movement, cracking, and structural damage. In this study, an attempt was made to enhance the strength and stability of expansive clay by using lime, fly ash, and silica fumes as stabilizing additives. The soil was first identified and tested in accordance with IS 2720 standards, and the results showed that it belongs to the highly plastic clay (CH) category, characterized by a high plasticity index of 42.89% and a free swell index of 54.12%. After classification, the soil was stabilized using different proportions of the selected additives lime (2%, 4%, 6%), fly ash (10%, 20%, 30%), and silica fume (5%, 10%, 15%) along with three combined mixes to study their collective effect. All prepared samples were compacted at their Optimum Moisture Content (OMC), cured for 7 days, and tested under Unconfined Compressive Strength (UCS) conditions to assess the improvement in load-bearing capacity. The results revealed a clear improvement in strength with the inclusion of stabilizing materials. Among the individual additives, 4% lime, 20% fly ash, and 15% silica fume produced the most favourable strength values. When used together, the combined proportion of 4% Lime + 20% Fly Ash + 10% Silica Fume (Mix 2) exhibited the highest UCS value of 185kN/m²which is 340.47% higher than control sample, demonstrating a strong synergistic interaction between the materials. The improvement in performance can be attributed to better inter-particle bonding, denser packing, and chemical interactions that occurred during curing.

Introduction

This study investigates the stabilization of expansive clay soil using a ternary blend of lime, fly ash, and silica fume to improve its engineering properties and suitability for construction. Expansive soils, particularly black cotton soils, undergo significant swelling and shrinkage due to moisture variations, causing damage to pavements, foundations, and buildings. To address these issues, the study explores a sustainable stabilization approach using industrial by-products instead of conventional materials.

The natural soil collected from Coimbatore, India, was classified as highly plastic clay (CH) with a high plasticity index (42.89%) and free swell index (54.12%), indicating severe expansiveness. Stabilization was carried out using hydrated lime, Class F fly ash, and silica fume in both individual and combined proportions. Laboratory tests, particularly Unconfined Compressive Strength (UCS) tests after 7 days of curing, were performed to evaluate soil improvement.

Results showed that for individual additives, optimum dosages were 4% lime (0.112 N/mm² UCS), 20% fly ash (0.105 N/mm² UCS), and 15% silica fume (0.141 N/mm² UCS), with silica fume providing the highest strength enhancement. For combined stabilization, three mixes were tested, and Mix 2 (4% Lime + 20% Fly Ash + 10% Silica Fume) achieved the highest UCS value of 0.185 N/mm², significantly outperforming individual additives.

The strength improvement is attributed to pozzolanic reactions that form cementitious compounds such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), along with the filler effect of silica fume that densifies the soil structure. The study concludes that the ternary stabilization approach creates a beneficial synergistic effect, enhancing strength, reducing plasticity, and improving soil stability. The optimum mix of 4% lime, 20% fly ash, and 10% silica fume was identified as the most effective and sustainable solution for stabilizing expansive clay soils.

Conclusion

The present investigation aimed to enhance the engineering behavior of expansive clay by stabilizing it with lime, fly ash, and silica fume.The untreated soil was identified as highly plastic clay (CH) with considerable swelling and plasticity characteristics, indicating the necessity for stabilization treatment. The incorporation of lime increases the soil strength effectively up to an optimum content of 4%. Beyond this percentage, a slight decline in strength was observed, likely due to excess lime reducing the efficiency of pozzolanic reactions.Samples treated with fly ash exhibited the highest strength improvement at 20% addition. Further increase in fly ash content resulted in a small reduction in strength because of the lower proportion of reactive soil particles available for bonding.The addition of silica fume continuously enhanced the soil strength owing to its ultra-fine particles, which filled micro-voids and improved particle interlocking and bonding within the soil structure.The combined application of lime, fly ash, and silica fume produced much greater strength improvement compared to the use of individual stabilizers, demonstrating a beneficial combined interaction among the additives.Out of all the tested combinations, Mix 2 containing 4% Lime + 20% Fly Ash + 10% Silica Fume achieved the highest UCS value of 0.185 N/mm² and was identified as the optimum stabilization mix.The improvement in strength can be linked to mechanisms such as cation exchange, pozzolanic reactions, and the void-filling capability of silica fume particles, all of which contributed to stronger bonding and denser soil structure.The stabilization process successfully improved the short-term compressive strength and handling characteristics of the expansive clay, making it more appropriate for geotechnical applications including pavement subgrades and foundation works.The findings also demonstrate that the utilization of industrial by-products such as fly ash and silica fume provides a cost-effective and environmentally sustainable approach for soil stabilization in construction projects.

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Copyright © 2026 Mukesh Chellam M, Jeyapriya S P. 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.