A Laboratory Study on the Influence of Steel Slag and Potassium Chloride on Improving the Properties of an Expansive Soil for the Foundation Bed

Abstract

Black cotton soil covers large parts of central and southern India and gives foundation engineers a recurring headache: it swells when it gets wet and shrinks badly when it dries out, and a structure sitting on it can heave or crack long before any load-bearing failure ever happens. This paper reports what happened when such a soil, dug from a site known for this behaviour, was mixed with steel slag - a waste product from steel plants - together with potassium chloride, a simple salt that is known to interfere with the swelling chemistry of montmorillonite clay. Before any treatment, the soil was put through the standard battery of index and strength tests: hydrometer analysis, Atterberg limits, free swell, specific gravity, compaction and CBR. It turned out to be a CH soil with 110% differential free swell, a liquid limit above 83%, and an almost unusable soaked CBR of 1.395%. Steel slag was then fixed at 15% of the dry mix, and potassium chloride was varied from 0 to 2% in half-percent steps. Every one of these five mixes was re-tested for free swell, liquid limit, compaction, soaked CBR and triaxial compressive strength, the last of these at both 7 and 14 days of curing. The numbers moved in a consistent direction: free swell fell from 90% (soil + slag alone) down to roughly 40% at the highest KCl dose, the liquid limit dropped to the low seventies, and density, CBR and triaxial strength all climbed steadily up to 1.5% KCl before easing off slightly at 2%. Soaked CBR rose from 1.395% in the raw soil to close to 8.7% at the best mix, and triaxial deviator stress improved further with longer curing. Taken together, the results point to 15% steel slag with 1.5% potassium chloride as a practical, cheap and waste-reusing way to turn this problematic clay into a workable foundation bed.

Introduction

The text discusses the stabilization of expansive black cotton soil using steel slag and potassium chloride (KCl). Black cotton soil contains montmorillonite clay minerals that absorb water and expand during the rainy season and shrink when dry. This repeated swelling and shrinking can damage foundations, walls, slabs, pavements, and canal linings. Since deep foundations and soil replacement are expensive, chemical and industrial by-product additives are investigated as economical alternatives.

Steel Slag

Steel slag is an industrial by-product containing calcium, iron, silica, and magnesium oxides. When mixed with expansive soil, its granular particles improve friction and density, while released calcium promotes clay-particle flocculation. Consequently, steel slag can reduce plasticity and swelling while increasing dry density, bearing capacity, and strength. Previous studies generally found that around 10–20% slag provides substantial improvement.

Potassium Chloride (KCl)

KCl reduces swelling through a chemical mechanism. Potassium ions replace more highly hydrated sodium and calcium ions around montmorillonite particles, reducing the diffuse double layer and bringing clay particles closer together. This results in lower swelling, liquid limit, and plasticity, along with improved compaction and density. KCl therefore primarily addresses the chemical cause of swelling, while steel slag contributes more strongly to mechanical strength and density.

Conclusion

1) The laboratory programme confirms that this black cotton soil, troublesome as it was in its raw state, responds well to a combined steel slag–potassium chloride treatment, and the results support the following conclusions. 2) The untreated soil classified as CH, with a differential free swell of 110%, a liquid limit above 83%, a plasticity index above 51%, and a soaked CBR of only 1.395%, confirming it as unsuitable for direct use as a foundation bed without treatment. 3) Steel slag alone, fixed at 15% of the dry soil weight, produced a worthwhile improvement even before any potassium chloride was added, reducing the free swell to 90% and raising the soaked CBR to roughly 4.2%, showing that the granular bulking and mild cementing action of the slag has real value on its own. 4) Adding potassium chloride to this slag-treated soil improved every property tracked in the study, consistently up to a dosage of 1.5%, beyond which each parameter eased back slightly at 2% KCl. This turning point was observed independently in the free swell, Atterberg limits, compaction, CBR and triaxial results, which is strong evidence that 1.5% KCl represents a genuine optimum for this soil-slag combination rather than an artefact of any single test. 5) At the identified optimum of 15% steel slag with 1.5% potassium chloride, the differential free swell fell from 110% to 45% (a reduction of just over 59%); the liquid limit dropped from about 83.3% to 71.3%; and the plasticity index came down from 51.2% to roughly 24.1%, cutting the swelling potential of the soil by more than half. 6) Compaction characteristics improved alongside the plasticity results, with the maximum dry density rising from 1.511 g/cc to 1.642 g/cc while the optimum moisture content needed to reach it fell from 26.6% to 25.0%, indicating a more efficient, better-packed soil structure at the optimum dosage. 7) The soaked CBR arguably the single most important parameter for foundation and pavement design rose from 1.395% in the untreated soil to 8.701% after 14 days of curing at the optimum mix, an improvement of more than six times, taking the soil from a practically unusable subgrade to one that meets reasonable strength expectations for light structures and pavements. 8) Shear strength parameters from the triaxial tests followed the same trend: cohesion and the angle of internal friction both rose steadily with KCl content up to 1.5%, reaching 1.75 kg/cm² and 20041’ respectively after 14 days, before easing back slightly at 2% KCl, mirroring the pattern seen in every other test. 9) The gap between the 7-day and 14-day CBR and triaxial results at every KCl dosage shows that the strength gain is not just an immediate mechanical effect of mixing but continues to develop with curing time, most likely reflecting ongoing ion-exchange and mild cementation reactions between the clay, the slag and the potassium chloride. Overall, the combination of 15% steel slag and 1.5% potassium chloride offers a practical, economical and comparatively fast-acting means of converting an otherwise problematic expansive soil into a stable foundation bed, while also making productive use of an industrial by-product that would otherwise go to waste.

References

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Copyright

Copyright © 2026 S. Om Sri Pandit, Dr. K. Ramu. 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.