Coal Waste Gets a Second Life: Fly Ash Boosts Soil Strength by Up to 34 Percent

In a finding that could reshape how engineers build roads and embankments across South Asia, researchers in Bangladesh have shown that a modest dose of coal combustion waste can dramatically strengthen the very soils that foundations and highways rest upon. The study, published in Discover Soil, examined fly ash from the Barapukuria Coal-Fired Thermal Power Plant, the only coal-based power facility currently operating in Bangladesh, which churns out roughly 52,000 metric tons of the powdery by-product every year. Most of that ash ends up in ash ponds, consuming land and raising environmental concerns. The new research suggests a far more productive fate: mixing it directly into weak soils to make them stronger, cheaper to build on, and less dependent on carbon-intensive cement and lime.

The research team, led by MD. Moin Akon of the University of Asia Pacific in Dhaka, set out to answer a question that has lingered in geotechnical literature: how does the same fly ash perform when blended into two fundamentally different soil types under identical laboratory conditions? Most previous studies had tackled either clay or sand in isolation. The researchers collected a clay of intermediate plasticity from Matidali in Bogra and a poorly graded sand from Gabtoli in Dhaka, both from depths of one to one and a half meters to avoid organic-rich surface material. The sand, classified as SP under the Unified Soil Classification System, was uniformly graded with a coefficient of uniformity of just 1.67, while the clay carried a liquid limit of 47 percent and a plasticity index of 22.37 percent, marking it as moderately plastic and vulnerable to moisture-driven volume change.

Characterizing the fly ash itself proved crucial to interpreting everything that followed. Drawing on published X-ray fluorescence analyses of Barapukuria ash, the team confirmed that the combined silicon dioxide, aluminum oxide, and iron oxide content reached 92.9 percent, while calcium oxide sat at a mere 0.56 percent. Those figures comfortably satisfy the ASTM C618 criteria for Class F fly ash, the low-calcium variety that lacks self-cementing ability and needs an external calcium source to drive pozzolanic reactions. In practical terms, this ash would not behave like a glue on its own. Any strength gains would have to come from physical rearrangement of particles or from limited chemical reactions drawing calcium from the surrounding soil, a distinction that shaped the entire experimental design.

The experimental program was methodical. Oven-dried soil and fly ash were blended at proportions of 0, 8.5, 12, and 15 percent by dry weight, moistened to the optimum moisture content determined by Standard Proctor compaction testing, and compacted into specimens. Treated samples were then sealed in plastic wrap and cured for 28 days at a controlled temperature of 25 plus or minus 2 degrees Celsius, giving any potential pozzolanic chemistry time to develop. Sandy specimens went into a direct shear apparatus under normal stresses of roughly 31, 62, and 93 kilopascals to extract the internal friction angle, while clay cylinders were crushed in unconfined compression tests to determine undrained shear strength, calculated as half the peak axial stress. At least two specimens were tested for every condition, with mean values used throughout.

The sandy soil told a strikingly non-linear story. Untreated sand, whose shear resistance comes almost entirely from grain-on-grain interlocking, produced a friction angle of 29 degrees. At 8.5 percent fly ash, essentially nothing changed; the failure envelope remained virtually identical, suggesting the modest ash dose did little to alter how load transferred between grains. But at 12 percent, the friction angle jumped to 37 degrees, an improvement of roughly 27.6 percent. The researchers attribute this to three complementary physical mechanisms: the fine ash particles fill the voids between sand grains, densifying the packing and multiplying the contact points that carry load; the irregular, angular morphology of ash particles roughens the contact interfaces, raising sliding resistance; and at the optimum dose the balance between these effects peaks. Push the dose to 15 percent, however, and the friction angle slips back to 36 degrees, still a healthy 24.1 percent gain but a clear sign that excess fines begin to separate sand grains rather than nestle between them, replacing efficient grain-to-grain contacts with weaker sand-to-ash interactions.

The clay responded through a different route and delivered an even larger headline number. Untreated clay averaged an undrained shear strength of 79.63 kilonewtons per square meter, corresponding to a firm to firm-to-stiff consistency. Adding 8.5 percent fly ash lifted that to 97.60 kilonewtons per square meter, a 22.6 percent gain, and at 12 percent the strength climbed to 106.36 kilonewtons per square meter, a 33.6 percent improvement with remarkably tight agreement between duplicate specimens, the coefficient of variation sitting at just 1.5 percent. In a low-calcium Class F system, the authors argue, this improvement most likely reflects a combination of reduced plasticity, improved compaction behavior, and limited early-stage pozzolanic activity, in which reactive silica and alumina from the ash react with whatever calcium hydroxide is available to form modest quantities of cementitious calcium silicate hydrate and calcium aluminate hydrate.

The 15 percent clay results, however, exposed a messy complication that anyone who has worked with fine powders will recognize. One specimen recorded a strength of 67.03 kilonewtons per square meter, actually 15.8 percent below the untreated baseline, while its twin reached 121.45 kilonewtons per square meter, a 52.5 percent improvement. The coefficient of variation ballooned to roughly 39 percent, compared with 13 percent at 8.5 percent ash and 1.5 percent at the optimum dose. The team attributes this scatter to the difficulty of manually dispersing a large volume of fine ash uniformly through a clay matrix: localized ash-rich and ash-deficient zones create wildly different reaction environments within nominally identical specimens. The average value at 15 percent, 94.24 kilonewtons per square meter, was used for trend analysis but with an explicit caveat that it masks substantial within-treatment variability.

To turn these observations into something a designer could use, the researchers fitted quadratic regression models to the strength data, a natural choice given the parabolic rise-peak-decline pattern in both datasets. For sand, the friction angle was modeled as 29 plus 0.62 times the fly ash percentage minus 0.022 times its square; for clay, cohesion was modeled as 79.6 plus 3.45 times the dosage minus 0.12 times its square. Differentiating each equation and setting the derivative to zero yields theoretical optimum dosages of about 14.1 percent for sand and 14.4 percent for clay, hinting that the true mathematical peak may lie slightly above the experimentally tested 12 percent, in the 13 to 14 percent window. The authors are candid about the limits of these models: each was fitted to only four data points, the bare minimum for a second-order polynomial, and they should be read as empirical trend-fitting tools for these specific soils rather than validated design equations.

The engineering and environmental implications travel together. A 27 to 34 percent gain in shear strength parameters translates into meaningful increases in bearing capacity and reductions in deformation for subgrades and embankments, exactly the infrastructure elements where weak deltaic soils cause the most trouble in Bangladesh and similar regions. Meanwhile, every ton of ash diverted into stabilization is a ton kept out of an ash pond, attacking a waste disposal problem and a weak-soil problem with a single intervention. The optimum range of roughly 10 to 15 percent also aligns with earlier findings by Prabakar and colleagues and by Sengul and colleagues for comparable soils, lending the result broader credibility. The authors are equally clear about what remains unknown: no scanning electron microscopy or X-ray diffraction was performed, so the proposed mechanisms are inferred from macroscopic behavior and literature rather than directly observed at the particle scale, and field-scale validation with wetting-drying cycles and long-term loading has yet to be carried out.

What emerges is a compelling, if carefully hedged, case for turning industrial waste into ground improvement. The Barapukuria results suggest that low-calcium Class F fly ash works primarily as a physical strength modifier, excelling in cohesionless soils through void filling and interlocking while offering more modest chemical benefits to clays under ambient curing. Future work, the researchers recommend, should test the unexplored 13 to 14 percent dosage range, adopt a minimum of three specimens per condition, add microstructural characterization, and extend curing durations before the approach moves from the laboratory to the highway. If those steps confirm what the shear boxes and compression machines have already shown, one of South Asia’s most stubborn waste streams may find itself quietly holding up the region’s roads.

Subject of Research: Fly ash stabilization of cohesive and cohesionless soils for enhanced shear strength

Article Title: Shear strength enhancement and optimum dosage prediction of fly ash-stabilized cohesive and cohesionless soils

Article References: Akon, M. M., Shovon, K. M. M. I., Chowdhury, A. A., & Ahmed, S. (2026). Shear strength enhancement and optimum dosage prediction of fly ash-stabilized cohesive and cohesionless soils. Discover Soil, 3(1), Article 128. https://doi.org/10.1007/s44378-026-00288-9

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00288-9

Keywords: fly ash, soil stabilization, shear strength, geotechnical engineering, Class F fly ash, direct shear test, unconfined compression, quadratic regression, sustainable construction, Bangladesh, coal combustion by-products, subgrade improvement