Fly Ash and Slag Turn Dredged Mud Into Low-Carbon Embankment Filler

Every year, port expansions, highway projects and river dredging operations generate staggering volumes of waste soil that must be dumped, buried or otherwise disposed of at considerable cost to builders and the environment. A new study published in Case Studies in Construction Materials offers a compelling alternative: transform that unwanted sludge into a strong, water-resistant embankment filling material using a binder made almost entirely from industrial waste. The research, led by Chen Zhang and colleagues, demonstrates that an alkali-activated blend of fly ash and blast furnace slag can stabilize dredged silt so effectively that it rivals conventional cement treatment while slashing greenhouse gas emissions by roughly 99 percent.

The team’s starting material was dredged sludge from Nanjing, in China’s Jiangsu Province, a silty clay with a water content of 33 percent, a liquid limit of 35.3 percent and a plasticity index of 16.6. Classified as silty clay because of its 73 percent silt fraction, the soil in its natural state has a loose microstructure and poor mechanical strength, making it unsuitable for load-bearing embankment fill without treatment. Compaction testing showed the soil reaches its maximum dry density at a water content of 14 percent, a figure the researchers used to design their mixtures.

Rather than relying on Portland cement, whose production accounts for a substantial share of global carbon dioxide emissions, the researchers turned to two abundant industrial by-products. Fly ash, captured from coal combustion flue gases, is rich in silica and alumina, while granulated blast furnace slag from ironmaking contributes abundant calcium. Both materials pose disposal problems of their own; uncontrolled fly ash can generate airborne dust and contaminate water. Under alkaline activation with sodium hydroxide and sodium silicate, however, these wastes dissolve and recombine into powerful cementitious gels, turning a liability into a resource.

To find the best formulation, the team ran an orthogonal test program examining four variables: the fly ash to slag ratio, the sodium hydroxide to sodium silicate ratio, the total alkali content and the water to solid ratio. Variance analysis revealed that the fly ash to slag ratio exerted the strongest influence on compressive strength, followed by alkali content, water to solid ratio and finally the activator balance. The optimal recipe emerged as an equal 5:5 blend of fly ash and slag, a sodium hydroxide to sodium silicate ratio of 1:7, an alkali content of 10 percent by mass of the binder, and a water to solid ratio of 0.35. The researchers attributed the strength gains to abundant calcium aluminate crystals that fill pores when the activator chemistry is properly balanced, while too much or too little water either leaves binder unreacted or dilutes the activator concentration.

With the optimal binder in hand, the team blended it into the dredged soil at doses ranging from 2 to 10 percent by mass and cured the compacted cylindrical specimens for 28 days under standard laboratory conditions of 20 degrees Celsius and 95 percent humidity, deliberately avoiding the high-temperature curing that pure fly ash geopolymers typically require. The results showed a clear threshold effect. Below about 4 percent binder content, the newly formed gel mainly repairs the cohesive soil structure disturbed by mixing, so properties improve only modestly. Above that threshold, enough gel is produced to bond soil particles into a genuinely new, stable skeleton, and performance climbs steeply. Compared with untreated soil, the 10 percent mixture delivered a 110 percent gain in compressive strength, a 106 percent increase in toughness index, a 243 percent boost in tensile strength and an 85 percent improvement in the water stability coefficient.

That last metric matters enormously for embankments, which must survive rainfall and saturation. The researchers saturated specimens after 21 days of curing and compared their strength with samples kept dry. Stress-strain curves showed the stabilized soil fails in a softening, plastic manner rather than shattering brittlely, absorbing energy before gradually losing load capacity. The toughness index, calculated as the area under the stress-strain curve up to peak strain, quantified this resistance to brittle fracture and rose steadily with binder dose.

Head-to-head against a conventional 4 percent cement-stabilized control, the picture was nuanced. At equal 4 percent dosage, the alkali-activated material lagged behind cement, with compressive strength, tensile strength and water stability reduced by 44, 53 and 21 percent respectively, because cement’s higher calcium content generates more calcium silicate hydrate gel. But at 10 percent dosage, the waste-based binder overtook cement on every measure, edging ahead by 4 percent in compression, 13 percent in tension and 14 percent in water stability. The trade-off is more binder, but one made entirely from wastes that would otherwise be landfilled.

The environmental accounting is where the new binder truly shines. Using life cycle assessment aligned with ISO 14040, the team quantified global warming potential, acidification, eutrophication, photochemical ozone creation and resource consumption for stabilizing one tonne of soil. The 4 percent alkali-activated mixture cut global warming potential by 99 percent relative to cement treatment, from 71.6 kilograms to 0.363 kilograms of carbon dioxide equivalent, while acidification, eutrophication and photochemical oxidation potentials fell by 100, 99 and 91 percent and resource consumption dropped 98 percent. Even the 10 percent mixture, using twice the binder mass, reduced global warming potential by 99 percent and resource consumption by 95 percent compared with the cement control.

To understand why the material performs so well, the researchers deployed an arsenal of microscopic techniques: X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy with energy dispersive spectroscopy, and nuclear magnetic resonance. Electron images revealed that untreated soil consists of blocky, flaky and flocculated particle aggregates riddled with voids, while the 10 percent mixture forms a dense fabric of flocculent gel bonding particles into lumpy aggregates. Image analysis showed porosity falling by 25 percent at 4 percent binder and 70 percent at 10 percent. Nuclear magnetic resonance confirmed that large pores declined by 75 percent and small pores by 28 percent at the highest dose, indicating the gel primarily bridges particles and eliminates large voids while also filling finer ones. Notably, no spherical fly ash particles survived, evidence that the ash dissolved and reacted almost completely.

The spectroscopic data converged on a clear chemical mechanism. Elemental analysis detected oxygen, sodium, aluminum, silicon and calcium, with sodium, calcium and aluminum to silicon ratios rising sharply with binder dose. Under the alkaline conditions created by sodium hydroxide and sodium silicate, Ca-O, Na-O, Al-O and Si-O bonds break, releasing calcium ions, sodium ions, aluminate and silicate species that then geopolymerize. Slag-derived calcium forms calcium silicate hydrate and calcium aluminosilicate hydrate gels, while fly ash contributes to sodium aluminosilicate hydrate, the signature N-A-S-H gel of alkali-activated systems. X-ray diffraction identified these new gel phases alongside the soil’s native quartz, illite and kaolinite; infrared spectra showed strengthened Si-O and Al-O absorption bands; and thermal analysis recorded additional mass losses from gel decomposition and bound water, cross-validating the mechanism. The gels weld soil particles into a stable skeleton and fill pores, explaining the dramatic mechanical and durability gains. The authors note that future work must test the material against sulfate attack, high temperatures and freeze-thaw cycling, but the study establishes that two of the world’s most troublesome industrial wastes can jointly convert dredged mud into a durable, genuinely low-carbon construction material.

Subject of Research: Alkali-activated fly ash and blast furnace slag stabilization of dredged waste soil for sustainable embankment construction

Article Title: Applicability and chemical mechanism of alkali-activated fly ash and blast furnace slag stabilized soil for sustainable embankment

Article References: Zhang, C., Wang, W., Zhu, Z., Li, N., Jang, P., & Ma, B. (2026). Applicability and chemical mechanism of alkali-activated fly ash and blast furnace slag stabilized soil for sustainable embankment. Case Studies in Construction Materials, 25, Article e06619. https://doi.org/10.1016/j.cscm.2026.e06619

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06619

Keywords: alkali-activated materials, fly ash, blast furnace slag, soil stabilization, embankment, geopolymer, dredged sludge, low-carbon construction, life cycle assessment, N-A-S-H gel, C-A-S-H gel, waste recycling