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

The research, led by Wen Zheng and colleagues at Yulin University, focuses on flowable backfill materials, known in engineering practice as controlled low-strength materials or flowable fill. These self-compacting slurries are pumped into utility trenches, abandoned pipelines, mine voids, and foundation excavations where mechanical compaction is impossible. Conventional formulations rely on ordinary Portland cement and virgin sand, which makes them technically reliable but environmentally costly. The team’s alternative replaces up to 40 percent of the cement with calcined coal gangue and fly ash, while using local excavated soil as the bulk filling phase rather than quarried aggregate.

The key innovation is what the authors call a resource-coupling mode, demonstrated at Jinjie Industrial Park in the Shanxi–Shaanxi–Inner Mongolia energy triangle. There, the coal preparation plant supplying gangue, the coal-fired power plant supplying fly ash, and the cement plant all sit within a two-kilometer radius. This geographic proximity allows the gangue to be calcined at around 700 degrees Celsius using waste heat from cement production, then ground and blended with fly ash and cement into a standardized binder that is mixed with excavated soil and water on site. Transport distances, and the emissions and costs they carry, effectively collapse to zero.

The chemistry behind the system is a carefully balanced three-way partnership. Calcining coal gangue transforms its kaolinite content into reactive metakaolin-like aluminosilicate phases. When cement hydrates, it releases calcium hydroxide, and these reactive phases consume it in pozzolanic reactions that form additional calcium silicate hydrate and calcium aluminosilicate hydrate gels, the glues that bind the matrix together. Fly ash plays a different role: its smooth spherical particles act as internal ball bearings that dramatically improve slurry flow, while also contributing to particle packing and later-age strength. Each material compensates for the other’s weaknesses.

To quantify this complementarity rather than merely assert it, the researchers ran a two-stage statistical experiment. A constrained mixture design first varied the proportions of cement, calcined gangue, and fly ash while holding soil content and water-to-solid ratio fixed. The fitted models showed that fly ash contributed most to flowability, while calcined gangue outperformed fly ash in strength: at equal 50 percent cement replacement, the gangue blend reached 3.61 megapascals at 28 days against 2.63 megapascals for the fly ash blend. A second central composite design then explored how soil fraction and water content interact, revealing that soil content was the dominant strength-limiting factor because the soil acts purely as an inert filler.

From these models the team identified a feasible processing window of 80 to 84 percent excavated soil and a water-to-solid ratio between 0.39 and 0.45, satisfying thresholds of at least 200 millimeters of flow spread and 0.8 megapascals of seven-day strength. The optimized reference formulation, 60 percent cement, 25 percent calcined gangue, and 15 percent fly ash, achieved a flow spread of 222 millimeters, a bleeding rate of 3.52 percent, and 28-day strength of 3.62 megapascals, beating both binary blends by 14.6 and 27.5 percent respectively.

Microscopic analysis explained why the ternary blend wins. X-ray diffraction and thermogravimetry showed that the optimized mix retained moderate calcium hydroxide levels, evidence that the calcined gangue was actively consuming the cement’s hydration byproduct in secondary reactions. Scanning electron microscopy revealed unreacted gangue and fly ash particles bridged together by needles of ettringite and gel-like hydration products, forming an interconnected load-transfer network. Nuclear magnetic resonance confirmed that the ternary system refined the pore structure, shifting the distribution toward finer pores relative to either binary system, and regional elemental mapping showed calcium-rich zones consistent with abundant cementitious gel.

The economic and environmental accounting is striking. Under a cradle-to-gate boundary, the optimized formulation reduced material cost by 34.6 percent and carbon emissions by 40.3 percent, from 217.2 to 129.6 kilograms of carbon dioxide equivalent per cubic meter, compared with a pure cement reference. At a production scale of 100,000 cubic meters per year, the authors estimate the approach would valorize roughly 99,000 tonnes of excavated soil, 4,700 tonnes of calcined coal gangue, and 2,800 tonnes of fly ash, saving about 20 mu of land and avoiding approximately 8,760 tonnes of carbon dioxide equivalent annually. The team also proposed scenario-specific recipes: a strength-oriented mix for load-bearing backfill, a flow-oriented mix for pumpable filling, and a cost-oriented mix for bulk disposal.

The authors are careful to note the limits of their work. The results rest on laboratory testing up to 28 days with specific local raw materials, and long-term durability under freeze–thaw cycling, sulfate attack, drying shrinkage, and contaminant immobilization remains unverified. The leaching screening of raw fly ash showed chromium slightly above a groundwater reference value, though hardened-material leaching was not assessed. Even so, the study offers a compelling template for energy-intensive regions worldwide: instead of shipping waste away and hauling virgin materials in, co-located industries can couple their residual streams into locally usable, performance-tuned construction materials, turning three disposal burdens into a single low-carbon product.

Subject of Research: Resource utilization of excavated soil and coal-based solid wastes in flowable backfill materials

Article Title: Resource utilization of excavated soil and coal-based solid wastes under resource coupling mode in energy agglomeration areas: A case study on flowable backfill materials

Article References: Zheng, W., Liu, W., Wei, F., Li, Y., Liu, N., & Ai, X. (2026). Resource utilization of excavated soil and coal-based solid wastes under resource coupling mode in energy agglomeration areas: A case study on flowable backfill materials. Case Studies in Construction Materials, 25, Article e06569. https://doi.org/10.1016/j.cscm.2026.e06569

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06569

Keywords: flowable backfill, controlled low-strength materials, coal gangue, fly ash, excavated soil, solid waste utilization, low-carbon construction, pozzolanic reaction, mixture design, carbon emissions, energy agglomeration areas, resource coupling

The research team, led by Désiré Ndahirwa of UniLaSalle with colleagues including Hélène Lenormand, Hafida Zmamou and Nathalie Leblanc, focused on a question that has dogged the field for years. Wood ash contains reactive silica, alumina and calcium-bearing phases that can, in principle, behave like cement itself, reacting with water through hydraulic and pozzolanic pathways. In practice, however, pastes made from pure wood ash are weak and porous. Previous studies had shown that replacing moderate amounts of cement with wood ash often reduces compressive and flexural strength, and most work had examined substitution levels below 50 percent. Almost nothing was known about what happens at very high replacement levels, where wood ash dominates the mixture, and the stiffness of such materials, measured as Young’s modulus, had rarely been quantified despite its importance for structural design.

To close that gap, the researchers gathered four locally sourced wood ashes from the Normandy region of France. Two of them, designated WFA3 and WBA, came from the combustion of wood pellets and were delivered as wet sludge, requiring oven drying and crushing before use. The other two, WFA8 and WFA9, arrived as dry fine powders from a local heating plant operated by Coriance in Mont-Saint-Aignan, where boilers with power inputs of 6 and 8 megawatts burn forestry wood chips, bocage wood chips and pallet residues at temperatures between 900 and 1100 degrees Celsius. The team prepared seventeen paste formulations in total: four containing only wood ash, twelve blending 80 to 95 percent wood ash with 5 to 20 percent ordinary Portland cement, and a reference paste of pure cement, all compacted with a mini-Proctor device to boost density and cured for up to 28 days.

The chemical analysis alone revealed why wood ash is such a tricky raw material. The four ashes were dominated by calcium oxide, silica, potassium oxide and sulfur trioxide, but in wildly varying proportions. Sulfate contents reached 15.4 percent in WFA8 and 14 percent in WFA9, far above the 4 to 5 percent limit set by ASTM standards for pozzolans, and their combined pozzolanic oxides fell well below the required thresholds. In plain terms, these ashes do not qualify as conventional pozzolans, yet X-ray diffraction showed they carry crystalline phases such as portlandite, calcite, albite, alite, dolomite and sylvite that can still participate in binding reactions. The variability is a direct consequence of differing feedstocks, boiler designs and combustion temperatures, and it means each ash must be evaluated on its own terms rather than lumped into a single category.

When it came to mechanical performance, the effect of the small cement additions was unmistakable. Pure wood ash pastes managed compressive strengths of only 0.13 to 1.65 megapascals at 28 days, but raising the cement content to 20 percent lifted those values substantially. The best performer was the WFA8-based blend, which reached 5.49 megapascals, while WFA9 and WFA3 pastes achieved 4.01 and 3.87 megapascals respectively at the same dosage. The researchers attribute the gains to a richer supply of hydration products, including calcium silicate hydrate gel, portlandite and ettringite, formed as the cement’s tricalcium silicate reacts with water and progressively densifies the paste matrix. Stiffness told the same story: the modulus of elasticity, estimated from the linear portion of stress-strain curves, climbed with cement content, curing time and bulk density in three of the four ash families, with the highest values consistently recorded in mixtures containing 20 percent cement.

One ash refused to follow the script. The wood bottom ash, WBA, behaved atypically across every measurement. Its pastes lost compressive strength between 7 and 28 days at certain dosages, its modulus of elasticity peaked at 7 days and then declined, and thermogravimetric analysis found no detectable portlandite whatsoever. X-ray diffraction offered an explanation: the WBA pastes contained no alite, the calcium silicate phase that drives strength development in hydrating cement, and their dominant crystalline phases were calcite and quartz. Adding 20 percent cement pushed the estimated calcite content up from about 47 to 63.5 percent while quartz fell, a signature of carbonation of calcium silicate hydrate or the formation of complex, less efficient hydrate phases, both of which are associated with increased porosity and weaker binding.

Scanning electron microscopy added a visual dimension to the story. Under the microscope, pure cement paste appeared dense and well packed, bristling with the products of hydration, while pastes made from 100 percent wood ash showed loosely arranged particles riddled with interparticle voids, along with unreacted ash grains and dark fragments of unburnt wood. With 20 percent cement added, the microstructure tightened, porosity dropped and hydration products proliferated, with needle-like ettringite crystals, gel-like calcium silicate hydrates, plate-shaped portlandite and rhombohedral calcite all visible. The ashes from the heating plant, WFA8 and WFA9, consistently produced more compact matrices than the pellet-derived WFA3 and the bottom ash WBA, underlining how much origin and processing shape a material’s destiny.

The mineralogical detective work also turned up some genuinely unexpected chemistry. In the WFA8 blends, introducing cement promoted the formation of alunite, a potassium aluminum sulfate hydroxide phase not present in the unblended paste, alongside an array of compounds including syngenite, arcanite, serandite and harmotome. Thermogravimetric analysis at 7 and 28 days complemented the diffraction data, identifying calcium silicate hydrates and ettringite dehydrating between 50 and 200 degrees Celsius, AFm phases such as calcium monocarboaluminate and hemicarboaluminate decomposing between 200 and 300 degrees, portlandite dehydroxylating between 400 and 500 degrees, and carbonates releasing carbon dioxide from 500 to 800 degrees. The two techniques agreed closely, with the single discrepancy being ettringite in the bottom ash pastes, which thermal analysis detected but diffraction did not, likely because its concentration fell below the instrument’s detection limit.

What emerges from the study is a nuanced but practical message. Low doses of ordinary Portland cement, between 5 and 20 percent, can meaningfully upgrade pastes in which wood ash makes up as much as 95 percent of the solid content, provided the ash is fine and reasonably reactive. The improvements in stiffness, strength and microstructure are real, even if the resulting materials remain suited to low-strength applications such as lightweight binders rather than load-bearing concrete. The decisive variable, the authors conclude, is the variability of the ash itself: its origin, chemistry and mineralogy govern everything from phase development to porosity. That insight carries weight well beyond Normandy. With millions of tons of biomass ash generated annually and cement production responsible for a major share of global carbon dioxide emissions, even modest cement dosages that transform a landfill-bound waste into a functional building material represent a meaningful step toward circular, lower-carbon construction. The next challenge will be standardizing how ashes are characterized and selected, so that builders can trust what is in the bag before it ever reaches the site.

Subject of Research: The effect of low ordinary Portland cement content on the stiffness, microstructure and mineralogical composition of wood ash-based pastes

Article Title: Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes

Article References: Ndahirwa, D., Lenormand, H., Zmamou, H., Chenot, E., Potel, S., & Leblanc, N. (2026). Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes. Cleaner Engineering and Technology, 34, Article 101303. https://doi.org/10.1016/j.clet.2026.101303

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101303

Keywords: wood ash, ordinary Portland cement, Young's modulus, compressive strength, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, calcium silicate hydrate, ettringite, porosity, biomass ash, sustainable construction