The core problem with recycled aggregates is well understood. When old concrete is crushed, fragments of the original natural stone remain encrusted with hydrated cement paste, the soft, capillary-rich residue of the demolished structure. This adhered mortar is far weaker and more porous than the parent rock, so it acts as a built-in weak zone. Water penetrates readily, the interfacial transition zone between aggregate and new cement paste is compromised, and the resulting concrete loses compressive strength, tensile strength, and long-term durability. Previous strategies have attacked the problem in various ways: mechanical grinding, thermal treatment, carbonation, polymer impregnation, and, most directly, acid soaking. But the acids used so far have mostly been strong inorganic acids such as hydrochloric acid and sulfuric acid, and the literature shows these can do as much harm as good, dissolving cementitious phases deep inside the aggregate and undermining its structural integrity.

The Indian team, led by Anchal Aggarwal together with Sanjeew Kumar Singh and Gaurav Goel, took a different chemical route. They chose oxalic acid, an organic acid with the formula C2H2O4, best known as the compound that gives rhubarb its sharp taste and, less glamorously, as a chief constituent of kidney stones. The choice is chemically shrewd. When oxalic acid meets the calcium-rich phases of cement mortar, it does not simply dissolve them; it reacts with calcium ions to precipitate calcium oxalate, an insoluble crystalline salt that deposits directly on the aggregate surface. Rather than etching the aggregate away, the treatment converts the troublesome outer mortar into a dense, mineralized skin. The researchers describe this as surface activation rather than surface removal, a conceptual shift that turns out to be the key to the method’s success.

To understand exactly what the acid was doing at the microscale, the team subjected the treated aggregates to a battery of advanced characterization techniques, including X-ray diffraction, field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy with elemental mapping. Together, these tools painted a consistent picture. The treated surfaces were coated with calcium oxalate crystals, calcium had been redistributed across the interfacial region, and pozzolanic crystalline deposits had formed, all of which contribute to a denser, better-bonded interface between the recycled aggregate and the surrounding cement paste in fresh concrete. In effect, the treatment builds a chemically compatible bridge between old mortar and new cement rather than leaving a porous, weak seam.

The mechanical evidence was equally striking. In impact tests, oxalic acid-treated aggregates outperformed aggregates treated with sulfuric acid by 4.6 percent and hydrochloric acid-treated aggregates by 5.9 percent, confirming that the organic treatment preserves and even enhances aggregate integrity where inorganic acids degrade it. Water absorption, the single most telling indicator of aggregate porosity, told a similar story. Untreated recycled aggregates absorbed 3.68 percent water by weight, a figure that dropped to 2.80 percent after sulfuric acid treatment and 2.75 percent after hydrochloric acid treatment. Oxalic acid pushed the figure down to 2.45 percent, evidence of genuine surface pore refinement and reduced accessible porosity rather than mere surface cleaning.

The most consequential results came when the treated aggregates were cast into concrete and tested at 28 days. Compared with concrete made from untreated recycled aggregates, the oxalic acid-treated material showed a 29.5 percent increase in compressive strength and a 35.2 percent increase in splitting tensile strength. Those are not incremental gains; they represent the difference between recycled aggregate concrete that must be derated and blended with virgin stone and concrete that can genuinely compete with conventional mixes in structural applications. The authors attribute the superior performance to the combined effects of adhered mortar modification and the formation of a calcium oxalate-rich surface, emphasizing that the benefit comes not from simply washing the old mortar away but from chemically transforming it into something useful.

Beyond raw performance, the choice of oxalic acid carries practical and safety advantages that could matter enormously for industrial adoption. Concentrated hydrochloric and sulfuric acids are among the most hazardous reagents in routine industrial use, demanding corrosive-resistant equipment, specialized storage, and rigorous worker protection protocols. Oxalic acid, while still requiring sensible handling, is comparatively less hazardous to store and manage, which lowers both operational costs and the regulatory burden of scaling the process to the enormous throughputs that aggregate processing plants demand. For an industry that consumes billions of tons of material annually, even modest simplifications in chemical handling translate into significant economic and safety dividends.

The environmental case was reinforced by a preliminary screening-level carbon and life-cycle assessment, which indicated a potential environmental advantage for the oxalic acid route relative to conventional treatments. Life-cycle thinking is essential here, because a treatment that improves concrete strength but imposes a heavy chemical or energy footprint would simply shift the environmental burden rather than reduce it. The screening assessment suggests that the mild conditions, lower hazards, and improved durability of the resulting concrete combine to make the overall proposition favorable, though the authors are careful to frame this as a preliminary, screening-level result rather than a definitive full life-cycle audit.

The work also fits into a broader and rapidly evolving research landscape. Other groups have pursued accelerated carbonation of recycled aggregates, slag-coated carbonation, nano-silica hydrophobic treatments, cement slurry coating, and thermo-mechanical upgrading, each with its own trade-offs in cost, energy, and scalability. Calcium oxalate chemistry itself has been gaining attention in materials science, from biomimetic conservation treatments for carbonate building stones to newly developed oxalate-activated calcium silicate cements, suggesting that the Indian team’s approach taps into a wider vein of promising mineral chemistry. What distinguishes the new study is its application of that chemistry directly to the adhered mortar problem, converting the recycled aggregate’s greatest liability into a strengthening feature.

If the process can be scaled economically, the implications for urban mining and circular construction are considerable. Cities effectively sit atop vast above-ground mines of demolished concrete, and technologies that upgrade this waste into high-performance raw material could simultaneously relieve landfill pressure, reduce demand for virgin sand and gravel extraction, and cut the carbon intensity of new construction. The authors position oxalic acid treatment as a sustainable route to producing high-performance recycled aggregates for circular concrete applications, and their data lend that claim real substance. Much work remains, from optimizing acid concentration and treatment duration to validating long-term durability in field structures, but the central insight is likely to endure: sometimes the best way to deal with a weak surface is not to remove it, but to grow a better one in its place.

Subject of Research: Oxalic acid surface activation of recycled construction and demolition waste aggregates for high-performance circular concrete

Article Title: Upcycling of construction and demolition waste by oxalic acid surface activation of recycled aggregates: a novel processing approach

Article References: Aggarwal, A., Singh, S. K., & Goel, G. (2026). Upcycling of construction and demolition waste by oxalic acid surface activation of recycled aggregates: a novel processing approach. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38250-5

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38250-5

Keywords: construction and demolition waste, recycled aggregates, oxalic acid, calcium oxalate, concrete, circular economy, surface activation, water absorption, compressive strength, life-cycle assessment, sustainable construction, adhered mortar

A team of researchers led by Jie Wang and Kunfeng Kong collected mixed demolition waste from a recycling source in Zhengzhou, Henan Province, China. The material, derived from demolished brick-concrete buildings, was crushed, screened, and cleaned of impurities before testing. Particle-by-particle analysis showed that recycled crushed stone, recycled concrete, and recycled clay brick together accounted for more than 95 percent of the mass in every size fraction examined, with impurities below 3.5 percent. Because demolition waste from brick-concrete structures naturally contains all three components mixed together rather than separated, the researchers argue this composition is representative of what recycling plants actually supply, making large-scale reuse more practical than processes demanding careful component sorting.

The material’s physical properties reveal why it behaves so differently from natural aggregates. Its water absorption of 8.5 to 9.8 percent and crushing value of 29.5 percent are far higher than those of conventional fill materials, reflecting the porous, weaker nature of recycled concrete and clay brick particles. Its optimum moisture content of roughly 13.5 percent is considerably higher than that of natural granular fills of similar gradation, while its maximum dry density of 1.827 grams per cubic centimeter is markedly lower. In practical terms, this means field crews must control moisture carefully during compaction, allow time for water to redistribute within the absorbent particles, and accept a lighter but more void-rich skeleton that is more sensitive to drainage conditions.

To characterize the material’s static strength, the team prepared cylindrical specimens 150 millimeters in diameter and 300 millimeters tall, compacted at three degrees of compaction representing well-compacted, insufficiently compacted, and relatively loose states, and tested them in a large-scale triaxial apparatus under confining pressures of 50, 100, and 150 kilopascals. The saturated drained shear tests showed clear strain-softening behavior: specimens reached a peak deviatoric stress and then weakened as shearing continued. Both peak strength and residual strength rose substantially with confining pressure and compaction. When confining pressure increased from 50 to 150 kilopascals, residual strength climbed by 106.8 percent, 87.3 percent, and 121.0 percent for specimens compacted to 96, 92, and 88 percent, respectively, demonstrating that even loosely packed waste fill gains considerable post-peak capacity when laterally confined.

The stiffness results carried a subtler message. The secant modulus at 50 percent of peak stress, a standard measure of deformation stiffness, was far more sensitive to compaction at low confining pressure than at high. At 50 kilopascals, the modulus of 96-percent-compacted specimens exceeded that of 88-percent-compacted specimens by more than 111 percent. Yet raising confining pressure from 50 to 150 kilopascals improved the modulus by only 6.8 percent for the densest specimens, compared with 77.3 percent for those at 92 percent compaction. The implication is that loosely compacted waste fill benefits enormously from lateral confinement, making it best suited to subgrade zones where surrounding material can provide strong restraint, such as well-compacted internal fill layers or track-supporting layers.

Volumetric behavior added another dimension. Under shear, the specimens first contracted slightly and then dilated, with the maximum dilation exceeding that of conventional natural subgrade fills. Denser specimens dilated more because their tight initial contacts left little room for further compression, forcing deformation to proceed through particle rotation and interlocking. Looser specimens contracted instead, densifying progressively. The researchers note that this contraction-dilation response matters for railway engineering: contraction signals ongoing settlement accumulation, while dilation indicates a well-interlocked skeleton but, near critical stress levels, may disturb interfaces with overlying ballast or track layers.

The heart of the study, however, lay in its long-term cyclic triaxial tests, which subjected specimens to 10,000 loading cycles at 1 hertz under saturated drained conditions. Here the team introduced the shear stress ratio, or SSR, defined as the applied cyclic deviatoric stress divided by the material’s static peak strength measured under identical compaction and confinement. This strength-normalized measure proved decisive. When SSR remained below roughly 0.5, cumulative plastic deformation grew slowly and stabilized as the particle skeleton compacted and rearranged. Once SSR exceeded about 0.5, the fitted curves steepened sharply and deformation accelerated toward incremental failure. Grey relational analysis confirmed SSR as the single most influential variable, ranking ahead of confining pressure and degree of compaction.

Perhaps the most striking finding concerned densely compacted specimens under high stress. At an SSR of 0.8, the final cumulative deformation of the 96-percent-compacted specimens was about 3.7 times that of the 92-percent specimens, the largest relative increase among all compaction states. The explanation lies in the dense skeleton’s limited remaining void space: once cyclic stress approaches the mobilized strength capacity, particle sliding, abrasion, and breakage are repeatedly activated, and with no room left for gradual compaction, the structure responds through dilation and abrupt loss of interlocking. Looser fills, by contrast, can still absorb cyclic deformation through progressive densification. In other words, compaction alone does not guarantee stability if the traffic-induced stress level creeps too close to the material’s strength reserve.

The resilient modulus, which quantifies the recoverable elastic stiffness under repeated loading, generally increased with compaction, confining pressure, and cyclic deviatoric stress. Dropping compaction from 96 to 88 percent reduced the modulus by roughly 13 to 25 percent across the tested stress states. But the researchers caution against a tempting misreading: a rising resilient modulus does not necessarily indicate a healthy structure. Correlation analysis across 21 matched loading conditions found only a moderate linear association between modulus evolution and final cumulative plastic deformation, meaning the recoverable and irreversible components of deformation can evolve independently. A subgrade may appear stiff while quietly accumulating permanent settlement.

From these results the authors distill a preliminary design framework: compaction near 96 percent is preferred when cyclic stresses stay within the stable range; an SSR at or below 0.30 offers a conservative reference for long-term service; the range between 0.30 and 0.50 warrants caution; and SSR at or above 0.50, associated with accelerated deformation, should be avoided under conditions comparable to those tested. The team stresses that these thresholds are laboratory observations specific to the tested gradation, moisture state, and loading regime, not universal design limits, and that field-scale and in-situ verification remain necessary. Unresolved questions include the effects of wetting-drying cycles, freeze-thaw weathering, particle breakage quantification, and varying train speeds. Still, the study offers railway engineers a concrete, stress-based recipe for turning demolition debris into dependable subgrade, a step that could simultaneously ease landfill pressure and relieve the aggregate shortage confronting infrastructure construction worldwide.

Subject of Research: Strength and deformation behavior of brick-concrete construction waste used as railway subgrade fill

Article Title: Analysis of the strength and deformation characteristics of brick-concrete construction waste used as subgrade fill

Article References: Wang, J., Kong, K., Li, Y., Lin, L., Wang, Z., Chen, F., Yao, J., Wang, M., & Xiao, Y. (2026). Analysis of the strength and deformation characteristics of brick-concrete construction waste used as subgrade fill. Case Studies in Construction Materials, 25, Article e06565. https://doi.org/10.1016/j.cscm.2026.e06565

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06565

Keywords: construction waste, railway subgrade, recycled aggregates, triaxial testing, shear stress ratio, cumulative deformation, resilient modulus, strain softening, compaction, cyclic loading, geotechnical engineering, sustainable materials