The review, led by a team at Guangdong University of Technology with collaborators at Walailak University and Shenzhen University, synthesizes recent advances in multiscale characterization and computational modeling of geopolymer materials. Its central message is that the field is undergoing a structural transition, documented through bibliometric analysis, away from conventional phase and composition identification and toward precise atomic-scale analysis, dynamic in situ monitoring, and data-driven material design. In other words, geopolymer science is maturing from a descriptive craft into a quantitative, predictive discipline, and the tools driving that shift are the same ones that transformed semiconductor physics and structural biology: synchrotron radiation, neutron scattering, advanced electron microscopy, molecular simulation, and machine learning.

What makes geopolymers so difficult to characterize is their peculiar structural personality. Unlike crystalline ceramics, they are ordered over short and medium ranges but completely disordered over long ranges, forming three-dimensional aluminosilicate networks in which alkali cations balance the negative charge of aluminum tetrahedra. This amorphous nature defeats the workhorse technique of traditional materials science, X-ray diffraction, which relies on periodic lattice order to produce interpretable patterns. The situation is compounded by the growing trend of blending multiple industrial solid wastes into a single binder, which introduces substantial compositional complexity, heterogeneous gel formation, and dynamic structural reconstruction as the material cures and ages.

To pierce this amorphous fog, the review highlights the power of advanced spectroscopic techniques. Solid-state nuclear magnetic resonance spectroscopy, in particular, has become indispensable for resolving local atomic configurations, distinguishing silicon coordination environments, quantifying network connectivity, and identifying how heteroatoms such as iron, magnesium, and heavy metals are incorporated into the gel framework. Infrared spectroscopy, long used to track the progressive shift of silicon-oxygen-aluminum stretching bands during geopolymerization, now operates in time-resolved and spatially resolved modes that can watch gel nucleation unfold in real time. For iron-rich precursors such as red mud and volcanic ash, Mössbauer spectroscopy reveals the oxidation state and coordination of iron species, a critical variable because iron can either participate in the binder network or remain as inert inclusions that dictate strength and durability.

Perhaps the most striking technical development the review documents is the rise of pair distribution function analysis, a total-scattering method that extracts structural information from the diffuse scattering that conventional diffraction discards. Applied with synchrotron X-rays and neutrons, pair distribution function analysis has allowed researchers to reconstruct the atomic structure of geopolymer gels directly, resolving debates about the role of charge-balancing extra-framework aluminum and tracking how local structure evolves during the earliest stages of gel formation. In situ neutron pair distribution function experiments have even captured the structural evolution of geopolymer gels as they form, while related work has mapped how local structure changes with temperature, carbonation, sulfate attack, and high-temperature exposure, connecting atomic-scale chemistry to the degradation mechanisms that matter in service.

At larger length scales, advanced microscopy is revealing the hierarchical architecture that atomic spectroscopy cannot see. Scanning and transmission electron microscopy, increasingly combined with focused ion beam sample preparation and cryogenic transfer, expose the nanoscale morphology of gel particles and the interfacial transition zones that govern composite behavior. Cryo-electron microscopy has delivered a genuinely viral result in the cement science community: direct observation of multistep nucleation and growth of aluminosilicate gel, showing that these amorphous binders assemble through intermediate phases rather than simple precipitation. Meanwhile, atomic force microscopy and nanoindentation map mechanical heterogeneity across gel phases and interfaces, and quantitative backscattered electron imaging paired with energy-dispersive spectroscopy now allows full-component characterization of interfacial zones in recycled and fiber-reinforced geopolymer concretes.

Pore structure, the hidden skeleton that controls transport, durability, and freeze-thaw resistance, has likewise come into sharper focus. X-ray and neutron tomography reconstruct three-dimensional pore networks non-destructively, from laboratory micro-computed tomography to hard X-ray nanotomography capable of resolving features tens of nanometers across. Low-field nuclear magnetic resonance relaxometry has emerged as a rapid, non-destructive probe of pore size distribution and water states, tracking setting, reaction kinetics, and even the influence of paramagnetic iron in waste-derived binders. Small-angle neutron scattering resolves bimodal pore evolution during early curing, and neutron radiography and tomography visualize capillary water absorption and moisture transport in real time, exploiting the exceptional sensitivity of neutrons to hydrogen.

The review also emphasizes that simulation and artificial intelligence are no longer peripheral add-ons but central pillars of the field. Molecular dynamics simulations of sodium aluminosilicate hydrate and calcium aluminosilicate hydrate gels now illuminate dissolution mechanisms, adsorption of heavy metals and radionuclides, ion migration in nanopores, and the interfacial bonding between gels and aggregates or fibers, often in direct dialogue with experimental scattering data. On the data side, machine learning models predict compressive strength, freeze-thaw degradation, bond strength, and multi-objective performance trade-offs involving cost and carbon dioxide emissions, with interpretable and transfer-learning frameworks extending predictions to ultra-high-performance geopolymer systems. Emerging machine learning interatomic potentials promise to bring quantum-level accuracy to simulations of cementitious systems at unprecedented scale.

Why does all this matter beyond the laboratory? Geopolymers sit at the intersection of two global imperatives: decarbonizing construction and managing mountains of industrial waste. They have demonstrated potential in marine structures, road construction, nuclear and heavy metal waste immobilization, fire-resistant panels, 3D-printed elements, and even neutron-shielding composites for radiation protection. But every one of those applications depends on reproducible performance, and reproducibility demands structure-property relationships grounded in measurement rather than recipe. The characterization toolkit assembled in this review is precisely what converts a variable waste stream into an engineering material whose atomic network, gel hierarchy, and pore architecture can be tuned deliberately.

The authors’ synthesis points toward a future in which synchrotron beamlines, neutron facilities, automated microscopy, and machine learning pipelines operate as an integrated design engine, closing the loop from composition to structure to property to prediction. Their bibliometric evidence shows the literature itself shifting toward dynamic monitoring and data-driven design, a signal that the community is already building that engine. For a material class once dismissed as too messy for fundamental science, geopolymers are becoming a showcase for how modern multiscale characterization can tame even the most disordered matter, and the payoff may be a generation of sustainable, high-performance binders designed on the computer before they are ever mixed in the lab.

Subject of Research: Multiscale structural characterization and structure-property relationships of geopolymer materials

Article Title: Review: advanced characterization of geopolymer materials-from multiscale structural understanding to structure-property relationships

Article References: He, X., Pantongsuk, T., Chen, W., Deng, R., Yu, T., & Zhang, B. (2026). Review: advanced characterization of geopolymer materials-from multiscale structural understanding to structure-property relationships. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13826-1

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13826-1

Keywords: geopolymers, aluminosilicate binders, low-carbon cement, solid-state NMR, pair distribution function, synchrotron radiation, neutron scattering, electron microscopy, molecular dynamics, machine learning, industrial solid waste, structure-property relationships

The review classifies the candidate materials into six families: bio-derived supplementary cementitious materials and ashes, bio-based lightweight aggregates, natural-fiber reinforcement, expanded-perlite aggregate, perlite powder and fines, and hybrid systems that combine several of these. Each family attacks the carbon problem from a different direction. Supplementary cementitious materials replace clinker directly, reducing the most carbon-intensive component of the binder. Lightweight aggregates substitute for quarried sand and gravel, easing pressure on primary resources. Natural fibers add tensile reinforcement without steel. Perlite, meanwhile, contributes something ordinary concrete cannot: trapped air. When the volcanic glass is heated, it expands like popcorn, forming a honeycomb of glassy pores that insulates far better than dense mineral aggregate.

The physics underlying all of these substitutions is captured in what the authors call the strength–insulation trade-off, and it recurs with remarkable consistency across the 139 studies. Introduce more porous, lightweight constituents into a mix and two things happen simultaneously: density falls and thermal conductivity falls with it, which is exactly what builders of energy-efficient envelopes want. But the same pores interrupt the continuous skeleton of hardened cement paste that carries compressive load. Matrix continuity weakens, moisture sensitivity rises, and compressive strength tends to decline. Across the reviewed datasets, compressive strength generally increased with density, while thermal conductivity declined as porosity rose. In other words, the two most desirable properties of a building material pull against each other, and every formulation sits somewhere on that spectrum.

What makes the review valuable is that it does not stop at the trade-off; it identifies the levers that shift where a formulation lands on it. Constituent type, replacement level, particle size distribution, binder chemistry, curing regime, moisture condition, and pore structure all emerged as decisive variables. A mortar that swaps in too much porous aggregate collapses mechanically, but a controlled substitution with well-graded particles and a binder designed to bond around the pores can retain most of its strength while gaining substantial insulating capacity. Curing matters too, particularly for bio-based systems, where fibers and ashes can interfere with hydration or release sugars that retard setting if the mix design is not adjusted to compensate.

The headline numbers from the review demonstrate how far the best-performing systems have come. A perlite-powder mortar with 15 percent cement replacement and a 2 percent dose of nanosilica reached a compressive strength of 68.3 megapascals, a figure that comfortably exceeds many structural-grade conventional mortars and shows that pozzolanic perlite fines, aided by nanosilica’s ability to densify the interfacial zones, need not sacrifice mechanical performance. On the bio-based side, a corn-stalk-ash mortar achieved 50.15 megapascals at just 5 percent cement replacement, turning an agricultural waste stream into a functional binder component. At the opposite end of the performance spectrum, lime–hemp composites reported thermal conductivity as low as 0.070 watts per meter-kelvin, roughly an order of magnitude below conventional concrete, illustrating the insulating potential of plant-aggregate systems even though they serve non-structural roles.

These figures matter because the building envelope is where the embodied carbon of construction meets the operational carbon of heating and cooling. A wall material that insulates well reduces energy demand for decades, and if that same material also displaces clinker or quarried aggregate, it cuts emissions at both ends of its life cycle. The review’s environmental analysis emphasizes that these benefits were most evident precisely when the alternative materials displaced clinker or primary aggregates. However, the authors caution that carbon outcomes depend heavily on assumptions about processing energy, transportation distances, service life, and end-of-life handling. A bio-ash that must be dried and calcined at high temperature, or a lightweight aggregate hauled across continents, can erode much of its theoretical advantage. Life-cycle thinking, not just lab performance, determines whether a formulation is genuinely low-carbon.

The circular-economy framing is central to the review’s argument. Bio-based residues such as corn stalk ash are waste products of agriculture, and valorizing them in construction diverts them from burning or landfill while substituting for manufactured inputs. Perlite, though a mined natural material, is abundant and requires relatively modest energy to expand compared with the 1,450-degree-Celsius clinker kiln. Hybrid systems that pair the two, using perlite for insulation and bio-ashes for pozzolanic reactivity, showed particular promise in the synthesis because each constituent compensates for the other’s weaknesses: the ash contributes strength-giving chemistry while the expanded glass supplies lightweight porosity, allowing formulators to balance insulation, mechanical adequacy, moisture resistance, and carbon reduction simultaneously.

Moisture emerges as a recurring caveat in the evidence base. Porous bio-aggregates and plant fibers can absorb water, swell, and degrade, and their presence can raise the moisture sensitivity of the composite, with consequences for durability, freeze–thaw resistance, and indoor air quality. The review indicates that binder design and curing protocols are the main defenses: binders that encapsulate fibers thoroughly, particle treatments that reduce water uptake, and curing regimes that ensure full hydration before exposure all improve long-term performance. This is where laboratory results diverge most sharply from field behavior, and the authors note that the most favorable systems are those engineered with moisture performance in mind from the outset rather than treated as an afterthought.

The practical implication is that the future of low-carbon construction may be less about a single replacement for Portland cement and more about a portfolio of tailored mixes matched to application. Structural elements can tolerate modest insulation penalties and should favor high-strength formulations like the nanosilica-modified perlite-powder mortar. Non-structural envelope infill, roof insulation, and partition walls can prioritize thermal performance with lime–hemp and other highly porous composites. Hybrid bio–perlite systems occupy the middle ground, offering balanced multifunctional performance for building-envelope applications where both load capacity and insulation matter. The review’s synthesis suggests that controlled substitution, careful particle grading, deliberate binder design, and appropriate curing are the tools that let engineers move deliberately along the strength–insulation spectrum rather than accepting whatever compromise a naive mix delivers.

For a sector under intense pressure to decarbonize, the message of this synthesis is quietly radical: the raw materials for a circular, low-carbon building industry may already be sitting in volcanic deposits and farm fields. Expanded perlite and bio-based residues cannot eliminate the cement industry’s carbon problem on their own, and the trade-offs they impose are real. But 139 studies of accumulating evidence show that with disciplined engineering, waste streams can become binders, quarried stone can become insulating pores, and the walls of future buildings might simultaneously lock up agricultural residue and cut heating bills. The trade-off between strength and insulation is not a barrier; it is a design space, and the map to navigate it has just become considerably more detailed.

Subject of Research: Expanded perlite and bio-based residues as low-carbon cementitious building materials

Article Title: Expanded perlite and bio-based residues for circular low-carbon cementitious materials: a systematic review

Article References: Ashraf, U., Khattab, T., & Abu-Rayash, A. (2026). Expanded perlite and bio-based residues for circular low-carbon cementitious materials: a systematic review. Discover Sustainability. https://doi.org/10.1007/s43621-026-04834-x

Image Credits: AI Generated

DOI: 10.1007/s43621-026-04834-x

Keywords: expanded perlite, bio-based residues, low-carbon cement, supplementary cementitious materials, compressive strength, thermal conductivity, circular economy, lightweight aggregates, hemp-lime composites, waste valorization, building envelope, sustainable construction

The research, conducted by Ghazi Bahroz Jumaa of the Civil Engineering Department at the University of Garmian in the Kurdistan Region of Iraq, tackles one of the most safety-critical questions in reinforced concrete design: how much shear force can a beam resist before it fails catastrophically? Shear failure is notoriously brittle and sudden, unlike the gradual, warning-giving flexural failure that designers prefer to govern. For beams without stirrups, the closed steel hoops that normally provide shear reinforcement, the entire resistance must come from the concrete itself, making accurate prediction of that resistance a matter of structural life and death.

The heart of the problem lies in the messy, heterogeneous nature of RCA. When old concrete is crushed, each recycled particle is a composite: natural stone wrapped in a layer of adhered mortar that is often weaker, more porous, and riddled with micro-cracks from the crushing process. In a loaded beam, diagonal cracks propagate through the concrete web, and the rough crack faces resist sliding through a mechanism engineers call aggregate interlock. Weaker, more irregular recycled particles interlock less effectively, and the old mortar attached to them introduces additional weak zones. The result is a material whose shear behavior is inherently more variable than that of natural-aggregate concrete, and whose performance depends on the percentage of recycled content, a variable that most established design equations were never formulated to include.

Existing design provisions, such as those in the American Concrete Institute’s ACI 318 code and Europe’s Eurocode 2, were calibrated decades ago on natural-aggregate concrete. When applied to RCA beams, they can be either unconservative or wastefully conservative, and the experimental literature over the past two decades, from studies by Han and colleagues in 2001 through work by Rahal and Alrefaei, Ignjatović and co-workers, and many others, has documented a wide scatter in measured shear strengths. Empirical equations such as the classic Zsutty formulation, dating back to 1971, capture broad trends but struggle with the added complexity of recycled content. This is precisely the kind of nonlinear, multi-variable prediction problem where machine learning has begun to shine.

Jumaa assembled a database of 179 experimental RCA beam specimens drawn from published laboratory studies spanning more than two decades. Each specimen was characterized by the key parameters that govern shear capacity: beam width, effective depth, the shear span-to-depth ratio, the longitudinal reinforcement ratio, the concrete compressive strength, and, crucially, the percentage of recycled concrete aggregate replacing natural stone. The measured shear capacities ranged from small laboratory beams failing at around 12 kilonewtons to massive 400-millimetre-wide specimens carrying more than 860 kilonewtons, giving the models a rich and demanding training landscape.

Five machine learning algorithms were trained and compared under a rigorous validation protocol: k-Nearest Neighbors, Random Forest, Support Vector Machines, Gaussian Process Regression, and Artificial Neural Networks. Rather than relying on a single train-test split, which can flatter a model by luck of the draw, the study employed a 10-fold cross-validation scheme repeated 20 times. In this procedure the data are repeatedly shuffled and divided into ten subsets; the model trains on nine and is tested on the held-out tenth, with the process cycled through all folds and then repeated from scratch twenty times to average out randomness. This is the gold standard for demonstrating that a model’s accuracy reflects genuine learning of underlying physics rather than memorization of individual specimens.

The results were striking. Support Vector Machines emerged as the top performer, achieving a coefficient of determination of 0.933, meaning the model explained more than 93 percent of the variance in measured shear strength, with a root mean square error of 29.98 kilonewtons and a mean absolute error of just 13.08 kilonewtons. Gaussian Process Regression followed closely with an R-squared of 0.912 and the lowest mean absolute error of the trio at 11.81 kilonewtons, while Artificial Neural Networks achieved an R-squared of 0.892. All three outperformed both the newly proposed empirical equations, including a modified Zsutty equation and an ACI 318-style equation incorporating RCA content, and the traditional design codes themselves. Support Vector Machines, which work by mapping inputs into a high-dimensional feature space where a maximum-margin boundary separates the data, proved especially adept at handling the nonlinear interactions between beam geometry, material strength, and recycled content.

Beyond raw accuracy, the study included a parametric analysis that interrogated how the trained models respond when individual variables change, a crucial sanity check for any data-driven engineering tool. The machine learning predictions reproduced physically sensible trends: shear capacity increased with beam depth and width, and decreased as the recycled aggregate content rose, consistent with the known degradation of aggregate interlock in RCA concrete. This alignment between statistical learning and structural mechanics matters enormously, because a black-box model that contradicts physical intuition would be dangerous to trust, no matter how impressive its error metrics appear on paper.

The practical implications reach well beyond the laboratory. As governments worldwide push circular-economy mandates and green building standards, the volume of RCA entering structural applications is set to grow sharply, and the fib Model Code 2020 and the next generation of Eurocode 2 are actively working toward codified design rules for recycled aggregate concrete structures. Reliable predictive tools are the prerequisite for such codification: engineers need confidence that a beam designed with, say, 50 or 100 percent recycled coarse aggregate will carry its intended shear load with the same margin of safety as a conventional beam. A validated machine learning model, trained on nearly two hundred real experiments, offers exactly that confidence, and could inform both code committees drafting new provisions and practitioners evaluating existing structures built with sustainable materials.

The study also fits into a broader movement sweeping through structural engineering, in which data-driven methods complement, and sometimes surpass, hand-derived empirical formulas. Researchers have already applied neural networks and related techniques to predict the shear capacity of fiber-reinforced polymer concrete beams, the compressive strength of lightweight and recycled concretes, and bond strength in reinforced concrete members. What distinguishes the present work is its focus on the specific, under-addressed case of stirrup-free RCA beams and its head-to-head comparison of five algorithms under a demanding cross-validation protocol. As Jumaa notes in the paper, the work represents a step forward in using machine learning to enhance structural engineering, with the potential to make a significant impact in the field of sustainable materials. If the construction industry is to recycle its own waste at scale, it may well need algorithms, as much as new mix designs, to make the resulting structures safe.

Subject of Research: Machine learning prediction of shear strength in reinforced recycled aggregate concrete beams without stirrups

Article Title: Shear strength prediction of reinforced recycled aggregate concrete beams without stirrups using soft computing and empirical methods

Article References: Jumaa, G. B. (2026). Shear strength prediction of reinforced recycled aggregate concrete beams without stirrups using soft computing and empirical methods. Neural Computing and Applications, 38(19), Article 771. https://doi.org/10.1007/s00521-026-12488-z

Image Credits: AI Generated

DOI: 10.1007/s00521-026-12488-z

Keywords: machine learning, recycled concrete aggregate, shear strength, reinforced concrete beams, stirrups, Support Vector Machines, artificial neural networks, Gaussian Process Regression, ACI 318, Eurocode 2, sustainable construction, structural engineering

The research team, led by Xiaojun Zhou, Chenxi Wang, and Chang Cai, systematically tested how dosages of iron oxide red (Fe₂O₃) ranging from 0 to 10 percent by mass of the cementitious materials affect the workability, mechanical performance, color, and long-term stability of cement mortar. Unlike many pigments that interfere with cement chemistry, iron oxide red turned out to be a largely inert guest in the system, and its physical presence produced a surprising cascade of beneficial effects at the right dose.

The workability results followed a predictable but important pattern. Without pigment, the mortar achieved a flow spread of 315 millimeters at a fixed superplasticizer dosage. Up to 4 percent pigment, the flow barely changed. Beyond that, however, flow declined linearly, dropping to 260 millimeters at 8 percent and 240 millimeters at 10 percent, the point at which casting became impractical. The culprit is geometry: the pigment particles are significantly finer than cement grains, so each added dose raises the total specific surface area of the powder blend, adsorbing more mixing water and leaving less free water to lubricate the paste.

Strength told a more interesting story. Both compressive and flexural strengths at 7 and 28 days rose and then fell with increasing pigment content, peaking at 8 percent. At that optimum, the 28-day compressive strength reached 71.9 megapascals, an 11.1 percent gain over the pigment-free reference, while flexural strength hit 12.3 megapascals, a 16.0 percent improvement. Earlier studies on iron oxide colorants had reported strength losses of up to 30 percent, making this finding notable. The team attributes the gains not to any chemical transformation but to particle packing: the micron-scale pigment grains fill the voids between cement particles and hydration products, densifying the matrix.

Microstructural analysis backed this interpretation. X-ray diffraction showed no new crystalline phases forming at any dosage, confirming that the pigment does not participate in hydration reactions. Scanning electron microscopy revealed progressively denser paste microstructures with increasing pigment content, and quantitative air-void analysis using the ASTM C457 method showed total air content falling by 46 percent at the 8 percent dosage. Most strikingly, the proportion of harmful pores larger than 200 micrometers dropped from 68 percent to 52 percent, while fine pores below 100 micrometers increased from 15 percent to 22 percent, a wholesale shift of the pore network toward smaller, better-connected scales.

Color performance was tracked using the CIELAB system, the international standard for quantifying perceived color. As dosage climbed from 0 to 10 percent, the redness coordinate a surged from 0.23 to 23.54 at 7 days, while lightness L fell as the pigment’s strong absorption of visible light reduced surface reflectance. The total color difference ΔE reached 20.1 at 8 percent, far beyond the threshold of 3 at which the human eye clearly distinguishes two colors. Importantly, the gains saturated above 6 percent dosage, as higher paste viscosity caused pigment particles to agglomerate and stop contributing to light reflection, meaning more pigment eventually buys diminishing chromatic returns.

Two mix-design variables proved surprisingly irrelevant to color. Varying the water-to-binder ratio from 0.32 to 0.40 produced a maximum ΔE of just 1.84, and sweeping the binder-to-sand ratio from 0.54 to 0.94 kept total color differences below 3.0. The decisive factor was instead the color of the cementitious matrix itself. When ordinary gray Portland cement and dark fly ash were replaced with white cement and pale yellow phosphorus slag, redness values jumped dramatically, and even at half the pigment dosage the light-colored system outperformed the dark one. A bright background simply lets the pigment’s selective reflection of red wavelengths shine through unmasked by gray hydration products.

Durability questions were addressed through a 56-day natural exposure test conducted through summer conditions, with 26 rainy days and mean temperatures rising from 22 to nearly 27 degrees Celsius. After an initial week in which surface lightness rose about 30 percent as free water evaporated and particles packed more tightly, both lightness and redness stabilized, with subsequent changes never exceeding one unit. The pigment’s photochemical inorganic structure resists degradation and fading. Shrinkage data added a bonus: at 56 days, pigmented mortar showed 790 microstrain versus 880 in the reference, a 10.2 percent reduction, again credited to the micro-filling effect that lowers capillary porosity and the capillary tension driving shrinkage.

The environmental accounting complicates the picture in an instructive way. A cradle-to-gate life cycle assessment following ISO 14040 and 14044 found that cementitious materials dominate the carbon footprint, contributing 82.7 to 87.1 percent of total emissions, while the pigment, despite representing only 8 percent of the binder mass, accounted for 12.5 to 16.7 percent because its emission factor of 1.6 kilograms of CO2-equivalent per kilogram vastly exceeds that of sand or industrial by-products. Sensitivity analysis showed that even a ±50 percent swing in the pigment’s emission factor moved total emissions by only ±8.34 percent, keeping the conclusions robust. The clearest decarbonization lever remains substituting clinker with fly ash and yellow phosphorus slag, which simultaneously lowers emissions and, by brightening the matrix, enhances the pigment’s coloring efficiency.

Taken together, the study reframes iron oxide red from a mere cosmetic additive into a multifunctional mineral admixture. Within the tested range, 8 percent dosage emerges as the sweet spot, delivering saturated red color, double-digit strength gains, reduced shrinkage, refined pore structure, and stable appearance under weathering, all without altering the fundamental hydration chemistry of cement. For architects and engineers seeking facades that are colored to their core, the message is that the pigment in the mix may be doing far more work than its hue suggests, provided the dosage, the binder brightness, and the clinker content are chosen with care.

Subject of Research: Effects of iron oxide red pigment on the color, mechanical performance, microstructure, and carbon footprint of colored cement mortar

Article Title: Study on the influence of iron oxide red colorant on the color and performance of colored mortar

Article References: Zhou, X., Wang, C., Cai, C., Jiang, J., Zhang, Y., & Luo, H. (2026). Study on the influence of iron oxide red colorant on the color and performance of colored mortar. Case Studies in Construction Materials, 25, Article e06596. https://doi.org/10.1016/j.cscm.2026.e06596

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06596

Keywords: iron oxide red pigment, colored mortar, cement-based materials, compressive strength, CIELAB color system, pore structure, shrinkage, life cycle assessment, carbon emissions, white cement, fly ash, construction materials

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

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

The team’s starting point was the engineered cementitious composite, or ECC, a class of material designed to bend without shattering. Instead of failing with a single catastrophic crack, a well-designed ECC strain-hardens: once the matrix cracks, embedded fibres bridge the gap and the load keeps rising as a network of fine, tightly controlled microcracks spreads through the material. Polyethylene (PE) fibres are the stars of this show. Previous studies have reported tensile strain capacities of six to eight percent in PE-reinforced ECCs—hundreds of times the stretch of ordinary concrete—along with crack widths fine enough to keep water and corrosive agents out. But PE fibres are slippery and relatively compliant, which means they do little to raise the stress at which the very first crack forms.

That is where the hybrid strategy comes in. The researchers combined PE fibres with alkali-resistant (AR) glass fibres, which are stiffer, bond more strongly to the matrix, and are durable in the alkaline chemical environment of cement. The logic is complementary: glass fibres stiffen the composite and delay crack initiation, while PE fibres carry the load through the long post-cracking phase. Both fibre types were 12 millimetres long, with the glass fibres slightly thinner at 18 micrometres versus 24 micrometres for PE, and the glass fibres carrying more than 1000 megapascals of tensile strength against 3000 megapascals for PE. Crucially, the total fibre volume was held constant at two percent across all mixes, so any change in behaviour could be attributed purely to the ratio between the two fibre types.

The binder itself was chosen with the climate in mind. Rather than ordinary Portland cement, the mixes used a blend of 30 percent OPC, 20 percent reactive magnesium oxide and 50 percent ground granulated blast-furnace slag, a by-product of steelmaking. The high slag content slashes clinker consumption, while the reactive MgO promotes the formation of brucite and magnesium silicate hydrate phases that densify the matrix and improve strength retention. Water-to-binder and sand-to-binder ratios were fixed at 0.25 and 0.36. Five mixes were tested: an unreinforced control, a conventional 2 percent PE mix, and three hybrids in which 0.5, 0.75 and 1.0 percent of the PE was swapped for glass fibre.

The mechanical results reveal a clear trade-off. In compression, the plain control reached 71.90 megapascals, the highest of all mixes, thanks to its superior flow and compaction. Adding fibres reduced compressive strength modestly for the PE-only mix (67.31 megapascals, a 6.4 percent drop) but progressively more as glass content rose: 64.02, 60.11 and 54.47 megapascals for the three hybrids. The researchers attribute this to fibre agglomeration and trapped air, which increase porosity and weaken the fibre-matrix interface. Still, two of the three hybrids remained above the 55-megapascal threshold that American Concrete Institute guidelines use to define high-strength concrete.

In tension, the picture flipped. The unreinforced matrix cracked at about 3.1 megapascals and immediately failed, with a negligible strain capacity of 0.014 percent. Every fibre-reinforced mix, by contrast, satisfied the ECC ductility criterion of exceeding 0.5 percent tensile strain. The PE-only mix delivered the greatest stretch at 5.91 percent strain with a peak stress of 6.55 megapascals, but its first-cracking strength of 2.80 megapascals was the lowest among the fibre mixes. Replacing a quarter of the PE with glass fibre produced the most balanced tensile response: the 1.25P0.75G mix combined the highest first-cracking strength of 4.13 megapascals with a peak stress of 6.12 megapascals and a still-respectable 2.84 percent strain capacity. Beyond that ratio, ductility fell steadily, dropping to 2.08 percent when the fibres were split evenly.

Flexural testing told a similar story with a twist. The plain beam snapped at 2.99 megapascals after just 0.35 millimetres of deflection. All fibre mixes showed deflection-hardening, but the sweet spot was the 1.5P0.5G mix, which paired a first-cracking strength of 7.81 megapascals and a peak flexural stress of 13.78 megapascals with the largest ultimate deflection of any mix, 21.66 millimetres—actually exceeding the PE-only mix’s 19.95 millimetres. The small dose of glass fibre delayed crack localisation while the remaining PE fibres sustained large post-cracking deformation. Pushing glass content higher raised first-cracking resistance further, up to 10.68 megapascals in the 1P1G mix, but shrank deflection capacity to 14.82 millimetres, confirming that PE fibres are the indispensable ingredient for bending ductility.

Perhaps the most technically impressive part of the study is the imaging. X-ray micro-computed tomography is a powerful non-destructive tool, but PE fibres attenuate X-rays so weakly that they are nearly invisible against a cementitious matrix, which is why most micro-CT studies have focused on steel fibres. The researchers solved this with a dedicated workflow on a SkyScan 1273 scanner: 100 kilovolts, an aluminium filter, 2500-millisecond exposures and a voxel size of about 6 micrometres, followed by segmentation that combined grey-value thresholds with an aspect-ratio filter to pick out elongated PE-fibre-like features geometrically. The result was full three-dimensional maps of pores, glass fibres and PE fibres. These showed fibre clustering worsening as glass content increased, and detectable porosity climbing from 3.47 percent in the plain matrix to 5.71 percent in the 1P1G mix, with the pore-size distribution broadening toward larger voids. A Pearson correlation analysis reinforced the trends: glass fibre content correlated strongly and positively with porosity and flexural first-cracking strength, but negatively with compressive strength and flowability, while PE content tracked tensile strain capacity and deflection with correlation coefficients around 0.98 and 0.94.

The sustainability accounting adds an honest caveat. The binder dominated the carbon footprint, with reactive MgO the largest single contributor, and fibres added further burden: embodied carbon rose from 0.864 tonnes of CO2-equivalent per cubic metre for the plain mix to 0.963 for the best hybrid, while primary energy climbed from 4.462 to 6.854 gigajoules per cubic metre, driven largely by the energy-intensive PE fibre. Even so, the 1.5P0.5G mix emerged as the best overall compromise, delivering high flexural strength, exceptional deflection capacity and acceptable compressive performance at a moderate environmental cost. For serviceability-critical elements such as thin façade panels, where crack control and deformation tolerance matter more than raw strength, hybrid PE-glass fibre reactive magnesia-slag composites offer a credible path toward low-clinker infrastructure that bends instead of breaking—and, thanks to the adapted micro-CT method, engineers can now see exactly what is happening inside.

Subject of Research: Hybrid polyethylene and glass fibre reinforcement of reactive magnesia-slag cementitious composites

Article Title: Effect of hybrid fibres on reactive magnesia-slag composites: mechanical behaviour, 3D microstructure by micro-CT and sustainability

Article References: Iqra, Soe, K., Yang, R. C., & Zhang, Y. (2026). Effect of hybrid fibres on reactive magnesia-slag composites: mechanical behaviour, 3D microstructure by micro-CT and sustainability. Case Studies in Construction Materials, 25, Article e06576. https://doi.org/10.1016/j.cscm.2026.e06576

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06576

Keywords: engineered cementitious composites, polyethylene fibres, alkali-resistant glass fibres, reactive magnesia, ground granulated blast-furnace slag, strain-hardening, micro-CT, porosity, flexural behaviour, embodied carbon, low-clinker binder, fibre dispersion

The research, led by Rachida Idir of Cerema and University Gustave Eiffel together with colleagues from the University of Montpellier’s LMGC laboratory and IMT Mines Alès, including Katerina Ioannidou and Gwenn Le Saout, examined two commercially important types of carbon black in cement pastes. The first, furnace black, is the industrial standard, produced by partially burning heavy petroleum feedstocks in controlled furnaces. The second, acetylene black, is made by decomposing acetylene gas and is prized for its high purity and distinctive structure. The team varied the amount of carbon black, the water-to-cement ratio, and the dosage of superplasticizer, the chemical admixture used to keep workable concrete flowing without excess water, to map out how each parameter shaped the electrical behavior of the resulting paste.

The physics at the heart of the work is the percolation threshold, a concept borrowed from statistical physics that describes when disconnected particles suddenly link up into a continuous network. Ordinary cement paste is a poor conductor, relying mostly on ions dissolved in its pore water to carry current. Dispersed carbon black particles change that. Below a critical loading, the particles remain isolated islands, and the paste barely conducts better than before. Once that threshold is crossed, conductive pathways snake through the matrix and resistance plummets. In these experiments, the threshold generally appeared around 2 percent carbon black by weight of cement, and conductivity values above 1 siemens per meter, a level useful for practical sensing and heating applications, were typically reached at dosages of about 3 to 4 percent.

Under the conditions tested, acetylene black proved more effective than furnace black at building these conductive networks. That advantage matters because it means less additive is needed to reach a target conductivity, which in turn reduces both the material cost and the environmental footprint of the composite. The finding aligns with a growing body of literature on carbon-modified cementitious materials, which has explored everything from carbon nanotubes and nanofibers to carbon fibers and graphite as conductive fillers. Carbon black occupies an attractive middle ground in that landscape: it is far cheaper than nanotubes, easier to disperse than many high-aspect-ratio fillers, and available at industrial scale, making it a realistic candidate for construction applications where cost per cubic meter is decisive.

Conductive cement is not a laboratory curiosity. Researchers have already demonstrated electrically heated pavement systems that keep airport runways ice-free without salt, and self-sensing composites whose electrical resistance shifts as cracks form or loads change, allowing structures to report their own health. The piezoresistive effect in carbon-loaded cement means that squeezing the material alters its conductivity, so a bridge deck embedded with such sensors could, in principle, flag overloads or hidden damage continuously. Joule heating, where current passing through the resistive material generates warmth, underpins the deicing concept. What has been missing from many of these demonstrations is a rigorous accounting of the climate burden that the conductive additive adds to an already carbon-intensive material.

That accounting is the second pillar of the new study. The team performed a life cycle assessment focused on the global warming potential, or GWP, of the paste formulations, considering both the production of the carbon black itself and the full paste recipe. At the scale of the raw materials, the differences between acetylene black and furnace black were significant, reflecting the very different industrial processes behind them. But once the analysis moved to the scale of the complete paste, those differences shrank dramatically. The reason is sobering: cement clinker production, with its limestone calcination and kiln fuel emissions, so dominates the carbon footprint of the paste that the choice between two carbon blacks becomes a secondary consideration. Cement remained the overwhelming contributor to GWP across all formulations.

To connect functionality with climate impact in a single metric, the researchers introduced a novel indicator they call the Performance Impact Indicator, or PII, defined as the ratio of global warming potential to electrical conductivity. This simple ratio captures a design tension: adding more carbon black raises the GWP of the paste, but it also raises conductivity, and the question is which effect wins. The experiments delivered a counterintuitive and encouraging answer. Higher carbon black contents led to lower PII values, meaning that the conductivity gains outpaced the added emissions. In other words, once past the percolation threshold, each additional increment of carbon black buys more electrical performance per unit of climate impact, at least within the range investigated. For designers of smart concrete, this suggests that under-dosing the filler may be the worst of both worlds, delivering neither strong conductivity nor an efficient carbon-to-performance trade.

The study also looked beyond conventional supply chains to two alternative routes for producing carbon black, examining them from an environmental perspective. The first is a process associated with hydrogen co-production, in which methane is pyrolyzed, splitting natural gas into solid carbon and hydrogen gas rather than burning it. This route, sometimes linked to so-called turquoise hydrogen, has attracted attention as a way to produce both a low-carbon fuel and carbon black with potentially far lower emissions than the furnace process. The second alternative is based on waste tire pyrolysis, in which end-of-life tires are heated in the absence of oxygen to recover oil, gas, steel, and a recycled carbon black. Both scenarios suggest that lower-impact carbon black supply routes may deserve serious consideration for conductive cement applications, potentially decoupling the growth of smart concrete from additional fossil feedstock consumption while giving waste tires a second life.

The broader significance of the work lies in its framing. Multifunctional materials are often evaluated on performance alone, with sustainability treated as an afterthought or a marketing claim. By pairing systematic electrical measurements with life cycle thinking, and by proposing a metric that binds the two together, the French team offers a template for how emerging construction technologies should be assessed. The PII approach echoes earlier eco-efficiency work in cement science, which sought to measure how much functional service a structure delivers per unit of environmental burden. Applying that logic to conductive cement reveals a genuinely useful insight: the environmental case for these materials improves as the conductive network matures, provided the dosage is chosen wisely.

Challenges remain before carbon-black cement becomes a routine building material. Dispersion of nanoparticles in the harsh, alkaline environment of fresh cement is notoriously difficult, and researchers continue to explore techniques such as sonication and optimized admixture chemistry to achieve uniform networks. Durability over decades of weathering, long-term stability of the conductive pathways, and the cost of scaling up alternative carbon black production routes all require further study. The authors also note that their environmental comparison at the paste scale was limited to the GWP indicator, leaving other impact categories for future work. Yet the direction of travel is clear. As infrastructure ages and cities demand smarter, more resilient materials, cement that can carry both loads and current may find its place, and this study shows that the greenest version of that future depends not just on what goes into the concrete, but on where its carbon comes from.

Subject of Research: Conductive cement pastes modified with carbon black and their electrical performance and global warming potential

Article Title: Carbon black from multiple production routes in conductive cement pastes: balancing multifunctional performance and environmental burden

Article References: Idir, R., Souane, S. F., Touati, F., Ioannidou, K., & Le Saout, G. (2026). Carbon black from multiple production routes in conductive cement pastes: balancing multifunctional performance and environmental burden. Journal of Industrial Ecology, 30(4), 2071-2088. https://doi.org/10.1007/s44498-026-00140-x

Image Credits: AI Generated

DOI: 10.1007/s44498-026-00140-x

Keywords: carbon black, conductive cement, cement paste, percolation threshold, electrical conductivity, life cycle assessment, global warming potential, furnace black, acetylene black, waste tire pyrolysis, turquoise hydrogen, smart materials

The core challenge lies in something invisible to the naked eye: the interfacial transition zone, or ITZ, the thin band of cement paste that bonds directly to the surface of each aggregate particle. In ordinary concrete, the ITZ is often the weakest region of the whole material, but coal gangue makes the problem dramatically worse. The rock is riddled with pores, bedding-related defects, pre-existing microcracks, and weak mineral regions, and its exposed surface is dominated by aluminosilicate minerals that react only weakly under normal curing conditions. Although the angular, rough texture of crushed gangue does provide good mechanical interlocking with the surrounding paste, that advantage cannot compensate for the discontinuous, mechanically feeble boundary that forms where aggregate meets binder. When water finds its way into these surface-connected defects, differences in pore structure and stiffness between the aggregate and the paste concentrate stress near the interface and invite cracks to begin exactly where the material can least afford them.

The research team attacked this weakness with three humble modifiers, all readily available in coal-mining regions: sodium silicate, silica fume, and limestone powder. Each works through a different route. Sodium silicate can penetrate and seal open pores and microdefects, while its soluble silicate species participate in reactions near the aggregate surface that improve interfacial continuity. Silica fume, an ultrafine powder of nearly pure amorphous silica, physically fills surface irregularities and interfacial voids and can later react pozzolanically to refine the surrounding cementitious products. Limestone powder contributes mainly through particle packing, heterogeneous nucleation of hydration products, and carbonate-related interactions with aluminate-bearing phases. The researchers applied each modifier at dosages of 2, 3, and 4 percent by aggregate mass, dry-mixing the powder onto washed, surface-dried gangue particles before the concrete was batched.

The concrete itself was deliberately unconventional. Instead of ordinary Portland cement, the team used a coal-based solid-waste binder containing 50 percent benchmark cement, 25 percent calcined coal gangue powder, 10 percent limestone powder, 10 percent fly ash, and 5 percent flue gas desulfurization gypsum, a formulation that turns more mining and power-plant waste into a useful building material. The calcined gangue powder was produced by heating ground raw gangue first to 500 degrees Celsius and then to 800 degrees, activating its aluminosilicate content. All mixtures shared a fixed water-to-binder ratio of 0.40 and an aggregate-to-binder ratio of 1.50, so that the only variable was the type and dosage of the surface coating. Compressive strength screening at 3, 7, and 28 days identified the best dosage for each modifier: 4 percent sodium silicate, 3 percent silica fume, and 4 percent limestone powder.

The most revealing part of the study came from nanoindentation, a technique that presses a microscopic diamond tip into a polished specimen surface to measure local stiffness point by point. The researchers ran indentation paths of 25 points, spaced 10 micrometers apart, across the aggregate, the ITZ, and the paste on either side. In every mixture, the modulus held steady at roughly 50 to 55 gigapascals inside the aggregate, plunged within the interfacial zone, bottomed out near 110 micrometers from the boundary, and then recovered toward the paste. But the depth of that plunge depended strongly on the treatment. The unmodified concrete reached a minimum of about 18 gigapascals; limestone powder raised it to 22, silica fume to 26, and sodium silicate to 33 gigapascals, the latter producing the smoothest mechanical transition across the entire interface.

Scanning electron microscopy gave those numbers a visible structure. The unmodified interface showed coarse residual particles, loose agglomerates, open pores, and discontinuous patches. Sodium silicate treatment, by contrast, produced abundant flocculent, fibrous, and fine granular products distributed around the aggregate surface, yielding a comparatively continuous and compact boundary. Silica fume yielded a more uniform fine-grained morphology, though some spherical residual particles remained visible. Limestone powder produced a heterogeneous picture, with compact product-rich regions coexisting alongside loose, porous zones and needle-like, plate-like reaction products morphologically consistent with carbonate-bearing AFm-type phases such as monocarboaluminate and hemicarboaluminate, though the authors caution that SEM alone cannot confirm phase identity.

Then came the durability trials, framed in an unusual but physically precise vocabulary. Rather than the conventional labels of wet-dry and freeze-thaw cycling, the researchers speak of liquid-vapor and liquid-solid phase cycling, emphasizing what the pore water itself is doing. In the liquid-vapor test, specimens spent 8 hours immersed in water, 8 hours drying naturally, and 8 hours in a 75-degree-Celsius oven, repeating the absorption-evaporation cycle up to 30 times. In the liquid-solid test, a rapid freeze-thaw apparatus swung the specimen core temperature between minus 35 and plus 20 degrees Celsius every 5 hours, forcing pore water to freeze, redistribute, and melt again and again. The results diverged sharply. After 30 liquid-vapor cycles, silica fume-modified concrete lost only 9.3 percent of its compressive strength, dropping from 27.9 to 25.3 megapascals, while the unmodified control lost 18.8 percent. Freezing proved far more brutal: after 15 liquid-solid cycles, the unmodified and limestone-modified specimens had lost roughly 70 and 67 percent of their strength, and by 30 cycles both had collapsed beyond the point of valid testing.

The survivors told the most important story. Sodium silicate-modified concrete, which had not been the best performer in the wet-dry regime, proved the champion of freeze-thaw resistance, retaining the highest residual strength, 9.7 megapascals after 30 cycles, and enough structural integrity to keep testing. Post-cycling nanoindentation after 15 cycles confirmed that the damage was concentrated precisely where the team expected: the aggregate region barely changed, the paste weakened moderately, and the ITZ deteriorated most severely. Silica fume’s interface lost only 3.8 percent of its minimum modulus under liquid-vapor cycling, while sodium silicate’s interface retained both the highest absolute modulus and the smallest relative loss, 24.8 percent, under liquid-solid cycling. The lesson is that different coatings build different kinds of interfacial quality, and the best choice depends on the environment the concrete will face.

The mechanistic explanation ties the whole picture together. Surface modification does not eliminate water-induced deterioration; it delays its initiation, localization, and propagation within the ITZ. Silica fume’s fine particles plug the small voids where moisture redistribution would otherwise nucleate microcracks, making it ideal for hot, drying climates with repeated wetting. Sodium silicate, supplying soluble silicate species into an alkaline local environment, appears to foster a more continuous, product-rich interfacial fabric that can distribute the internal pressures generated by ice formation and delay debonding and crack coalescence, making it the better choice for cold regions. Limestone powder, with its more heterogeneous product distribution, offered the least protection against either regime. The authors note that water-migration pathways were inferred from combined macroscopic, microscopic, and nanoindentation evidence rather than directly measured, and that local alkalinity changes and individual reaction products were not quantified.

What makes this work resonate beyond the laboratory is its double dividend: it converts an environmental liability into infrastructure material while simultaneously making that material tougher than anyone expected. Because the binder itself is built from calcined coal gangue, fly ash, and desulfurization gypsum, an entire concrete could one day be assembled almost entirely from industrial byproducts, with nothing more exotic than sodium silicate, silica fume, or ground limestone standing between a weak interface and a durable one. For coal regions searching for a future after mining, and for a construction sector under pressure to cut its carbon and resource footprint, the humble act of coating a waste rock particle before it enters a mixer may prove to be one of the most consequential steps in sustainable materials engineering.

Subject of Research: Surface modification of coal gangue aggregate to strengthen the interfacial transition zone in coal-based solid-waste concrete

Article Title: Surface modified coal gangue aggregate in coal-based solid-waste concrete: ITZ strengthening and resistance to water phase-transition cycling

Article References: Wang, Y., Yang, Y., Yu, C., Li, R., Chang, J., & Zhang, T. (2026). Surface modified coal gangue aggregate in coal-based solid-waste concrete: ITZ strengthening and resistance to water phase-transition cycling. Case Studies in Construction Materials, 25, Article e06572. https://doi.org/10.1016/j.cscm.2026.e06572

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06572

Keywords: coal gangue, concrete, interfacial transition zone, surface modification, sodium silicate, silica fume, limestone powder, freeze-thaw resistance, solid waste recycling, nanoindentation, sustainable construction, coal-based binder

The motivation is twofold. Cement production is one of the most energy-intensive industrial processes on the planet, responsible for an estimated 7 to 8 percent of global anthropogenic carbon dioxide emissions, while concrete itself is the most widely consumed manufactured material on Earth, with annual usage exceeding 4 billion metric tons. Replacing even a modest fraction of cement with an industrial by-product therefore carries enormous environmental leverage. At the same time, self-compacting concrete, first developed in Japan in the late 1980s, has become a favored technology for densely reinforced and geometrically complex structures because it flows under its own weight and compacts without vibration. Its precise mix design requirements make it particularly receptive to supplementary cementitious materials such as fly ash, slag, and silica fume, which have long been used to cut cement content. Coal slurry powder, however, has remained largely unexplored, especially at high temperatures.

The raw material in this study was air-dried and ball-milled before use, yielding a powder with a specific surface area of roughly 25 to 30 square meters per gram and a specific gravity of 2.35. X-ray fluorescence analysis revealed a predominantly siliceous-aluminous composition, containing approximately 35 to 45 percent silicon dioxide, 8 to 12 percent alumina, 4 to 6 percent iron oxide, and 2 to 4 percent calcium oxide, with the combined silica, alumina, and iron oxide content exceeding 50 percent. The loss on ignition was 13.9 percent, a figure the author treats cautiously because it may include volatile components beyond residual carbon. Importantly, the study stops short of classifying the powder as a confirmed pozzolan, since a standardized pozzolanic activity index and quantitative mineral-phase analysis were not performed; its contribution is interpreted as physical filler action combined with possible secondary reactions.

The experimental program replaced ordinary Portland cement with coal slurry powder at 0, 10, 20, and 30 percent by mass, keeping the total binder content constant at 450 kilograms per cubic meter and the water content at 180 kilograms per cubic meter, for a constant water-to-binder ratio of 0.40. Because the cement fraction shrank as replacement increased, the effective water-to-cement ratio rose from 0.40 in the control to 0.571 in the 30 percent mixture, a detail that helps explain the strength reductions observed at higher dosages. Fresh concrete properties were assessed with slump-flow, V-funnel, and L-box tests following European guidelines for self-compacting concrete. All mixtures fell within the SF2 slump-flow class, with values declining from 720 millimeters for the control to 660 millimeters at 30 percent replacement, while V-funnel times rose from 8.2 to 10.4 seconds and L-box passing ratios dropped from 0.98 to 0.90, all still within acceptable limits thanks to mixture-specific superplasticizer adjustment.

The hardened results told a nuanced story. At ambient temperature, the control mix reached compressive strengths of 30.3, 45.29, and 57.1 megapascals at 7, 28, and 56 days respectively, while the 30 percent replacement mixture managed only 19.7, 29.44, and 37.1 megapascals at the same ages, reflecting straightforward cement dilution. Splitting tensile strength followed the same pattern, falling from 5.64 megapascals in the control at 28 days to 3.67 megapascals at 30 percent replacement. Yet the study’s statistical analysis, a two-way analysis of variance on 28-day compressive strength, showed that exposure temperature was actually the dominant factor controlling performance, with a partial eta squared of 0.993, compared with 0.964 for replacement level and 0.791 for their interaction. Coefficients of variation remained below 6 percent across all conditions, indicating solid experimental repeatability.

Thermal exposure was conducted in a programmable furnace at a deliberately slow heating rate of 2 degrees Celsius per minute, with four-hour holds at 300, 600, and 900 degrees Celsius followed by gradual cooling. The author is careful to note that this protocol characterizes material-level degradation rather than reproducing a standardized structural fire test such as ISO 834 or ASTM E119. At 300 degrees Celsius, strength losses were modest, around 8.5 to 8.6 percent. At 600 degrees, all mixtures lost roughly 41 to 43 percent of their strength as calcium hydroxide decomposed and the calcium silicate hydrate gel began to break down. At 900 degrees, only about one-third of the original ambient strength remained in every mixture; the 28-day control retained 14.90 megapascals and the 30 percent mix just 9.69. Crucially, the proportional losses were nearly identical across all replacement levels, at roughly 67 percent, meaning the coal slurry powder did not improve percentage strength retention at extreme temperatures.

Where the moderate replacement levels genuinely shone was in durability and microstructure. At ambient conditions, the 20 percent mixture absorbed less water than the control, 3.6 percent versus 4.2 percent, showed lower sorptivity at 0.09 versus 0.12 millimeters per square root of minute, passed a lower chloride charge in the rapid chloride permeability test, 2200 versus 2800 coulombs, and exhibited higher electrical resistivity at 16.8 versus 12.5 kilohm-centimeters. These gains are consistent with improved particle packing and possible secondary reactions that reduced pore connectivity. By contrast, the 30 percent mixture performed worse than the control on every transport measure, confirming that excessive substitution dilutes the cementitious phases. Scanning electron microscopy reinforced the picture: at 300 and 600 degrees, the 10 and 20 percent mixtures showed less cracking and better preserved interfacial transition zones than the control, while the 30 percent mix deteriorated fastest, and at 900 degrees all samples suffered severe damage.

The multi-technique microstructural analysis traced the degradation mechanisms in detail. Energy-dispersive X-ray analysis showed that the control’s calcium-to-silicon mass ratio of about 2.17 at ambient temperature dropped to roughly 0.85 to 0.95 with 10 to 20 percent replacement, reflecting the influx of silica-rich phases, then climbed to above 4 in all mixes at 900 degrees as silicate hydration products were destroyed and calcium-rich decomposition products concentrated. Fourier-transform infrared spectroscopy documented the progressive loss of hydroxyl and water-related bands, while thermogravimetric analysis recorded total mass losses of 6 to 8 percent at low temperatures, 18 to 22 percent after intermediate degradation, and 30 to 40 percent across the full range, with distinct events marking water evaporation below 200 degrees, portlandite dehydroxylation at 400 to 500 degrees, and carbonate decomposition between 650 and 800 degrees. X-ray diffraction confirmed the near-complete disappearance of calcium hydroxide and calcium carbonate peaks and the collapse of the amorphous binding phase at 900 degrees.

The practical takeaway is a carefully bounded one. Within the conditions investigated, 10 to 20 percent coal slurry powder offers a favorable balance: it cuts cement consumption by up to 90 kilograms per cubic meter at the 20 percent level while maintaining acceptable mechanical performance and delivering the clearest durability benefits of any mixture tested. The author emphasizes, however, that these material-level results do not establish a structural fire-resistance rating, and that fire-critical load-bearing applications would require standardized structural fire testing, realistic heating curves, applied loading, and appropriate fire protection measures. The study also flags important caveats for real-world deployment: only a single coal-processing source was examined, slurry composition varies widely with geology and beneficiation methods, and coal-processing residues may contain environmentally relevant trace metals, so total elemental analysis and standardized leaching tests should precede large-scale structural use, particularly where concrete may contact groundwater.

Future work, the study suggests, should tackle long-term durability under chloride exposure, freeze-thaw cycling, and chemical attack, explore blending coal slurry powder with other supplementary materials, apply advanced tools such as nano-indentation and in-situ thermal characterization, and conduct full-scale structural testing and life-cycle assessments that account for drying, grinding, and transportation of the powder. Until then, the research stands as a compelling proof of concept that one of coal mining’s most troublesome waste streams can, at moderate dosages, become a functional ingredient in high-performance concrete, turning an environmental liability into a small but meaningful wedge against the cement industry’s carbon footprint.

Subject of Research: Use of coal slurry powder as a partial cement replacement in self-compacting concrete evaluated for mechanical, durability, and elevated-temperature performance

Article Title: Valorization of coal slurry powder in self-compacting concrete: Mechanical, durability, and elevated-temperature performance

Article References: Almutairi, A. (2026). Valorization of coal slurry powder in self-compacting concrete: Mechanical, durability, and elevated-temperature performance. Case Studies in Construction Materials, 25, Article e06537. https://doi.org/10.1016/j.cscm.2026.e06537

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06537

Keywords: coal slurry powder, self-compacting concrete, cement replacement, sustainable construction, elevated temperature, durability, compressive strength, microstructure, supplementary cementitious materials, thermal degradation, chloride permeability, carbon emissions