The clay was classified as lean clay and had a natural water content of 47.6%, a liquid limit of 36.4% and a plasticity index of 18.9. The investigators mixed it with reactive magnesium oxide, metakaolin and water, then cast cylindrical specimens 37.6 millimetres in diameter and 80 millimetres high. A preliminary series varied the magnesium oxide-to-metakaolin mass ratio, binder content and liquid-to-solid ratio, with three parallel specimens prepared for each condition. Mechanical results were based on the arithmetic mean of three valid measurements, and statistical comparisons used one-way analysis of variance followed by Tukey’s test. Carbonation was conducted at 20 degrees Celsius, 70% relative humidity and 20% carbon dioxide, an accelerated laboratory condition rather than an atmospheric exposure.

Mix design strongly affected the clay’s strength before carbonation. At 30% binder content and a liquid-to-solid ratio of 0.65, the mixture with equal masses of magnesium oxide and metakaolin produced the highest mean seven-day strength, 0.580 megapascals. This was not a simple case of adding more magnesium oxide being better: the magnesium-only mixture reached just 0.044 megapascals, while reducing magnesium oxide too far also lowered strength. The researchers selected a 6:4 magnesium oxide-to-metakaolin ratio because its seven-day strength was not significantly different from that of the 5:5 mixture, while X-ray observations indicated that it retained more brucite as a potential carbonation reserve. Increasing binder content raised seven-day strength from 0.278 megapascals at 20% to 0.580 and 0.803 megapascals at 30% and 40%, respectively.

The selected binder also remained less alkaline than an ordinary Portland cement reference. At 28 days, the magnesium oxide–metakaolin clay had a pH lower than the 11.89 measured for the Portland cement mixture, although the paper did not test effects on surrounding soils, groundwater or ecosystems. Water content also influenced performance: both strength and pH decreased approximately linearly as the liquid-to-solid ratio rose from 0.60 to 0.70. More initial water meant weaker particle contacts and could impede carbon dioxide transport through pore films. The 0.65 ratio was therefore retained for the principal carbonation experiments as a compromise between mechanical performance and carbonation conditions.

Most of the strength in the composite came from hydration rather than from brucite alone. Magnesium oxide releases magnesium ions and hydroxide during hydration, producing brucite and establishing an alkaline environment that helps dissolve metakaolin. The metakaolin supplies reactive silicon and aluminum that combine in low-crystallinity magnesium aluminosilicate hydrate, or M-(A)-S-H, gel. Clay stabilized with magnesium oxide alone remained below 0.05 megapascals after 28 days, while the selected magnesium oxide–metakaolin mixture reached 0.740 megapascals. The gel-rich matrix was therefore interpreted as the principal load-bearing skeleton, wrapping and bridging clay particles, while brucite contributed more limited pore filling and alkalinity buffering. Hydrotalcite-like layered double hydroxides were also detected and may have contributed to layered pore filling and ion regulation, although their contribution was not independently quantified.

Carbonation produced a secondary gain only when it followed enough hydration. For specimens pre-cured for seven days, carbonation initially increased strength, but five days of exposure returned the compressive strength to a level not significantly different from that of non-carbonated specimens. In contrast, all carbonated 14-day groups were stronger than their non-carbonated controls, and specimens pre-cured for 28 days gained strength after one day of carbonation and improved further after three days. Extending carbonation from three to five days did not produce a significant additional increase at the longer curing ages. This apparent optimum suggests that sufficient pre-cure is needed for carbonate crystals to become integrated with a continuous gel framework, rather than accumulating as discrete products in a still-open pore network.

The phase evidence supported that two-stage interpretation. Before carbonation, X-ray diffraction identified brucite, residual periclase and a broad feature consistent with poorly crystalline M-(A)-S-H gel, as well as a broad hydrotalcite-like reflection. After exposure to carbon dioxide, nesquehonite reflections intensified while brucite reflections weakened, indicating progressive conversion of brucite into hydrated magnesium carbonate. Thermogravimetric measurements showed that apparent carbon dioxide uptake was consistently greater near the specimen exterior than in the interior, confirming that transport through the material was incomplete. Yet the mature 28-day matrix remained comparatively stable: its gel-associated thermal mass loss did not decrease appreciably during carbonation, suggesting that the M-(A)-S-H skeleton was retained and combined with the new carbonate phase.

Microscopy showed how the products fit together. Brucite appeared as loosely stacked plates in early specimens, while longer curing produced denser material with continuous gel-like regions between clay particles. After carbonation, larger needle-like or hexagonal prismatic nesquehonite crystals appeared alongside the gel. Some crystals grew inside cracks and bridged opposing surfaces. Elemental maps showed broad overlap of magnesium, silicon, aluminum and oxygen in gel-rich regions, while magnesium, carbon and oxygen were concentrated in the prismatic crystals. A gel-like region had a magnesium-to-silicon atomic ratio of about 1.18, consistent with M-(A)-S-H-related material, and a crystal had a magnesium-to-carbon ratio of about 1.02, supporting its identification as nesquehonite when combined with the diffraction and thermal data. The specimens also underwent slight axial expansion during carbonation, consistent with crystal growth and pore filling.

Under the study’s assumptions, the magnesium oxide–metakaolin system also had a lower estimated carbon footprint than Portland cement. For treating one tonne of clay with 300 kilograms of binder, the authors estimated a net footprint of 231.12 kilograms of carbon-dioxide equivalent for magnesium oxide–metakaolin, compared with 285.28 kilograms for Portland cement, a reduction of 18.98%. Apparent carbon dioxide uptake deducted from this balance was 33.71 kilograms per tonne of clay. After 28 days of curing and five days of carbonation, the magnesium oxide–metakaolin specimens had a mean strength of 1.309 plus or minus 0.079 megapascals, versus 1.120 plus or minus 0.080 for the Portland cement reference. The balance depended on specified production, transport and processing assumptions, and the accelerated 20% carbon dioxide treatment did not establish performance under natural weathering or field-scale carbonation. The central laboratory result is instead a clear mechanistic pairing: hydration creates the load-bearing gel network, and subsequent carbonation adds interlocking crystals that reinforce an already developed matrix.

Subject of Research: Hydration-controlled carbonation strengthening of clay stabilized with a magnesium oxide–metakaolin binder

Article Title: Hydration regulation and carbonation-induced strengthening of MgO–metakaolin binders in clay-based composites

Article References: Xiao, Z., Zhu, C., Cao, K., An, K., & Hu, A. (2026). Hydration regulation and carbonation-induced strengthening of MgO–metakaolin binders in clay-based composites. Case Studies in Construction Materials, 25, Article e06624. https://doi.org/10.1016/j.cscm.2026.e06624

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06624

Keywords: clay stabilization, magnesium oxide, metakaolin, carbonation, nesquehonite, cement replacement, M-(A)-S-H gel, CO2 mineralization, unconfined compressive strength, construction materials, Hydration, regulation

The choice of an L16 orthogonal array is central to the efficiency of the experimental program. Orthogonal designs, rooted in the work of Genichi Taguchi and classical factorial design theory, allow the independent estimation of main effects for multiple factors using a fraction of the full factorial run count. With five mix parameters under investigation—slag content, metakaolin content, sodium hydroxide molarity, the sodium silicate to sodium hydroxide ratio, and the alkali-to-binder ratio—a full factorial exploration at even modest resolution would require dozens to hundreds of paste batches. The L16 array compresses this into sixteen carefully balanced runs in which each factor level appears an equal number of times and factor levels are distributed orthogonally across columns. This balance means that, on average, the influence of one factor is not confounded with the linear influence of another, enabling analysis of variance to attribute portions of observed variance to individual parameters. The trade-off, as the authors themselves note, is that certain two-factor interactions cannot be fully disentangled from main effects within a resolution-limited array, a limitation that is well understood in design-of-experiments practice and typically addressed through follow-up experiments at the identified optimum.

The finding that the alkaline activator ratio exerted the greatest influence on compressive strength aligns with the fundamental chemistry of geopolymerisation. The sodium silicate to sodium hydroxide ratio controls the availability of dissolved silica in the activating solution, which in turn governs the Si/Al ratio of the forming gel network. Higher soluble silica content generally promotes denser, more cross-linked aluminosilicate gels and, in calcium-rich systems, modifies the composition of C-(A)-S-H phases toward higher chain length and greater mechanical integrity. Dissolved silica also moderates the highly caustic environment of sodium hydroxide, affecting the rate at which precursor particles dissolve and the degree to which dissolved species polycondense into stable networks. Meanwhile, the identification of the alkali-to-binder ratio as the dominant factor for flowability is consistent with rheological behavior in alkaline-activated pastes: the total volume of liquid relative to solid determines the spacing between particles and the viscosity of the suspending medium, and alkaline activator solutions are substantially more viscous than water, amplifying their effect on workability.

The economic dimension of the optimization deserves particular attention because it is frequently neglected in laboratory studies of alternative binders. Metakaolin, produced by calcining kaolinitic clay at moderate temperatures, carries an embodied energy cost and a market price considerably higher than that of fly ash or slag, which are industrial byproducts available at low cost near their points of generation. The study’s identification of metakaolin dosage as the dominant contributor to material cost therefore has direct practical consequences: even modest increases in metakaolin content can shift the economics of a mix substantially, which is why the optimal formulation converged on a low metakaolin fraction of ten percent. This cost sensitivity also highlights a general principle in sustainable materials engineering—the environmental and economic benefits of byproduct utilization depend heavily on local supply chains, transport distances, and competing demands for the same materials from other industries, factors that a laboratory cost model can only approximate.

The curing regime established in the preliminary single-factor experiments also merits scientific context. Geopolymerisation is a thermally activated dissolution-condensation process, and low-calcium fly ash systems in particular require elevated temperatures to achieve meaningful reaction within practical timeframes. Temperatures in the range of 60 to 80 degrees Celsius are commonly reported as favorable for fly ash activation, with the 60 degrees Celsius, 24-hour condition identified here as optimal balancing reaction extent against the risk of excessive water loss or gel coarsening at higher temperatures. The presence of slag in the ternary system partially relaxes this thermal requirement, since calcium-rich systems gain strength appreciably at ambient temperature, but heat curing remains beneficial for achieving early strength in blended systems. This thermal dependence has implications for field application, where ambient-temperature curing or modest heat treatment would be required, and it represents one of the practical hurdles that distinguishes geopolymer concretes from conventional Portland cement systems in cold-weather construction.

The microstructural observations reported through X-ray diffraction and scanning electron microscopy provide the mechanistic underpinning for the mechanical results. X-ray diffraction in geopolymer systems typically reveals the characteristic broad hump associated with short-range ordered aluminosilicate gel, superimposed on residual crystalline phases such as quartz and mullite inherited from the fly ash. Shifts in the position of this hump and changes in its relative intensity reflect alterations in gel composition and quantity. Scanning electron microscopy, in turn, allows direct visualization of gel morphology, unreacted particle remnants, and the pore network. The reported features of enhanced C-(A)-S-H and N-A-S-H gel formation, increased matrix densification, and reduced porosity in the optimized mixtures are exactly what reaction mechanism models would predict for a well-balanced ternary system: slag-derived calcium gel fills space and provides early binding, while fly ash and metakaolin-derived sodium aluminosilicate gel contributes to long-term framework development, and the coexistence of the two gel types in an intermixed morphology is widely associated with superior mechanical performance in hybrid systems.

The statistical methodology underlying the decision model also reflects current best practice in multi-criteria optimization. Entropy weighting is an objective technique that assigns greater weight to criteria exhibiting greater variability across the experimental alternatives, on the reasoning that a parameter that discriminates strongly between options carries more information for decision-making. Combining this with subjective weighting, which encodes engineering judgment about the relative importance of strength, workability, and cost, produces a synthetic weighting scheme that tempers purely data-driven assignment with domain expertise. This hybrid approach guards against the failure modes of either extreme: purely subjective weights can be arbitrary and difficult to defend, while purely entropy-based weights can overemphasize criteria whose variability is an artifact of the chosen experimental levels rather than of genuine practical importance. The convergence of the multi-objective analysis on a mixture with moderate values of most parameters—fifty percent slag, ten percent metakaolin, six molar sodium hydroxide, an activator ratio of one to one point five, and an alkali-to-binder ratio of zero point three five—is characteristic of Pareto-optimal solutions in multi-objective problems, where the best compromise rarely coincides with the extreme values that maximize any single objective.

The reported maximum compressive strength of 115.87 megapascals under one experimental condition is notable in the context of published geopolymer literature, where ambient-cured fly ash systems typically achieve strengths in the range of 30 to 70 megapascals and heat-cured or slag-rich systems can exceed 100 megapascals. The high value is consistent with the combination of substantial slag content, heat curing, and an activator chemistry favoring dense gel formation. However, the authors’ transparency about the replication structure of their testing is scientifically important. The three compressive strength specimens per mixture were subsamples drawn from a single batch rather than products of independent batch preparations, which means that the reported variability reflects within-batch homogeneity rather than between-batch reproducibility. Between-batch variation, arising from differences in mixing efficiency, temperature, activator preparation, and precursor sampling, is typically larger than within-batch variation, so the true uncertainty surrounding each mixture’s mean strength is likely wider than the within-batch statistics alone would suggest. The observation that repeat testing broadly preserved the relative ranking of selected mixtures is reassuring for the optimization conclusion, since ranking-based decisions are more robust to absolute strength uncertainty than threshold-based ones, but independent replication at the batch level remains a desirable confirmation step.

From a sustainability perspective, the study contributes to a growing body of evidence that industrial byproducts can substitute for Portland cement clinker at high replacement levels without sacrificing mechanical performance. Cement production is estimated to account for roughly seven to eight percent of global carbon dioxide emissions, driven both by the calcination of limestone and by the fossil fuel energy consumed in clinker kilns. Geopolymer binders eliminate the calcination step entirely, and while the alkaline activators—particularly sodium silicate—carry their own embodied emissions, life cycle assessments generally report substantial net reductions for geopolymer concretes relative to ordinary Portland cement equivalents, with the magnitude depending on activator dosage and transport distances. Ternary systems that incorporate slag, itself a finite byproduct facing increasing demand, raise questions about long-term resource availability, which strengthens the case for including calcined clays such as metakaolin in precursor blends, since clay reserves are abundant and calcination can be performed at temperatures far below those required for clinker production.

The workability findings also carry practical significance for construction practice. Flowability, typically assessed in paste systems through mini-slump or flow table tests, serves as a proxy for the fresh-state behavior of the corresponding concrete, where workability governs placement, compaction, and finishability. The strong influence of the alkali-to-binder ratio on flowability identified in the effect analysis suggests that contractors seeking to adjust workability in the field would have a more effective lever in the liquid content than in the activator composition, though such adjustments must be balanced against the strength consequences of diluting the alkaline environment. The interplay between workability and strength is a recurring theme in geopolymer technology, and the multi-objective framework applied here offers a structured way to navigate it rather than relying on iterative trial and error, which the authors correctly identify as laborious and inefficient for multi-parameter systems.

Finally, the methodological integration demonstrated in this study—combining screening-level single-factor experiments, an orthogonal array for efficient multi-factor assessment, analysis of variance for effect attribution, and a hybrid weighted decision model for multi-objective selection—constitutes a transferable template for other researchers working on complex binder systems. The approach is not limited to geopolymers; it applies equally to alkali-activated materials, limestone calcined clay cements, and other multi-component systems where formulation space is large and performance criteria are competing. The explicit acknowledgment of limitations, including the interaction confounding inherent in the chosen array and the subsample nature of the mechanical replicates, models the kind of methodological candor that allows subsequent studies to build incrementally on reported results, refining the optimum through targeted confirmation experiments and extending the framework to durability indicators, long-term strength development, and full-scale concrete applications where aggregate packing and interfacial transition zone effects introduce additional layers of complexity.

Subject of Research: Optimization of a ternary geopolymer paste through orthogonal experimental design, ANOVA and entropy-based multi-objective decision analysis

Article Title: Optimization of a ternary geopolymer paste through orthogonal experimental design, ANOVA and entropy-based multi-objective decision analysis

Article References: Murugeswari, S., & Meena, T. (2026). Optimization of a ternary geopolymer paste through orthogonal experimental design, ANOVA and entropy-based multi-objective decision analysis. Frontiers in Built Environment, 12, Article 1934627. https://doi.org/10.3389/fbuil.2026.1934627

Image Credits: AI Generated

DOI: 10.3389/fbuil.2026.1934627

Keywords: geopolymer, fly ash, GGBS, metakaolin, orthogonal experimental design, ANOVA, entropy weighting, multi-objective optimization, compressive strength, sustainable construction, XRD, SEM

The challenge is deceptively simple to state but notoriously difficult to solve. Backfill grouts must remain fluid enough to be mixed, transported and placed into large voids, yet cohesive enough to avoid bleeding and segregating once in place, and strong enough after hardening to contribute to ground stabilization. In highly diluted systems, the large water content needed for placement promotes sedimentation of solid particles and water separation, while simply adding more cement to compensate increases resistance to flow and inflates both cost and the carbon footprint of filling operations that can consume enormous material volumes.

A research team led by Rachida Idir, Nourredine Goufi, Minh Trinh Quang, Bernard Husson and Martin Cyr approached the problem with a sequential formulation strategy. They first pinned down the right dose of bentonite, a sodium-rich clay whose swelling montmorillonite platelets trap water and hold cement particles in suspension. Then they pushed metakaolin — a reactive aluminosilicate made by calcining kaolinitic clay — as a partial cement replacement up to a remarkable 80 percent by mass. Finally, they added quicklime as an external calcium source and mapped the resulting three-component binder space using mixture-design interpolation.

The bentonite results reveal a classic engineering compromise. With no clay at all, a reference grout containing 150 kilograms of cement per cubic meter flowed through a Marsh cone in just 9 seconds but bled a catastrophic 30 percent of its water. At 2 percent bentonite, bleeding dropped to 4 percent with a flow time of 12 seconds; at 3 percent, bleeding fell to 2 percent while flow time hit the 15-second limit adopted as a practical flowability criterion. Beyond that, the suspension stiffened dramatically: 4 percent bentonite stretched the flow time to 24 seconds, and at 5 percent the grout would not discharge at all.

Rheological measurements on sand-free equivalents, performed with a Couette rheometer at a constant volumetric solid concentration of 0.258, confirmed the picture. All mixtures behaved as shear-thinning, yield-stress fluids, but increasing bentonite shifted the entire flow curve upward — the shear stress at 120 reciprocal seconds rose from roughly 6 pascals at 1 percent clay to about 28 pascals at 5 percent. The hysteresis-loop area, an index of thixotropic breakdown, jumped nearly twelvefold across the same range, signaling a strongly connected particle network that resists flow and rebuilds structure after shearing.

Mechanically, the story took an unexpected turn depending on cement chemistry. The clinker-rich CEM I 52.5 R grout started stronger, at 2.85 megapascals at 28 days versus 2.19 for the slag-rich CEM III/C, but collapsed to 1.10 megapascals with just 1 percent bentonite and fell to 0.67 at 3 percent. The slag cement, which contains roughly 82 percent ground granulated blast-furnace slag and only about 15 percent clinker, proved far more tolerant, holding above the 1.5-megapascal target across the entire bentonite range. The authors attribute this to the harsh, ion-rich pore solution of fast-hydrating CEM I, which promotes flocculation of montmorillonite platelets and disrupts hydration product distribution.

Metakaolin alone, meanwhile, proved a fresh-state friend but a mechanical disappointment. Across replacement levels from 10 to 80 percent, bleeding stayed below 2 percent and Marsh cone flow times hovered between 14 and 16 seconds for both cements. Yet compressive strength fell steeply: only the CEM III/C reference with no metakaolin exceeded the 1.5-megapascal target. The reason is calcium starvation. Metakaolin’s pozzolanic reaction consumes calcium hydroxide released by clinker hydration, and once the clinker fraction shrinks too far, there simply is not enough calcium to sustain the formation of additional calcium silicate and calcium aluminosilicate hydrates that bind the matrix together.

Quicklime changed the equation. When calcium oxide hydrates, it generates fresh calcium hydroxide, feeding the pozzolanic reaction directly. Among eight ternary compositions tested with 2 percent bentonite, one stood out: 40 percent CEM III/C, 40 percent metakaolin and 20 percent quicklime delivered a 28-day compressive strength of 1.56 megapascals — more than four times the 0.37 megapascals of the unactivated reference — while flowing in 13 seconds and bleeding just 4 percent. Notably, strength did not scale with lime content: cement-free mixes with 30 or 50 percent quicklime managed only 0.60 and 0.54 megapascals, proving that a residual clinker fraction remains essential.

Interpolating the strength data across the ternary composition space, the team identified a candidate region meeting all three criteria — flow time at or below 15 seconds, bleeding under 5 percent, and strength of at least 1.5 megapascals — spanning roughly 0 to 45 percent metakaolin, 0 to 40 percent quicklime and 20 to 100 percent CEM III/C, interpreted jointly rather than as independent limits. The authors stress this map is exploratory, derived from a limited experimental matrix, and that hydration products and pore structure were not directly characterized, so the proposed reaction mechanisms remain plausible interpretations rather than demonstrated facts.

The implications reach beyond the laboratory. The 1.5-megapascal threshold corresponds to the minimum requirement of France’s Inspection Générale des Carrières for filling mortars used in underground consolidation by injection, making the results directly relevant to stabilizing the voids left beneath cities built over old quarries and mines. Before field deployment, however, the formulations must survive real-world trials of pumping, injectability and placement under pressure, along with assessments of durability and dimensional stability — quicklime hydration is exothermic and can drive early-age expansion, a particular concern in large-volume fills restrained by surrounding ground. If those hurdles are cleared, the study offers a practical recipe for filling the underground voids of the built environment with dramatically less cement, at lower cost and with a smaller carbon footprint.

Subject of Research: Rheological and mechanical formulation of low-cement backfill grouts with bentonite, metakaolin and quicklime for underground cavity filling

Article Title: Rheological and mechanical design of low-cement backfill grouts for underground cavities incorporating bentonite and metakaolin with quicklime

Article References: Idir, R., Goufi, N., Quang, M. T., Husson, B., & Cyr, M. (2026). Rheological and mechanical design of low-cement backfill grouts for underground cavities incorporating bentonite and metakaolin with quicklime. Results in Engineering, 32, Article 113492. https://doi.org/10.1016/j.rineng.2026.113492

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113492

Keywords: backfill grout, bentonite, metakaolin, quicklime, rheology, compressive strength, underground cavities, cement replacement, bleeding, Marsh cone, ground stabilization, supplementary cementitious materials

The material belongs to a family of substances known as geopolymers — amorphous, metastable aluminosilicates first developed by Joseph Davidovits in 1970. They are synthesized by reacting an aluminosilicate precursor, such as calcined clay, fly ash, or natural pozzolans, with an alkaline activator like sodium hydroxide at temperatures below 100 degrees Celsius. At the molecular level, the result is an anionic framework of corner-sharing SiO4 and AlO4 tetrahedra, into which charge-balancing cations such as sodium or potassium are incorporated from solution. This architecture gives geopolymers a set of properties that make them unusually attractive for water treatment: low-cost and abundant precursors, tunable porosity, chemical stability, and a synthesis route that is gentle on the environment. Their adjustable surface chemistry and robust structural framework also support adsorption efficiency and recyclability, addressing two of the biggest weaknesses of conventional adsorbents.

What distinguishes the new work is its dual pore-generation strategy and its systematic exploration of precursor composition. The team calcined kaolin from Zettlitz in the Czech Republic at 700 degrees Celsius for three hours to produce metakaolin, then blended it with silica fume — an ultrafine byproduct of ferrosilicon and silicon metal production, containing more than 96 percent amorphous silica and a surface area of roughly 20,000 square meters per kilogram. Silica fume replaced metakaolin at proportions ranging from 0 to 25 weight percent, serving as a reactive silica source within the alkaline activator system. To open up the internal architecture, the researchers added hydrogen peroxide at 5 weight percent as a foaming agent, which decomposes to generate gas-induced pores, and corn oil at 1 weight percent as a templating phase that promotes interconnected voids. The paste was cured at 60 degrees Celsius for 12 hours, followed by hydrothermal curing for another 24 hours, before being crushed, sieved to particles below 100 micrometers, washed with hot water, and dried.

The screening experiments revealed a striking optimum. When six adsorbent formulations were tested against a 10 milligram per liter methylene blue solution at natural pH with a dosage of 1 gram per liter, both adsorption capacity and removal efficiency climbed as silica fume content rose to 10 percent, where the S(10) formulation achieved roughly 90 percent removal. Beyond that threshold, performance declined. The explanation lies in the structural evolution of the geopolymer: at 10 weight percent, the additional reactive silica facilitates more complete polycondensation, producing a robust and well-distributed sodium aluminosilicate hydrate gel with a balanced silicon-to-aluminum ratio that maximizes negatively charged surface sites for cationic attraction. Above that level, particle agglomeration reduces available surface area, partially blocks mesoporous channels, and increases paste viscosity — which hinders uniform distribution of the hydrogen peroxide foam and yields a less interconnected pore structure riddled with physical defects.

With the optimal composition identified, the researchers mapped how operating conditions shape performance. Increasing the adsorbent dosage from 0.5 to 2.5 grams per liter raised removal efficiency from 60 to 93 percent, though the amount adsorbed per gram of material fell from 11.52 to 3.84 milligrams as sites began to overlap and aggregate. pH proved decisive: removal efficiency rose from 58.5 percent at pH 1 to 93 percent at pH 7, then slipped slightly to 90.5 percent at pH 9. The chemistry is intuitive — under acidic conditions, the geopolymer surface is surrounded by hydrogen ions and acquires a positive charge, electrostatically repelling the cationic dye, while neutral conditions maximize the attraction between dye cations and the negatively charged surface. Notably, the optimum coincides with the material’s point of zero charge, and because real textile effluents often require expensive pH adjustment before treatment, the ability to perform best at near-neutral conditions could meaningfully cut operational costs. Contact time experiments showed 73.05 percent removal within the first 30 minutes, reaching equilibrium at 60 minutes with 93.6 percent removal.

Kinetic modeling pointed to a chemical rather than purely physical binding process. The pseudo-second-order model fit the data with a correlation coefficient of 0.99, far outperforming the pseudo-first-order model at 0.84 and the Elovich model at 0.7 — a signature of chemisorption, in which electron sharing or exchange between adsorbent and adsorbate governs the rate. The intraparticle diffusion model fit poorly, and its non-zero intercept confirmed that pore diffusion is not the sole rate-limiting step; boundary layer resistance and film diffusion also contribute to mass transfer. On the equilibrium side, the Langmuir isotherm described the data best with a correlation coefficient of 0.98, ahead of the Freundlich model at 0.94 and the Temkin model at 0.92. Together, the kinetic and isotherm analyses indicate monolayer chemisorption on energetically equivalent sites, with a maximum adsorption capacity of 12.30 milligrams per gram and a separation factor of 0.064 confirming favorable adsorption.

Spectroscopic and microscopic analyses supplied direct evidence of how the dye molecules attach. Fourier transform infrared spectroscopy showed a reduction and shift of the primary silicon-oxygen-aluminum vibration band between 850 and 1200 wavenumbers after dye uptake, alongside the appearance of methylene blue-specific signals near 1243, 1100, and 1400 wavenumbers, attributed to C–N and N–N stretching and aromatic ring deformations. Changes in the broad hydroxyl stretching region near 3400 wavenumbers suggested hydrogen bonding between surface hydroxyl groups and the dye. X-ray diffraction revealed the characteristic broad amorphous hump of the N-A-S-H gel, which broadened slightly after adsorption but returned to its original profile after regeneration, confirming structural stability. Nitrogen sorption measurements classified the material as mesoporous, with a Type IV isotherm and H3-type hysteresis loop indicating slit-like or ink-bottle-shaped pores, a specific surface area of 12.515 square meters per gram, and an average pore diameter of 31.7 nanometers — dimensions that facilitate rapid transport of dye molecules to internal active sites. Scanning electron microscopy showed pores partially coated and filled after adsorption, with elemental mapping revealing increased carbon and nitrogen content that vanished after regeneration.

The adsorption mechanism that emerges is a layered one. Negatively charged siloxo and alumoxo sites on the geopolymer surface attract the cationic dye electrostatically, while charge-balancing sodium ions within the pores undergo direct ion exchange with methylene blue cations. The dye molecule adopts a more planar configuration, lying flat on the surface, which facilitates π–π interactions between its aromatic rings and silanol groups, and the presence of aluminum and sodium ions combined with the dye’s conjugated structure may induce additional n–π interactions. Hydrogen bonding with surface functional groups further enhances uptake. Initial capture is assisted by van der Waals forces, dipole–dipole interactions, and physical pore filling within the mesoporous network, but the rate is ultimately governed by the abundance of accessible surface sites rather than diffusion alone.

Perhaps most importantly for real-world deployment, the adsorbent proved reusable. Over five adsorption–desorption cycles, removal efficiency declined from 93.6 percent to approximately 73 percent when regenerated with hydrochloric acid and to about 80 percent when regenerated with ethanol. Ethanol proved the superior regenerant: its lower dielectric constant weakens the electrostatic grip between dye cations and the negatively charged surface, while it competes for silanol and aluminol binding sites through hydrogen bonding and solvates the dye’s aromatic rings. The residual capacity loss stems from irreversible chemisorption at high-energy sites and partial pore masking in narrow mesopores, where dye molecules bound through strong chemical interactions resist displacement. Even so, retaining 80 percent efficiency after five cycles is a commendable result that addresses the common criticism that adsorption merely transfers pollutants from water to a solid that must then be discarded.

The researchers are candid about the material’s limits. Its maximum capacity of 12.30 milligrams per gram trails some highly porous commercial adsorbents and engineered materials, a consequence of the relatively modest surface area and the absence of additional surface functionalization. Yet the comparison that matters may be economic rather than absolute: the removal efficiency of 93.6 percent rivals that of far more elaborate systems, and the synthesis route relies on low-cost, widely available industrial wastes — silica fume from ferroalloy furnaces and calcined clay — while operating at mild temperatures without energy-intensive processing. The team suggests that surface tailoring, chemical modification, or the incorporation of carbon-based materials, metal oxides, or nanoscale additives could push performance further, and that testing under real wastewater conditions, long-term reuse, and continuous-flow systems will be needed to establish broader applicability. For now, the study demonstrates that two industrial waste streams, a splash of hydrogen peroxide, and a spoonful of cooking oil can be transformed into a working water-cleaning material — a small but compelling example of circular chemistry aimed at one of the world’s most persistent pollution problems.

Subject of Research: Porous metakaolin–silica fume geopolymer adsorbents for removing methylene blue dye from contaminated water

Article Title: Eco-friendly production of porous metakaolin–silica fume geopolymer for adsorptive removal of methylene blue from water

Article References: Oshani, F., Mahinroosta, M., & Ghasemi, E. (2026). Eco-friendly production of porous metakaolin–silica fume geopolymer for adsorptive removal of methylene blue from water. Results in Engineering, 32, Article 113419. https://doi.org/10.1016/j.rineng.2026.113419

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113419

Keywords: geopolymer, methylene blue, adsorption, silica fume, metakaolin, wastewater treatment, dye removal, porous materials, chemisorption, water purification, industrial byproducts, sustainability

The choice of ingredients is deliberate and each component plays a distinct structural role. Diatomite, a silica-rich sedimentary rock formed from the fossilized frustules of microscopic algae and sourced from Lampang in northern Thailand, supplies an intrinsically porous mineral framework that facilitates mass transfer of dye molecules into the pellet interior. Metakaolin, produced by calcining kaolin clay from Ranong, serves as the reactive aluminosilicate precursor that forms the chemically stable geopolymer network when activated with a mixture of sodium silicate solution and 10 M sodium hydroxide. Water hyacinth residue collected in Chiang Mai brings oxygen-rich functional groups and a fibrous morphology that modify the pore architecture and surface reactivity of the final composite. The researchers cast pastes containing 20 weight percent calcined diatomite, 15 weight percent metakaolin, and the alkaline activator, adding water hyacinth at 0, 1, 2, and 3 weight percent to produce a series labeled DG-WH0 through DG-WH3, which were then cured at room temperature for up to 28 days.

Structural analysis revealed how the biomass reshapes the material from the inside out. X-ray diffraction showed that quartz remained the dominant crystalline phase in all compositions, inherited largely from unreacted portions of the diatomite and metakaolin precursors, while a broad diffuse feature between roughly 20 and 35 degrees two-theta confirmed the presence of a poorly ordered aluminosilicate geopolymer matrix. Fourier-transform infrared spectroscopy identified the characteristic Si-O-T asymmetric stretching band near 1016 to 1021 per centimeter, along with hydroxyl and carbonate features, and small shifts in these bands after dye exposure hinted at interactions between methylene blue and the composite surface. Scanning electron microscopy told a more visually striking story: the biomass-free pellet was relatively dense, but increasing water hyacinth content progressively introduced interfacial gaps, larger voids, and interconnected pore channels, particularly in the 2 and 3 percent formulations.

Quantitative measurements reinforced what the micrographs suggested. Apparent porosity rose from 58.57 to 62.03 percent across the series, while water absorption climbed sharply from 58.30 to 79.55 percent, and bulk density fell from 1.22 to about 1.08 grams per cubic centimeter. Nitrogen physisorption on the two endpoint compositions showed that the BET specific surface area of the 3 percent pellet reached 14.843 square meters per gram, roughly 86 percent higher than the 7.986 square meters per gram of the biomass-free control, while total pore volume increased by about 75 percent. In other words, the invasive weed was effectively blowing open the internal architecture of the pellet, creating more pathways and more exposed surface for dye molecules to reach.

Surface charge added a second, electrostatic dimension to the story. Zeta potential measurements at pH 7 showed that all pellets carried negative surface charges, becoming progressively more negative as water hyacinth content increased, from minus 67.3 millivolts for the control to minus 74.9 millivolts for the highest-biomass pellet. The point of zero charge of the best-performing composition was approximately 2.22, meaning that at the pH used in the adsorption experiments the surface was decisively negative and therefore electrostatically attractive to positively charged methylene blue cations. Deprotonated surface hydroxyl groups on the aluminosilicate framework, generating negatively charged siloxide and aluminate sites, are thought to contribute to this favorable electrostatic environment.

The adsorption experiments bore out the design logic. At pH 2, performance was poor, with removal efficiencies between roughly 49 and 54 percent, because the flood of hydrogen ions competed with dye cations for adsorption sites. Performance peaked at pH 4, where the 3 percent pellet removed 77.25 percent of methylene blue with an equilibrium capacity of 19.31 milligrams per gram, and remained high but essentially flat from pH 6 through 10, indicating that electrostatics alone do not govern the process. Kinetic measurements showed rapid initial uptake followed by a gradual plateau between about 800 and 1080 minutes, with 24-hour removal reaching 77.03 percent and a capacity of 19.26 milligrams per gram for the best pellet. Increasing the initial dye concentration from 50 to 250 milligrams per liter lowered the removal percentage from 95.26 to 77.40 percent while raising the per-gram capacity from about 4.76 to 19.35 milligrams per gram, the classic signature of a fixed number of sites being progressively saturated.

Equilibrium data fit the Langmuir isotherm model better than the Freundlich model, with a linearized correlation coefficient of 0.9998 and a maximum monolayer capacity of 22.32 milligrams per gram, while both linearized and nonlinear kinetic analyses consistently favored the pseudo-second-order model. Against the backdrop of published geopolymer adsorbents, the 22.32 milligrams per gram figure is competitive, outperforming lightweight geopolymer-expanded glass composites, metakaolin/slag geopolymers, red mud and rice husk ash systems, and coal gangue-based materials, and landing close to reduced graphene oxide/geopolymer composites, though some fly ash and metakaolin systems report higher capacities. The authors caution that direct comparisons across studies are complicated by differences in material form and experimental conditions, and their pellets carry an inherent mass-transfer penalty because dye molecules must diffuse through the pellet interior rather than contacting a loose powder directly.

That penalty is the price of practicality, and the study is unusually candid about the trade-offs involved. Compressive strength after 28 days of curing fell from 21.5 megapascals for the biomass-free pellet to 9.1, 6.5, and 5.5 megapascals at 1, 2, and 3 percent water hyacinth, because the fibrous additive and its interfacial discontinuities disrupt the continuity of the load-bearing geopolymer matrix. The modest gain in equilibrium capacity at the highest biomass loading does not fully compensate for the substantial loss of mechanical robustness, so the researchers suggest that an intermediate water hyacinth content may offer the best balance for applications where pellets must survive handling. Reusability testing over five adsorption-desorption cycles showed another honest limitation: removal efficiency declined to about 33 percent of its initial value by the fifth cycle, even though the pellets retained roughly 99 percent of their mass, pointing to a need for improved regeneration protocols.

What makes this work resonate beyond its numbers is the circular economy story it tells. Water hyacinth is a global nuisance, clogging waterways, blocking navigation, and starving aquatic ecosystems of light and oxygen, yet here its residue becomes a functional ingredient in a low-cost ceramic adsorbent built from abundant local minerals. The study also models a kind of scientific honesty that is increasingly valued in materials research: rather than claiming a single dominant mechanism, the authors attribute dye uptake to coupled effects, including electrostatic attraction to the negatively charged surface, possible hydrogen bonding through hydroxyl-containing groups, and improved pore accessibility, while explicitly declining to claim mechanisms, such as pi-pi interactions, that their characterization could not verify. They note that methylene blue served only as a model cationic dye, and that real textile wastewater, with its mixtures of dyes, salts, and organic matter, will pose stiffer competitive challenges. Future work on mixed-pollutant systems, thermodynamic analysis, and regeneration chemistry will determine whether these weed-powered pellets can graduate from elegant laboratory concept to working technology for the dye-polluted rivers that need them most.

Subject of Research: Pelletized diatomite-based geopolymer composites incorporating water hyacinth biomass for the adsorptive removal of methylene blue dye from wastewater

Article Title: Pelletized diatomite-based geopolymer-water hyacinth composites for methylene blue adsorption: insights into structure-property-performance relationships

Article References: Thammarong, S., Zhang, W., Pimpha, N., Saenkam, K., Boontakam, W., Tandorn, S., Randorn, C., Srichumpong, T., Pengpat, K., & Rujijanagul, G. (2026). Pelletized diatomite-based geopolymer-water hyacinth composites for methylene blue adsorption: insights into structure-property-performance relationships. Results in Chemistry, 31, Article 103923. https://doi.org/10.1016/j.rechem.2026.103923

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103923

Keywords: geopolymer, water hyacinth, diatomite, methylene blue, adsorption, wastewater treatment, dye removal, metakaolin, porous materials, biomass composites, isotherm modeling, mechanical strength

The study, published in Case Studies in Construction Materials, was led by Kari Steyn with Wibke De Villiers and Adewumi John Babafemi as supervisors. The motivation is urgent. Cement and steel production together account for roughly sixteen percent of global carbon dioxide emissions, and the United Nations Environment Programme has repeatedly warned that the building sector must transform rapidly to meet global climate targets. Bio-based materials such as hempcrete offer a rare opportunity to not merely reduce emissions but reverse them, because the hemp plant absorbs carbon dioxide as it grows and the lime binder continues to absorb the gas as it slowly carbonates over the life of the building.

The researchers prepared two hempcrete mixes with identical hemp-to-binder-to-water ratios of 1.0:2.8:2.8 by mass. The reference mix combined a dolomitic pressure-hydrated lime, a calcium hydrated lime and a highly reactive metakaolin in proportions of forty, twenty and forty percent of the binder respectively. The modified mix, designated M1, replaced three percent of that binder with potassium sulphate. The dosage was chosen deliberately, informed by prior studies showing that sulphate-based activators accelerate pozzolanic reactions in lime and slag systems. In related binders, sodium sulphate has produced strength gains of up to 274 percent at one day, and potassium sulphate has outperformed it in cement-slag mortars, lifting compressive strength by five to six megapascals at higher dosages.

The mechanism behind the enhancement is the accelerated precipitation of ettringite, a calcium aluminium sulphate hydrate mineral. Metakaolin supplies abundant reactive alumina, confirmed by X-ray fluorescence analysis showing 44.40 percent aluminium oxide in the material, while the limes supply calcium oxide at 35.94 and 71.06 percent respectively. The potassium sulphate delivers the sulphate ions needed for ettringite crystals to form rapidly during early curing. Scanning electron microscopy confirmed the prismatic and cuboidal crystal morphology of the activator and the angular, plate-like texture of the metakaolin, while energy-dispersive spectroscopy verified the elemental signatures of every constituent, including the hemp shives themselves, whose water-transporting vessels and tracheids were imaged in detail.

The mechanical results were striking. The activated mix achieved compressive strengths 50.47 percent higher than the reference at seven days, 99.73 percent higher at fourteen days, and 64.99 percent higher at twenty-eight days, reaching 0.681 megapascals compared with 0.413 megapascals for the control. Those figures may sound modest next to ordinary concrete, but for a material weighing roughly 548 kilograms per cubic metre, less than a quarter the density of conventional concrete, they represent a meaningful advance. The activated hempcrete outperformed comparable lime-metakaolin hempcretes reported in the literature, including mixes that incorporated Portland cement or were cured for ninety days, and it matched or exceeded sunflower bark and rice husk composites of similar density.

Equally important was what the additive did not do. Density, the property that underpins hempcrete’s thermal insulation, was essentially unchanged: ambient-cured specimens registered 547.27 kilograms per cubic metre for the reference mix and 548.59 for the activated mix at twenty-eight days. Oven-dried specimens were lighter still, at roughly 502 to 519 kilograms per cubic metre, placing the material in the same density class as autoclaved aerated concrete. The activated mix also retained its strength far better after oven drying, losing only 6.06 percent of its compressive strength compared with a 44.09 percent loss for the reference, suggesting that the sulphate activation produced a more stable internal microstructure.

Durability testing revealed further benefits. Using a digital image correlation technique with two cameras tracking microscopic displacements over six hundred hours, the researchers measured drying shrinkage, a property that has remained largely unexplored in hempcrete composites. The activated mix shrank by only 0.247 percent, against 0.344 percent for the reference, a clear sign of improved dimensional stability attributed to early ettringite formation. Capillary water absorption tests showed that the activated mix absorbed water significantly more slowly in the first minutes of exposure, with an initial rate of 4.290 kilograms per square metre per minute versus 5.840 for the control, though after twenty-four hours of immersion the two mixes converged and the activated mix absorbed slightly more water over the full 144-hour test.

The carbon accounting was the study’s most consequential finding. In a bubble column reactor through which a carbon dioxide-rich gas mixture was bubbled for sixty minutes, the reference binder sequestered 0.229 kilograms of carbon dioxide per kilogram of binder and the activated binder 0.207, with pH rising above thirteen within the first minute as calcium dissolved and then falling as carbonate precipitated. Scaling this up in a cradle-to-gate life-cycle assessment, each 290 by 140 by 90 millimetre specimen emitted about 0.75 kilograms of carbon dioxide equivalent during production, dominated almost entirely by lime calcination, but sequestered 0.920 kilograms through hemp growth plus roughly 0.3 kilograms through binder carbonation. The net result was negative: minus 0.491 kilograms of carbon dioxide equivalent per specimen for the reference mix and minus 0.470 for the activated mix.

That the potassium sulphate addition shifted the net footprint by only about four percent is the crux of the study’s argument. Previous attempts to strengthen hempcrete, whether by adding more binder or by densifying the mix, have typically raised embodied energy and degraded the hygrothermal performance that makes the material attractive in the first place. Here, a three percent chemical addition delivered near-doubled strength and measurably better dimensional stability at negligible environmental cost. The authors caution, however, that the proposed ettringite mechanism rests on compositional evidence from X-ray fluorescence and electron microscopy, and they recommend direct confirmation through X-ray diffraction and thermal analysis in future work.

The road to commercial viability still requires answers to harder questions. The researchers recommend expanding the dosage range to map the full response curve, and they stress that long-term performance, including freeze-thaw resistance, accelerated ageing and biodeterioration over multi-year cycles, remains untested. End-of-life pathways for hempcrete, and the degradation kinetics that would allow a true cradle-to-grave assessment, are also largely uncharted. But the direction of travel is clear. A material that grows in fields, absorbs carbon as it cures, and can now be made substantially stronger with a pinch of inexpensive fertiliser chemistry is edging closer to the mainstream of sustainable construction, where every kilogram of embodied carbon matters.

Subject of Research: Enhancing the mechanical and durability performance of hempcrete with potassium sulphate while maintaining carbon sequestration

Article Title: Performance of hempcrete for carbon-negative construction: Density, compressive strength, durability and carbon sequestration

Article References: Steyn, K., De Villiers, W., & Babafemi, A. J. (2026). Performance of hempcrete for carbon-negative construction: Density, compressive strength, durability and carbon sequestration. Case Studies in Construction Materials, 25, Article e06597. https://doi.org/10.1016/j.cscm.2026.e06597

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06597

Keywords: hempcrete, carbon-negative construction, potassium sulphate, compressive strength, ettringite, lime binder, metakaolin, drying shrinkage, carbon sequestration, life-cycle assessment, sustainable building materials, durability

The coating, dubbed Ag-Zeo by the team, begins with an unlikely raw material: metakaolin derived from halloysite clay nanotubes, a natural aluminosilicate precursor. Conventional ZSM-5 synthesis relies on organic structure-directing agents such as tetrapropylammonium hydroxide, chemicals that are expensive and environmentally unfriendly. By substituting metakaolin, Ludox silica, silver nitrate and a small amount of ZSM-5 seed crystals, the researchers avoided organic templates entirely. The polished steel coupons were cleaned, aged in the zeolite sol for 48 hours, and then crystallized in a high-pressure reactor at 175 degrees Celsius for two days, allowing zeolite crystals to nucleate and grow directly on the metal surface without any binder.

Characterization confirmed that the coating is the real thing. X-ray diffraction revealed the hallmark reflections of the MFI-type ZSM-5 framework, including the intense peak near 23 degrees two-theta, alongside weaker peaks at 38.1 and 44.8 degrees corresponding to face-centered cubic metallic silver. Nitrogen adsorption measurements showed a Type I isotherm typical of microporous materials, with an exceptionally high BET surface area of about 474 square meters per gram, a pore volume of 0.26 cubic centimeters per gram and an average pore diameter of 2.24 nanometers. Scanning and transmission electron microscopy captured the classic coffin-shaped micron-sized ZSM-5 crystals, roughly 1.21 micrometers across, deposited as a thin, homogeneous film of one to five percent by weight on the steel.

Perhaps the most important structural finding is that silver was incorporated into the zeolite framework rather than simply sprinkled on top. Diffuse reflectance ultraviolet-visible spectroscopy identified three distinct silver species: isolated Ag-plus ions absorbing near 210 nanometers, sub-nanometer silver clusters near 290 nanometers, and metallic Ag-zero nanoparticles showing a surface plasmon band at 418 nanometers. Fourier-transform infrared spectroscopy showed the preserved five-membered ring vibration at about 545 wavenumbers, while shifts in the silicon-oxygen and aluminum-oxygen stretching region between 970 and 1220 wavenumbers pointed to genuine isomorphous substitution. Energy-dispersive X-ray mapping confirmed silver at 1.03 percent by weight, uniformly distributed alongside silicon, aluminum, oxygen and sodium throughout the crystals.

On the coated implant, elemental mapping told an equally encouraging story. Signals from iron, chromium and nickel revealed the underlying alloy, while silver, silicon, oxygen and aluminum confirmed a continuous zeolite layer above it. Because the coating is thin, the electron beam still interacts with the substrate, indicating intimate contact between the two. The researchers argue that this uniform integration matters biologically: silver locked into the framework leaches slowly and evenly, avoiding the concentration spikes that plague surface-deposited nanoparticles, while the microporous architecture can wick up biological fluids and improve interactions between the implant and surrounding tissue.

Corrosion testing in a brutal acidic chloride bath, one molar sodium chloride at pH 1.0, showed the coating earning its keep. Bare stainless steel started with a corrosion current density of 0.014 amperes per square centimeter, which climbed to 0.046 within days as chloride ions attacked the passive chromium oxide film. The Ag-Zeo coated samples moved in the opposite direction, with corrosion current falling from 0.03 to 0.021 amperes per square centimeter over five days of immersion, a sign that the coating-substrate interface was stabilizing rather than degrading. Electrochemical impedance spectroscopy reinforced the picture, showing larger semicircles and higher charge-transfer resistance for coated samples, consistent with a barrier that limits ion diffusion to the metal.

The drug delivery results may be the most clinically provocative. Cisplatin, a platinum-based chemotherapy used against osteosarcoma and other cancers, was loaded into the zeolite with an encapsulation efficiency of 88 percent and a loading capacity of 4.4 percent. When the loaded coating was tested across a pH gradient, it behaved like a smart material. At pH 5.6, which mimics the acidic microenvironment of bone tumors, cumulative cisplatin release climbed steadily to about 15 percent over 72 hours. At physiological pH 7.4, release was slower, reaching only about 10 percent in the same period. Crucially, the profile showed gradual, sustained liberation rather than the dangerous burst release common to many polymer carriers, a behavior the team attributes to host-guest interactions inside the zeolite channels and to a synergistic effect between silver and the platinum complex.

Antifungal performance rounded out the platform’s credentials. Implant-associated Candida albicans infections are notoriously difficult to treat because the fungus builds resilient biofilms that resist azoles, polyenes and other standard drugs. In adhesion assays using Candida albicans ATCC 14053, surfaces treated with Ag-Zeo at 16 milligrams per milliliter showed dramatically fewer adherent, viable fungal cells than untreated surfaces or surfaces treated with zeolite alone, which performed no better than the control. That comparison isolates silver as the active agent, delivered steadily from its zeolite host. The authors point to the established mechanisms of silver action, including reactive oxygen species generation, oxidative and nitrosative stress, membrane disruption and interference with microbial proteins and DNA, all sustained by the controlled ion release that the framework provides.

Taken together, the study sketches a next-generation implant surface that is simultaneously protective, therapeutic and antimicrobial, all from a clay-derived, template-free synthesis that the researchers describe as a sustainable strategy for implant coating technology. The work remains at the laboratory stage, and questions of long-term biocompatibility, mechanical wear and silver dosing in living patients will need to be answered before clinical translation. But the concept, a single green coating that resists corrosion, fights fungal colonization and dispenses chemotherapy on demand in response to tumor acidity, offers a compelling glimpse of where biomaterials engineering is heading: away from passive hardware and toward implants that actively participate in their own defense and in the treatment of the diseases that put them there.

Subject of Research: A green-synthesized silver-integrated ZSM-5 zeolite coating on AISI 316 L stainless steel implants for corrosion resistance, controlled cisplatin delivery and antifungal activity

Article Title: Sustainable silver-integrated ZSM-5 coating on AISI 316 L implants: a multifunctional platform for controlled cisplatin delivery and antifungal activity

Article References: Ravinayagam, V., Cevik, E., Tamilvanan, M., Akhtar, S., Rehman, S., & Jermy, B. R. (2026). Sustainable silver-integrated ZSM-5 coating on AISI 316 L implants: a multifunctional platform for controlled cisplatin delivery and antifungal activity. Journal of Saudi Chemical Society, 30(4), Article 55. https://doi.org/10.1007/s44442-026-00108-3

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00108-3

Keywords: zeolite coating, ZSM-5, silver nanoparticles, AISI 316L stainless steel, cisplatin delivery, antifungal activity, Candida albicans, corrosion resistance, hydrothermal synthesis, metakaolin, drug release, biomaterials

The team, led by Cyrill Joël Ngally Sabouang and Jean Aimé Mbey and drawn from the University of Yaoundé I, the University of Bamenda and the University of Ngaoundere, started with kaolin clay from the Mayouom locality of Cameroon. Kaolinite, the dominant mineral in this clay, is a layered silicate made of one tetrahedral silica sheet stacked on one octahedral alumina sheet, with an ideal formula of Al2Si2O5(OH)4. That structure means kaolinite contains roughly 46.5 percent silicon dioxide by mass, making it an attractive and abundant feedstock for producing amorphous silica, the reactive form of the material prized for adsorption applications. Prior to synthesis, the raw clay was wet-sieved at 45 micrometers, dried, and characterized as containing more than 80 weight percent kaolinite with good structural organization.

The first critical transformation was thermal. Heating the kaolin in a muffle furnace to 650 degrees Celsius at 5 degrees per minute, holding for one hour, and then cooling converted the crystalline kaolinite into amorphous metakaolinite. X-ray diffraction confirmed the change: the characteristic kaolinite reflection at 7.16 angstroms vanished, replaced by a broad diffraction halo between 20 and 35 degrees two-theta that signals a disordered, glass-like atomic arrangement. Infrared spectroscopy told the same story from a different angle. The sharp O-H stretching bands of kaolinite at 3691 and 3620 inverse centimeters disappeared after calcination, evidence that the structural hydroxyls had been driven off and the crystal lattice disrupted. Thermal analysis added a third confirmation, showing the expected dehydroxylation event near 546 degrees with a 10.8 percent mass loss, and a later exothermic event near 1013 degrees marking the onset of mullite formation if the material were heated further.

Before gel synthesis, the metakaolin was purified by dispersing it in excess 3 molar hydrochloric acid for one hour under constant stirring, a treatment that dissolves metallic impurities such as iron oxide and calcium or magnesium compounds. After decantation and repeated washing with distilled water—until a silver nitrate test confirmed the supernatant was free of chloride ions—the cleaned solid was dried and stored airtight. The synthesis itself followed a classic sol-gel route: batches of 5, 10, 15 and 20 grams of treated metakaolin were dispersed in 100 milliliters of 8 molar sodium hydroxide, stirred for 20 minutes, and left for 24 hours. During this alkaline digestion, hydroxide ions attack the silicate network of the metakaolin, dissolving silicon species into solution as sodium silicate. The supernatant was filtered off, and the pH was dropped to 3 with hydrochloric acid, triggering the hydrolysis and condensation reactions that precipitate silica gel. The gels were collected by centrifugation, dried at room temperature, and labeled GS1:20 through GS4:20 according to their clay-to-solution ratios.

X-ray diffraction of the finished gels revealed a broad amorphous silica halo between 15 and 40 degrees two-theta in every sample, but the details differed in ways that turned out to be scientifically revealing. All gels contained crystalline reflections from halite—ordinary sodium chloride—formed either when excess sodium hydroxide reacted with the hydrochloric acid during the pH adjustment step, or when sodium silicate was directly neutralized during precipitation. Crucially, the intensity of the halite peak at 2.82 angstroms shrank as the metakaolin fraction increased, indicating that the dominant source of salt contamination was leftover alkali rather than the silicate itself. The researchers suggest that a post-centrifugation washing step could strip out much of this by-product, and note that the brief 3-minute spin at 2000 rpm may have left enough liquid in the cake for salt to precipitate during drying.

The highest metakaolin loading, GS4:20, produced the most distinctive mineralogy. Its halite peaks were faintest, its amorphous halo strongest, and its pattern showed traces of hydroxysodalite, an ordered aluminosilicate framework that the authors interpret as the beginning of crystallization from the silica oligomers in the gel. They attribute this to the mixture approaching a stoichiometric balance, leaving less free sodium available to form salt. The halo intensity ranked GS3:20 below GS2:20, below GS1:20, below GS4:20, consistent with more silica gel forming as more metakaolin was dissolved into the alkali.

Infrared spectroscopy then exposed how the ratio sculpted the gel’s molecular structure. All gels showed a broad band near 3385 inverse centimeters from O-H stretching of terminal silanol groups and adsorbed water, a Si-OH vibration near 930, an H-O-H bending band near 1640 from physically adsorbed water, and Si-O-Si stretching modes near 1010, 780 and 420. In GS4:20, the silanol and water bands were strongest, pointing to a network built from short oligomeric chains. Shorter chains mean more chain ends, and more chain ends mean more exposed hydroxyl groups available to hydrogen-bond with water molecules—an outcome directly beneficial for a desiccant. By contrast, gels made with less metakaolin, such as GS1:20, showed enhanced Si-O-Si bands without equivalent silanol signals, indicating longer polymer chains with fewer terminal hydroxyls, and trapped structural water instead.

Thermal analysis corroborated the spectroscopic picture. All samples lost physisorbed water below 100 degrees, but the temperature of that event increased in the order GS2:20, GS3:20, GS1:20, GS4:20, implying progressively stronger water-gel bonding forces. Only GS1:20 and GS2:20 showed additional water evaporation between 100 and 210 degrees, attributed to water trapped inside the growing silica network when more effective dissolution allowed optimal chain growth. Total mass loss rose with metakaolin content, again consistent with short-chain, hydroxyl-rich structures adsorbing more water. A thermal event near 800 degrees marked halite melting, weakest in GS4:20, matching the diffraction evidence of minimal salt in that sample.

The overall mechanistic narrative is one of competing interactions. When metakaolin is scarce, the abundant hydroxide solution interacts strongly and effectively with each silicate unit, dissolving species thoroughly and allowing silica particles to grow into extended chains that incorporate structural water. When metakaolin is plentiful, solid-solid interactions between particles stabilize them and limit their contact with the alkali, curtailing particle growth and freezing short oligomers rich in terminal hydroxyls into the network. Meanwhile, excess sodium hydroxide left over after sodium silicate formation reacts with the hydrochloric acid added for precipitation, generating the halite by-product that contaminates low-ratio gels.

The authors are careful to flag a limitation: the study did not specify sodium-to-silicon ratios in a normalized way, which may restrict how directly the conclusions transfer to metakaolins from other natural sources with different purities and reactivities. Even so, the practical takeaway is striking. By simply adjusting how much clay is loaded into the alkali bath, a producer can bias the product toward either long-chain gels with structural water or short-chain gels dense with adsorption-ready hydroxyl groups—tunable moisture retention from one of the cheapest raw materials on Earth. For a material that quietly protects everything from medicines to microchips against humidity, that kind of formulation control, derived from Cameroonian kaolin and conventional laboratory reagents, could reshape how low-cost desiccants are designed.

Subject of Research: Effect of metakaolin mass fraction in sodium hydroxide on sol-gel silica gel structure and water retention

Article Title: Influence of the metakaolin mass fraction in sodium hydroxide solution on the synthesis of silica gel from metakaolin

Article References: Ngally Sabouang, C. J., Kogaing Fonkou, B. W., Mache, J. R., Kong, S., & Mbey, J. A. (2026). Influence of the metakaolin mass fraction in sodium hydroxide solution on the synthesis of silica gel from metakaolin. Discover Industrial Chemistry and Materials, 1(1), Article 13. https://doi.org/10.1007/s44508-026-00015-w

Image Credits: AI Generated

DOI: 10.1007/s44508-026-00015-w

Keywords: metakaolin, silica gel, sol-gel, kaolinite, sodium hydroxide, amorphous silica, X-ray diffraction, FTIR, thermal analysis, desiccant, hydroxysodalite, materials chemistry