The work, carried out by Ferit Cakir, Pinar Yildirim, and Mohsen Shams, addresses a longstanding problem in structural rehabilitation. Aging concrete and masonry structures often suffer from deteriorated material, insufficient load-carrying capacity, and limited ability to deform without catastrophic failure. Traditional fixes—jacketing with reinforced concrete or steel—work, but they add significant weight and bulk and demand labor-intensive construction. Fiber-reinforced polymer composites have become the elegant alternative: they are corrosion-resistant, add almost no mass, and can be bonded directly onto existing surfaces. Yet each fiber family has its own Achilles heel. Carbon fibers are stiff and strong but fail suddenly and brittlely. Glass fibers stretch further but are less stiff. Aramid fibers, the stuff of bulletproof vests, absorb enormous energy but struggle in compression and degrade under moisture and ultraviolet light.

Hybridization is the attempt to have it all. Rather than relying on a single fiber type, engineers weave or laminate two fiber families together so that a stiff fiber carries load at small deformations while a tougher, more extensible fiber keeps working after damage begins, delaying unstable fracture. The fabrics used in this study take the concept to its finest scale: intraply hybrids, in which carbon, glass, and aramid yarns are interwoven within the same plain-weave layer, not stacked in separate plies. The team tested three combinations—carbon-glass, aramid-carbon, and glass-aramid—each with a nominal areal weight of 200 grams per square meter. Because both fiber families share the same bonded surface, they can participate simultaneously in load transfer and crack control, a property the authors argue makes intraply architecture especially promising for external strengthening of cementitious elements.

The experimental program was disciplined in its symmetry. Each fabric was bonded with a two-component epoxy in three orientations—horizontal, vertical, and diagonal at 45 degrees—producing nine strengthened configurations, each represented by three specimens. Mortar prisms of 40 by 40 by 160 millimeters were tested in three-point bending, 50-millimeter cubes under axial compression, and cured fabric coupons in direct tension, all following the EN 196-1 and ASTM D3039 standards. Unstrengthened reference specimens established the baseline: 6.66 megapascals of flexural strength and 46.29 megapascals in compression. Crucially, the amount of reinforcement was kept identical across orientations, so any difference in performance could be traced to fiber composition and geometry rather than quantity of material.

The tension tests delivered the first surprise. The aramid-carbon fabric pulled along its horizontal aramid direction was the champion, averaging 178.18 megapascals—nearly six times the 30.82 megapascals of the glass-aramid fabric pulled along its weak aramid direction. Intriguingly, carbon was not the strongest direction in either carbon-containing fabric, a sign that yarn count, crimp, and impregnation quality sculpt each fabric’s directional character as much as the fiber chemistry does. In every hybrid, the diagonal coupon fell between the horizontal and vertical results, exactly what the orthogonal weave predicts when both families share the load at 45 degrees. Statistical analysis confirmed that hybrid system, orientation, and their interaction all shaped tensile strength with overwhelming significance.

But it was the structural tests that rewrote the rankings. Every strengthened prism beat the unconfined reference, with flexural strengths climbing from 16.04 to 39.41 megapascals—gains of 141 to 492 percent. The star was not the aramid-carbon fabric but the carbon-glass hybrid laid diagonally, which reached 39.41 megapascals, almost six times the reference. Under bending, cracks rarely travel perfectly straight; they fork and incline as stresses redistribute. Diagonal reinforcement intercepts those wandering crack paths at multiple points, mobilizing both fiber families at once, spreading damage instead of concentrating it in one fracture plane. The researchers observed precisely this behavior: diagonal specimens developed distributed, inclined cracking, while horizontal and vertical layouts produced a single localized crack that the fabric bridged. In fact, within every hybrid system, the horizontal-versus-vertical ranking in bending mirrored the coupon tensile strength of whichever fiber family ran along the prism axis—a satisfying confirmation that post-cracking flexural capacity is governed by the crack-bridging family.

Compression told a very different and humbling story. Here the fabric does not pull across cracks; it acts as a passive corset, restraining the lateral expansion that Poisson effects generate as the mortar core crushes. Only the fiber family wrapped around the circumference matters, and its performance depended on fit rather than raw strength. Just two configurations beat the reference significantly: carbon-glass vertical, at 53.70 megapascals, a 16 percent gain, and glass-aramid horizontal, at 53.51 megapascals, a 15.6 percent gain. Both placed glass yarns in the hoop direction—a telling detail, because glass’s higher failure strain lets the wrap stretch around square corners and follow the dilating core without rupturing prematurely. Meanwhile, every aramid-carbon layout and the vertical glass-aramid layout significantly reduced compressive strength, with the worst case dropping 40.1 percent. The authors attribute the losses to sharp unrounded corners concentrating stress in the fabric, platens already providing passive restraint at the cube ends, and aramid yarns being difficult to impregnate with epoxy, weakening the confining action.

A two-way factorial analysis of variance formalized the pattern. For compressive strength, the interaction between hybrid system and orientation produced the largest effect of the entire study, with a partial eta-squared of 0.928—meaning the benefit of any given orientation is inseparable from the fibers it positions. The same held for tension and flexure. The researchers are careful about terminology: without single-fiber control fabrics, they cannot claim a quantified synergistic hybrid effect against a rule-of-mixtures baseline, so the differences among fabrics are interpreted as effects of hybrid composition—the specific fiber combinations, proportions, and arrangements within the weave—rather than proven synergy. This candor does not diminish the practical punch of the findings; it sharpens them.

The broader message is one that materials data sheets cannot convey. Direct tensile tests characterize the reinforcement itself, but once a composite is glued to a cementitious substrate, performance emerges from the interplay of matrix cracking, adhesive stress transfer, woven geometry, and the governing failure mechanism. The configuration that wins in tension lost in bending; the one that ruled bending fared poorly in compression. The authors therefore recommend matching the fabric to the demand: diagonal carbon-glass for flexure-dominated members, carbon-glass vertical or glass-aramid horizontal for compression-dominated ones, with carbon-glass vertical the most balanced choice overall—and vertical glass-aramid layouts to be avoided. They also caution that these are laboratory-scale results from homogeneous mortar, with bond behavior, environmental durability, cyclic seismic loading, and full-scale masonry geometry still to be verified. For the aging bridges, walls, and heritage structures that increasingly need help carrying their load, the study offers a deceptively simple design rule: orient the strongest direction of the fabric along the principal tensile stress, whether that runs down a beam or around a column, and let the weave do the rest.

Subject of Research: Structural strengthening of mortar elements using intraply hybrid composite fabrics with varied fiber combinations and reinforcement orientations

Article Title: Structural performance of mortar elements strengthened with intraply hybrid composite fabrics: Combined effects of fiber hybridization and reinforcement orientation

Article References: Cakir, F., Yildirim, P., & Shams, M. (2026). Structural performance of mortar elements strengthened with intraply hybrid composite fabrics: Combined effects of fiber hybridization and reinforcement orientation. Case Studies in Construction Materials, 25, Article e06610. https://doi.org/10.1016/j.cscm.2026.e06610

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06610

Keywords: hybrid composites, fiber-reinforced polymer, mortar strengthening, flexural strength, confinement, carbon fiber, glass fiber, aramid fiber, fiber orientation, structural rehabilitation, masonry, textile reinforcement

The premise behind so-called dual-scale or multiscale reinforcement is straightforward on paper. Glass fibers, typically tens of micrometers in diameter, are workhorses that carry load along their length and dramatically raise stiffness and strength. Organoclay, by contrast, consists of layered silicate platelets roughly one nanometer thick that, when properly dispersed as individual sheets in a process called exfoliation, can interfere with polymer deformation at the molecular scale. If the two fillers could each do their job without getting in each other’s way, the argument goes, a hybrid composite should outperform either single-filler system. That synergy has been reported in some polyamide systems, and it has fueled a large body of research on hybrid glass fiber and nanoclay composites across thermoplastic and thermoset matrices alike.

The Korean-American team set out to determine whether such synergy actually exists in polypropylene, one of the most commercially important thermoplastics but also one of the most chemically challenging matrices for nanocomposite work. Unlike nylon 6, whose polar chains can pry apart clay galleries directly, polypropylene is nonpolar and has no natural affinity for the silicate surface. Getting clay to exfoliate in polypropylene requires a compatibilizer, almost always maleic anhydride grafted polypropylene, known as PP-g-MA. The maleic anhydride groups interact with the organically modified clay surface while the polypropylene backbone entangles with the matrix, tethering the polymer to the platelets and giving the shear of melt processing a chance to peel the layers apart.

Before any hybrid specimens could be judged, the researchers had to solve a processing problem that plagues every short-fiber thermoplastic compounder: glass fibers break. Every pass through an extruder screw, every sharp corner in a melt channel, shears the brittle fibers into shorter fragments, and because the reinforcing efficiency of short fibers depends strongly on their length relative to a critical length, fiber breakage translates directly into lost stiffness and strength. The team compared two processing routes and found that a single extrusion pass followed by injection molding, with dry glass fibers fed into the process, preserved fiber length best. That condition was adopted as the optimum for all subsequent experiments, a deceptively simple but consequential methodological choice that maximized the contribution the fibers could make.

With processing settled, attention turned to the compatibilizer. Because the reinforcement from organoclay depends almost entirely on achieving exfoliation, the amount of PP-g-MA matters enormously: too little and the platelets stay stacked in useless tactoids, too much and the low-molecular-weight compatibilizer plasticizes the matrix and degrades properties. Preliminary experiments established an optimum of 4 weight percent PP-g-MA for the polypropylene organoclay system, a value consistent with the ratio-driven behavior documented in earlier polypropylene nanocomposite literature, including the influential work of Kim, Reichert, and colleagues on how the compatibilizer-to-clay ratio governs morphology. Armed with the right compatibilizer level and the gentlest viable process, the researchers molded specimens spanning the full experimental grid: montmorillonite contents from 0 to 7 percent and glass fiber contents from 0 to 30 percent.

The characterization campaign combined microscopy, tensile testing, and impact testing, and the results tell a story of competition rather than cooperation. When both fillers are present, they must share the same pool of PP-g-MA, and the data show that the glass fibers are by far the greedier partner. The compatibilizer is largely consumed by the fiber surfaces and by the matrix demands associated with them, leaving the organoclay starved of the interfacial chemistry it needs to exfoliate. The silicate platelets, deprived of their tether to the polypropylene, fail to deliver the nanoscale reinforcement that makes clay nanocomposites attractive in more polar matrices. Under the microscope, the clay remains poorly dispersed precisely in the formulations where it was supposed to add a second dimension of strengthening.

The mechanical measurements drive the point home. Across the composition matrix, the overwhelming majority of the observed reinforcement, in both stiffness and strength, traces back to the glass fibers, while the organoclay’s incremental contribution is minimal once the fibers are present. Perhaps most strikingly, when the team modeled the modulus of the hybrid composites using established micromechanical frameworks, the kind of composite theory built on the Eshelby inclusion concept, the Mori-Tanaka averaging scheme, and the widely used Halpin-Tsai equations, the experimental data were well described by purely additive contributions from the two fillers. There was no synergistic boost in modulus beyond what simple summing predicts. In the language of composite engineering, the hybrid system behaves as a linear combination of its ingredients, not as a new material with emergent properties.

Why does this matter beyond the laboratory? For one thing, it disciplines expectations. The hybrid nanocomposite concept has generated hundreds of papers and considerable industrial curiosity, partly because successful examples exist in polyamide 6, where clay and glass fiber can genuinely complement each other and where nanoclay has even been shown to strengthen the fiber-matrix interface. The new results make clear that such success does not transfer automatically to polypropylene, and they identify the mechanism: compatibilizer competition. Any formulator attempting a dual-scale polypropylene composite must budget PP-g-MA for both fillers, or accept that one of them, almost certainly the clay, will underperform. The study also delivers practical guidance in its own right, since the finding that a single extrusion with dry fiber feed minimizes breakage is directly actionable for compounders seeking to squeeze maximum performance from glass fiber reinforced polypropylene.

There is also a broader lesson about how materials science actually progresses. The romantic narrative of nanotechnology suggests that adding a dash of nanofiller will upgrade any conventional material, and early landmark results, from the Toyota nylon 6-clay hybrids of the early 1990s onward, encouraged exactly that hope. Three decades later, the field has matured to the point where careful, systematic studies like this one can say with confidence when the nano-addition is worth it and when it is not. Here, the honest answer is that in polypropylene, the micrometer-scale glass fibers do nearly all the mechanical work, the nanoclay mostly watches from the sidelines, and the compatibilizer chemistry that makes nanocomposites possible is a finite resource that the fibers win by default. For engineers designing lighter, stiffer polypropylene parts for automotive and consumer applications, that is a valuable, money-saving truth, and for researchers pursuing genuine multiscale synergy, it defines the real obstacle to overcome: not the fillers themselves, but the shared interfacial chemistry that both of them need to succeed.

Subject of Research: Dual-scale reinforcement of polypropylene with glass fiber and organoclay

Article Title: Influence of dual-scale reinforcement on the structure–property relationships of polypropylene composites

Article References: Kim, J., Paul, D. R., James, C. M., & Yoo, Y. (2026). Influence of dual-scale reinforcement on the structure–property relationships of polypropylene composites. Polymer Bulletin, 83(12), Article 646. https://doi.org/10.1007/s00289-026-06630-z

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06630-z

Keywords: polypropylene, glass fiber, organoclay, nanocomposites, montmorillonite, PP-g-MA, compatibilizer, fiber breakage, exfoliation, micromechanical modeling, hybrid composites, mechanical properties

The study, published in the journal Advances in Industrial and Engineering Chemistry, was conducted by Jong-Hyun Kim and Dong-Jun Kwon of Gyeongsang National University together with Pyeong-Su Shin of the Korea Institute of Convergence Textile. The researchers set out to answer a deceptively simple question: when you want to know how strongly an epoxy resin will grip a glass fiber, should you trust the angle formed by a droplet sitting on the surface, or the forces recorded as a sample is dipped into a liquid? Their answer, backed by hundreds of measurements, favors the dynamic approach.

The logic behind contact angle analysis is elegant. When a drop of water, diiodomethane, formamide or ethylene glycol is placed on a solid, the shape of that drop reflects the balance of surface tensions at the solid, liquid and air interfaces. By measuring angles with several liquids of known surface tension, researchers can decompose a material’s surface energy into dispersive and polar components using the Owens-Wendt equation, and from there calculate the work of adhesion between two materials. In theory, a higher work of adhesion should translate directly into a stronger interface, quantified as interfacial shear strength, or IFSS, the stress needed to shear the fiber out of its resin jacket.

The team compared two measurement philosophies. The static method is the one most laboratories know well: deposit a droplet, photograph it, and read the angle. For flat epoxy plates the droplet is about a millimeter across; for cylindrical glass fibers, the researchers had to work with droplets of roughly 100 micrometers, either resting on the fiber or embedded along it, and extract the angle from the droplet geometry using the Pythagorean theorem and Young’s equation. The dynamic method, by contrast, never looks at a droplet at all. Instead, a DCAT 11 instrument lowers the sample into a test liquid at a controlled speed of one millimeter per minute to a depth of two millimeters, and records the weight change. From the forces during immersion and withdrawal, the advancing and receding contact angles are calculated through the Wilhelmy principle.

The static method’s weakness revealed itself quickly. On glass fibers, the measured contact angle changed not only with the solvent but also with the size of the droplet, even when the surface tension stayed the same. Large droplets sag under their own gravity, distorting the geometry that the angle calculation depends on. Small droplets evaporate rapidly, so the angle captured in the photograph is often a receding angle rather than the advancing angle that theory requires. The researchers also noted that image quality, resolution, brightness, contrast and viewing angle all introduce errors, and that even a carefully polished surface can carry dust or microcracks that skew the result. In short, the static measurement is hostage to a long list of experimental subtleties.

To settle the question, the researchers built four epoxy systems from two resins with different numbers of epoxide groups, DGEBA and TGDDM, and four hardeners with distinct chemistries: an acid anhydride, an amino hardener, and two variants of diaminodiphenyl sulfone known as 33DDS and 44DDS. They measured surface energies for each cured resin and for the glass fiber, computed the work of adhesion for every fiber-resin pairing by both methods, and then measured the true interfacial shear strength with the microdroplet pull-out test. In that test, tiny resin droplets are cured onto single fibers and pulled off in a universal testing machine fitted with a custom microvice, forty times per condition, at a crosshead speed of 0.5 millimeters per minute. The debonding force, divided by the fiber circumference and embedded length, yields the IFSS.

The pull-out results crowned a clear winner among the resins: the TGDDM/33DDS system, whose high crosslink density and stiffness produced the strongest bond to the glass fiber. But when the researchers checked whether the work of adhesion values predicted that ranking, the two measurement methods diverged sharply. The static method produced work of adhesion values around 100 dyne per centimeter that failed to track the IFSS trend at all. The formulation with the highest measured shear strength showed a comparatively low static work of adhesion, while resins with low amino content scored high adhesion values despite weaker interfaces. The dynamic method returned lower absolute values, near 60 dyne per centimeter, but crucially its ranking of the four resins mirrored the ranking from the mechanical pull-out tests.

The explanation lies in how much contact each method actually creates. During dynamic immersion, the liquid wets a large and uniform area of the sample, so the recorded forces integrate genuine chemical interactions across the whole solid-liquid interface. A static droplet, by contrast, touches only a small patch, and its shape is at the mercy of gravity, evaporation and gradual impregnation of the liquid into the substrate over time. The researchers also observed measurement noise in the dynamic force curves for single fibers, a consequence of the tiny sensing area, yet the weight-based data still proved more quantitative and reproducible than droplet photography. For micro-scale fibers in particular, the sensitivity of the balance becomes the decisive factor in measurement quality.

The implications reach well beyond epoxy and glass. Contact angle measurements are a standard tool for evaluating sizing agents on carbon fibers, plasma treatments on basalt, and surface modifications across the fiber-reinforced composites industry. If static droplet measurements can invert the apparent ranking of materials, then design decisions based on them, from sizing formulations to resin selection, may rest on shaky ground. The Korean team’s conclusion is pragmatic: the static method remains attractive for its simplicity, but for reliable analysis of interfacial properties, the dynamic contact angle method is the more appropriate choice, because it is the one whose surface energy predictions actually agree with what happens when fiber meets resin under load.

There is a broader lesson in the result about the gap between thermodynamic prediction and mechanical reality. The work of adhesion is a theoretical quantity, calculated from idealized surfaces and equilibrium angles, while interfacial shear strength reflects crosslink density, matrix modulus, curing conditions and the messy mechanics of debonding. The dynamic method, by sampling a larger interface and delivering advancing angles uncontaminated by evaporation or gravity, comes closer to bridging that gap. As composites push into ever more demanding applications, the study suggests that the humble dipping experiment, harder to perform but richer in information, deserves a more central place in the materials laboratory than the familiar photograph of a droplet.

Subject of Research: Comparison of static and dynamic contact angle methods for predicting fiber-matrix interfacial properties in epoxy-glass fiber composites

Article Title: Evaluation of static and dynamic contact angle methods for their correlation with interfacial properties in composites

Article References: Kim, J.-H., Shin, P.-S., & Kwon, D.-J. (2025). Evaluation of static and dynamic contact angle methods for their correlation with interfacial properties in composites. Advances in Industrial and Engineering Chemistry, 1(1), Article 16. https://doi.org/10.1007/s44405-025-00016-x

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00016-x

Keywords: contact angle, composites, glass fiber, epoxy, surface energy, interfacial shear strength, work of adhesion, microdroplet pull-out test, dynamic contact angle, static contact angle, fiber-matrix interface, Owens-Wendt equation

The research, conducted by Fatih Kantarci and Moncef L. Nehdi, centers on geopolymer concrete, an alkali-activated alternative to Portland cement concrete that is synthesized from industrial by-products rich in aluminum and silicon, such as blast furnace slag, metakaolin, and fly ash. When these precursors are mixed with highly alkaline solutions like sodium hydroxide, a chemical process called geopolymerization forms three-dimensional Si-O-Al-O polymeric gels that bind aggregates into a solid mass. Depending on the precursor and activator chosen, geopolymer binders can cut CO2 emissions by up to 80 percent compared with Portland cement, while saving roughly 60 percent of the energy and reducing production costs by about 25 percent. Geopolymer concretes have already found their way into road pavements, precast elements, and fire-resistant construction in the United States, Australia, Europe, and India.

Like most cementitious materials, however, geopolymer concrete is inherently brittle and prone to cracking under moderate loads or shrinkage stresses. The established remedy is fiber reinforcement, which enhances crack resistance, tensile strength, ductility, and impact performance while redistributing stresses within the matrix. Among the many fiber types available, the researchers selected glass fiber for its affordability, ease of manufacture, corrosion resistance, and high tensile strength of 1300 megapascals. The glass fibers used in the study were just 6 millimeters long and 15 micrometers in diameter, with an elastic modulus of 72 gigapascals and, crucially, a melting point of approximately 850 degrees Celsius, meaning they retain structural stability throughout the temperature range examined.

The experimental program began with a careful optimization of the geopolymer mix itself. Blast furnace slag from a local plant, with a specific gravity of 2.84 and a cement-like fineness, served as the primary precursor at a dosage of 400 kilograms per cubic meter. The team varied the sodium hydroxide activator concentration across 10, 12, and 14 molar solutions and tested alkali activator solution-to-binder ratios of 0.50 and 0.60. Compressive strength measurements at 7, 28, and 90 days revealed a clear optimum: strength increased as the sodium hydroxide concentration rose to 12 molar, then declined at 14 molar. The researchers attribute the initial gain to higher alkalinity, which dissolves silicon and aluminum links in the raw precursor to form aluminosilicate gels, while the decline at 14 molar reflects inhibited condensation reactions of silicate species and the precipitation of geopolymer gels that ultimately weaken the matrix. Scanning electron microscopy confirmed the story, showing a dense, compact microstructure with low porosity in the strongest mixes and abundant large pores and cracks in the weakest.

With the optimum production parameters established at 12 molar sodium hydroxide and a 0.60 activator-to-binder ratio, the team incorporated glass fibers at volume fractions of 0.3, 0.6, and 0.9 percent. Notably, the concrete was cured entirely under ambient laboratory conditions at 23 degrees Celsius and 55 percent relative humidity, with no steam or heat curing, removing a major barrier to casting geopolymer concrete on real construction sites. After 90 days of curing, the specimens were exposed to temperatures of 150, 300, 450, 600, and 750 degrees Celsius for one hour in a furnace heated at roughly 2 degrees Celsius per minute, then cooled under two contrasting regimes: gradual air cooling inside the opened furnace, or rapid immersion in room-temperature water, simulating the thermal shock that firefighting operations inflict on burning structures.

The results reveal a nuanced interplay between fiber content, temperature, and cooling method. Glass fibers improved compressive strength in both heated and unheated specimens, with the optimum at 0.6 percent by volume. At this dosage, fibers wrapped in geopolymer gel bond strongly to the matrix, bridging cracks, reducing stress concentrations at crack tips, and retarding crack propagation. The residual compressive strength of the fiber-reinforced samples actually increased up to 150 or 300 degrees Celsius, a phenomenon attributed to polycondensation and further densification of the tetrahedral aluminosilicate gels as moisture evaporates, before declining at higher temperatures. Remarkably, after exposure to 750 degrees Celsius, the water-cooled specimen containing 0.6 percent glass fiber retained a compressive strength approximately 33 percent higher than the plain, fiber-free samples. Beyond 450 degrees Celsius, however, the mismatch in thermal expansion coefficients between glass fibers and the geopolymer matrix generated interfacial stresses and microcracks, while partial softening of the fibers, dehydration of the gels, and thermal phase transformations further eroded strength.

The cooling regime proved to be a decisive variable. Water-cooled samples consistently exhibited lower residual compressive and flexural strengths than their air-cooled counterparts, because the steep temperature gradients during rapid quenching induce thermal shock, microstructural damage, and an elevated risk of explosive spalling. Flexural strength, which is particularly sensitive to crack initiation and propagation, benefited even more visibly from fiber reinforcement, since the three-dimensionally dispersed fibers direct crack paths and transfer stresses through a bridging effect that preserves specimen integrity. At all temperatures, the 0.6 percent fiber content delivered the highest flexural values, and the relative improvement from fiber addition was more pronounced in flexure than in compression, underscoring the dominant role of crack bridging in bending behavior.

Complementary measurements of weight loss and water absorption traced the progressive thermal deterioration of the material. Weight losses remained modest at 150 and 300 degrees Celsius, driven by the evaporation of free and absorbed water, but increased sharply after 450 degrees Celsius as thermal stress generated microcracks, and again at 750 degrees Celsius, where thermo-chemical damage degraded the geopolymer gel itself. The fiber-free air-cooled specimen lost 1.3 percent of its mass at 150 degrees Celsius but 7.1 percent at 750 degrees Celsius, roughly a five-and-a-half-fold increase, while 0.3 and 0.6 percent fiber additions reduced these losses by preserving microstructural integrity. Water absorption told a parallel story: values stayed nearly unchanged up to 450 degrees Celsius thanks to the dense matrix, then climbed as thermally induced shrinkage and thermal-shock microcracking opened new transport pathways. The fiber-free water-cooled sample doubled its water absorption from 4.3 to 8.6 percent after exposure to 750 degrees Celsius. Interestingly, the highest fiber dosage of 0.9 percent proved counterproductive, increasing water absorption because of poor workability, uneven fiber dispersion, and fiber balling, a reminder that more fiber is not always better.

Visual and microstructural examinations completed the picture. Sample surfaces brightened to a light brown up to 600 degrees Celsius and darkened to brown-black at 750 degrees Celsius, a coloration attributed to the gehlenite phase identified by X-ray diffraction, which also detected calcium silicate, calcium oxide, akermanite, and ilvaite. Crucially, the glass fibers did not melt even at 750 degrees Celsius, and no specimen fragmented, chipped, or disintegrated under either cooling regime, although water-cooled samples displayed more surface cracks. Scanning electron microscopy after 750 degrees Celsius showed that the fibrous air-cooled sample retained a dense, compact microstructure, while the non-fibrous water-cooled sample exhibited large cracks and spherical pores, with fiber-matrix debonding and increased microcrack density explaining the measured strength losses.

The study’s conclusions carry practical weight for the construction industry’s decarbonization ambitions. An ambient-cured, slag-based geopolymer concrete reinforced with 0.6 percent glass fiber emerges as a promising candidate for fire-resilient structural applications, provided that workability and fiber dispersion are carefully controlled. The findings also deliver a clear warning for fire engineering: the way a structure cools after a fire matters nearly as much as the fire itself, with rapid water quenching inflicting measurable thermal-shock damage that gradual air cooling avoids. The authors point toward future research on long-term durability under diverse service environments and on extending the approach to other fiber types and cooling scenarios, steps that could help carry geopolymer composites from the laboratory into the load-bearing skeleton of sustainable cities.

Subject of Research: Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes

Article Title: Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes

Article References: Kantarci, F., & Nehdi, M. L. (2026). Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes. Cleaner Engineering and Technology, 34, Article 101311. https://doi.org/10.1016/j.clet.2026.101311

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101311

Keywords: geopolymer concrete, glass fiber, elevated temperature, fire resistance, cooling regime, thermal shock, blast furnace slag, compressive strength, flexural strength, sustainable construction, alkali-activated materials, microstructure