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

The research, led by Yeonhae Ryu and colleagues at Gyeongsang National University in the Republic of Korea, begins from a simple observation: different industries expose aramid films to fundamentally different stressors. Aerospace and defense components endure repeated thermal shocks, sometimes swinging more than 200 degrees Celsius during launch and re-entry. Optoelectronic devices and protective coatings face prolonged ultraviolet radiation that cleaves chemical bonds and discolors surfaces. Flexible and printed electronics suffer contamination from transition-metal ions shed by electrodes, solders and environmental exposure, which can erode dielectric reliability. Because these threats operate through entirely different chemical and physical pathways, the authors argue, a single compositional tweak cannot protect against all of them at once.

Previous attempts to harden aramids have largely relied on additives: inorganic fillers, polymer blends or nanostructured reinforcements grafted onto fiber surfaces. While such approaches can improve individual properties, the resulting performance is frequently inconsistent across stressors and shows weak or non-monotonic dependence on additive content. The Korean team instead modified the aramid backbone itself, incorporating three functionally distinct diamine co-monomers into the polymer chain through low-temperature solution polycondensation with p-phenylenediamine, 3,4′-oxydianiline and terephthaloyl chloride. Each co-monomer was selected for a molecular feature anticipated to counter a specific industrial threat, creating three copolymer systems designated BABP-ARP, PIPE-ARP and APA-ARP.

The first system, BABP-ARP, embeds 4,4′-bis(4-aminophenoxy)biphenyl, a rigid biphenyl unit that promotes chain stiffness and aromatic pi-pi stacking. The researchers reasoned that this enhanced cohesion would help the film resist the intermolecular disruption caused by thermal shock cycling. To test the idea, films containing 60 and 90 mole percent BABP were shuttled between chambers held at 250 degrees Celsius and minus 60 degrees Celsius, with one hour of dwell time at each extreme, for a cumulative 20 hours equivalent to ten full hot-cold cycles. The protocol mimics conditions encountered by structural films on high-altitude aerial systems and spacecraft thermal-management surfaces.

The results were strikingly composition-dependent. At 90 mole percent BABP, tensile strength, Young’s modulus and elongation at break were all retained within a few percent of their pre-cycling values, with narrow specimen-to-specimen scatter. At 60 mole percent, the same descriptors fluctuated far more widely, with elongation in particular showing both large mean changes and high variability. Thermogravimetric analysis reinforced the picture: the higher-loading films showed smaller post-cycling shifts in their five percent weight-loss temperature, and Fourier-transform infrared spectroscopy confirmed that the hydrogen-bond environment, quantified through deconvolution of the amide I region, remained essentially unchanged. The authors interpret this as a percolative mechanism: only when enough biphenyl-bearing units are present do the cohesive contributions of chain stiffness, hydrogen bonding and aromatic stacking span the entire polymer matrix continuously.

The second system, PIPE-ARP, incorporates 4,4′-(piperazine-1,4-diyl)dianiline, whose piperazine nitrogens were expected to coordinate with transition-metal cations. Films containing 20 mole percent PIPE, the practical upper limit before gelation destroys film formation, were immersed for 20 hours in aqueous solutions of iron(III) chloride, copper(II) chloride and zinc(II) chloride at 0.1 and 0.5 molar concentrations. All three metals stiffened and embrittled the films, reducing tensile strength and elongation while raising modulus, with iron producing the largest perturbation and zinc the smallest. But the spectroscopic evidence overturned the team’s initial hypothesis about why.

X-ray photoelectron spectroscopy revealed that the pristine films carried piperazine nitrogens in a protonated, piperazinium-chloride-like state near 402 electron volts in binding energy, a legacy of hydrochloric acid generated during polycondensation. Upon metal exposure, the nitrogen 1s peak shifted downward to roughly 399 to 400 electron volts, the range of neutral amine environments, while the chloride 2p signal largely vanished for copper and zinc exposure. The direction of the shift is opposite to what dative nitrogen-to-metal coordination would produce, which would deplete electron density at nitrogen and raise the binding energy. Instead, the data point to counter-ion exchange: the incoming metal cation displaces the proton from the piperazinium nitrogen, and the departing proton leaves with its chloride counter-ion. The metal then polarizes the now-neutral nitrogen electrostatically. Iron behaved distinctly, retaining most of its chloride signal, suggesting that iron-exposed films accumulate additional iron-chloride species that act as stress concentrators and drive the disproportionate embrittlement observed.

The third system, APA-ARP, replaces a backbone aromatic ring with a pyridine unit via 6-(4-aminophenoxy)pyridin-3-amine, chosen for its relatively photostable chromophore. Films at 40 and 60 mole percent APA were exposed to UV-A radiation centered at 340 nanometers for 100 hours in an accelerated weathering chamber. The films yellowed visibly, a consequence of increased absorption in the blue-light region between 400 and 500 nanometers, consistent with the formation of UV-induced chromophoric species that extend conjugation. Mechanically, however, the higher APA loading fared markedly better: tensile strength fell by only about 10 percent at 60 mole percent APA compared with roughly 25 percent at 40 mole percent, and modulus and elongation changes were similarly muted. The pyridine unit thus confers partial, composition-dependent photostability rather than complete immunity, and the authors caution that applications demanding strict color stability would still require additional photo-stabilizers.

Perhaps the most conceptually important finding is what did not differ. At baseline, before any stressor exposure, the three systems showed nearly indistinguishable mechanical, thermal and optical properties. Mean thermal decomposition temperatures clustered between 483 and 489 degrees Celsius across all compositions, visible-light transmittance exceeded 80 percent for every film type, and hydrogen-bond fractions from infrared deconvolution varied by less than 0.05. The authors attribute this uniformity to the disordered, largely amorphous microstructure of solvent-cast films, in which randomly entangled chains prevent the cooperative hydrogen-bond networks that would otherwise amplify architecture-specific effects. Only under stressor exposure do the structural contributions of biphenyl, piperazine and pyridine moieties become mechanistically visible, with the co-monomer identity setting the qualitative response mechanism and its loading modulating the response strength.

The study thus delivers a practical selection guideline for engineers designing aramid films for extreme environments: high-load biphenyl co-monomers for thermal-cycling-dominated settings, high-load pyridine co-monomers for UV-dominated settings, and, for metal-ion environments, an appreciation that the governing interaction is counter-ion-mediated electrostatics rather than nitrogen coordination, meaning the accompanying anion in a contaminant can qualitatively shape the mechanical outcome. The authors acknowledge the limitations of their diagonal experimental matrix, in which each film faced only its matched stressor, and point to off-diagonal exposures and high-temperature chain-alignment post-treatments as natural next steps. If those follow-up experiments confirm the framework, the era of one-size-fits-all aramid protection may give way to a new generation of task-specific films, each molecularly tailored to the precise hell it is destined to endure.

Subject of Research: Stressor-matched co-monomer design of para-aramid thin films for extreme environments

Article Title: Stressor-matched co-monomer design: a structure-driven framework for para-aramid thin films targeting industry-specific extreme environments

Article References: Ryu, Y., Nam, H., Im, J., Lee, J., Kim, D., & Choi, H. H. (2026). Stressor-matched co-monomer design: a structure-driven framework for para-aramid thin films targeting industry-specific extreme environments. Advances in Industrial and Engineering Chemistry, 2(1), Article 6. https://doi.org/10.1007/s44405-026-00046-z

Image Credits: AI Generated

DOI: 10.1007/s44405-026-00046-z

Keywords: para-aramid, thin films, co-monomer design, thermal shock, metal-ion exposure, UV photo-aging, XPS, hydrogen bonding, high-performance polymers, extreme environments, polymer chemistry, aerospace materials