The material at the center of the research is the thin spray-on liner, or TSL, a flexible support layer sprayed onto rock surfaces in layers typically 5 to 20 millimeters thick. Unlike rigid shotcrete, a well-designed TSL penetrates micro-fractures in the rock during application, binds fragmented rock into a continuous load-bearing composite, and forms a shallow load-bearing arch in partnership with the surrounding ground. Commercial TSLs fall into two families: latex-based products, which are highly ductile but prohibitively expensive, and polymer-modified cementitious liners, which are cost-effective but historically too stiff and brittle for the demands of deep tunnels with large deformation. The research team, led by Xing Gao and Jinlong Teng, set out to close that performance gap with fiber reinforcement.

The researchers formulated a two-component cementitious TSL from white cement, fine quartz sand, vinyl acetate-ethylene copolymer powder, and various admixtures, then added chopped glass fibers at four dosage levels: 0, 0.5, 1.0, and 1.5 percent of the powder mass. The fibers themselves were slender, 6-millimeter strands with a tensile strength of 555 megapascals and an aspect ratio of nearly 484. Specimens were cast, cured for seven days under controlled temperature and humidity, and then subjected to uniaxial compression and three-point bending tests, each group run with six parallel samples to ensure statistical reliability.

What distinguishes this study from earlier work is its insistence on measuring toughness across the entire loading-to-failure process, not just at the peak. Using an energy-based framework rooted in the first law of thermodynamics, the team partitioned the compressive response of each specimen into four stages: compaction, elastic deformation, stable crack expansion, and post-peak accelerated crack growth. By integrating the stress-strain curves, they separated the total input energy into stored elastic energy and dissipated energy, the latter consumed by crack closure, crack propagation, and frictional sliding on fracture surfaces. From this accounting they derived a pre-peak toughness index, a post-peak toughness index normalized against the unreinforced material, and a coupled comprehensive compressive toughness index formed by their product.

The results traced a clear arc of improvement followed by decline. Compressive strength rose from 12.64 megapascals in the unreinforced liner to a peak of 14.75 megapascals at 1 percent fiber content, then fell back to 12.85 megapascals at 1.5 percent. Energy absorption followed the same pattern, peaking at 317.9 kilojoules per cubic meter at the 1 percent dosage. Failure modes evolved in parallel: the unreinforced material split with two longitudinal tensile cracks and shed fragments, the 0.5 percent mix developed a combined tension-shear crack pattern, the 1 percent mix failed through a single diagonal shear band while remaining intact, and the 1.5 percent mix displayed a complex tri-directional shear crack network with pronounced ductile dilation. The comprehensive compressive toughness index nearly doubled, climbing from about 0.48 to 0.93 across the fiber range.

Flexural testing told an equally striking story. Flexural strength peaked at 8.47 megapascals at 1 percent fiber content, a 77.2 percent gain over the unreinforced matrix, while fracture energy surged more than fivefold, from 408.53 newtons per meter to 2189.69 newtons per meter at the highest fiber dosage. The flexural toughness ratio climbed steadily from 0.22 to nearly 0.68. Crack lengths in bending shrank progressively from 37.5 millimeters in the unreinforced beams to 27 millimeters in the most heavily reinforced ones, and the post-peak load response shifted from a cliff-like brittle drop to a gentle stress-tailing curve sustained by the dense fiber network bridging the main crack.

To unify these findings, the team constructed a comprehensive toughness index R that multiplies the pre-peak compressive index, the post-peak compressive index, and the flexural toughness ratio. The multiplicative, non-compensatory design ensures that weakness in any single dimension drags down the overall score. The index rose from roughly 0.10 to 0.63 as fiber content increased, and unlike the flexural-to-compressive strength ratio, which peaked at 1 percent and then declined, R kept rising because it captured the continuously improving post-peak energy dissipation. Independent validation against failure modes and crack patterns confirmed that R tracked actual toughness evolution more faithfully than strength-based measures alone.

Beneath the macroscopic numbers lies a more complicated microscopic story, one the authors describe as a competition between positive toughening and negative damage. Microscopy of fracture surfaces and helium pycnometry revealed that fibers hinder the migration and escape of air bubbles during mixing and compaction, a cofferdam effect that traps pores near fiber intersections and, at high dosages, produces fiber agglomeration and continuous defect zones. Porosity fluctuated in a rise-fall-rise pattern, reaching its lowest value of 10.54 percent at the 1 percent dosage, where the fiber network achieved its optimal spatial arrangement. This pore evolution explained the ladder-like drop in elastic modulus and the sharper strength penalty under bending, which is far more defect-sensitive than compression: flexural strength fell 25.9 percent at 1.5 percent fiber content, against only 13.2 percent for compressive strength.

Field-emission scanning electron microscopy exposed the three-stage energy dissipation mechanism that gives the material its resilience. First, hydration products, including calcium silicate hydrate gel, calcium hydroxide platelets, and polymer films, tightly encapsulate the fiber surface, forming a dense interfacial transition zone that debonds under load and converts mechanical energy into debonding work. Second, the debonded fiber slides against rough matrix pore walls, dissipating energy as frictional heat during crack opening. Third, when the stress on a bridging fiber exceeds its ultimate tensile strength, the fiber fractures in a flush, brittle break, completing the final stage of energy absorption. Crucially, the team found that this positive toughening mechanism dominates over the pore-related damage, which mainly erodes strength rather than toughness.

The practical upshot is a clear design recommendation: 1 percent glass fiber content delivers the best overall balance of strength, toughness, cost, and pumpability, since higher dosages raise the risk of pipe clogging during spraying. The authors caution that their results come from laboratory-cast specimens, and that fiber orientation, moisture migration, and substrate constraint in real sprayed layers may shift performance, so field validation on rock surfaces remains necessary. Even so, the study delivers something the field has lacked: a quantitative, multi-scale framework that links fiber dosage, pore structure, interfacial micromechanics, and full-process energy dissipation. For engineers contending with squeezing ground and violent rockbursts in ever-deeper tunnels, that framework turns a promising flexible liner into a rationally designed one.

Subject of Research: Multi-scale toughness evaluation and fiber toughening mechanisms in cementitious thin spray-on liners for deep tunnel support

Article Title: Study on multi-scale toughness evaluation and toughening mechanism of fiber-reinforced cementitious thin spray-on liner

Article References: Gao, X., Teng, J., Feng, J., Liu, Y., Ma, L., Bu, X., & Cheng, L. (2026). Study on multi-scale toughness evaluation and toughening mechanism of fiber-reinforced cementitious thin spray-on liner. Case Studies in Construction Materials, 25, Article e06593. https://doi.org/10.1016/j.cscm.2026.e06593

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06593

Keywords: thin spray-on liner, glass fiber reinforcement, cementitious composites, tunnel support, toughness evaluation, energy dissipation, fracture energy, porosity, fiber-matrix interface, deep underground engineering, polymer-modified cement, flexural strength

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: Evaluation of static and dynamic contact angle methods for their correlation with interfacial properties in composites. (n.d.). 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 appeal of basalt fiber begins with its origin. Unlike glass fiber, which requires carefully formulated raw materials, basalt fiber is produced by melting naturally occurring volcanic basalt rock at high temperatures and drawing it into continuous filaments. This single-source production route eliminates the need for complex batch formulations, and the resulting fiber offers a combination of properties that frequently matches or exceeds those of conventional glass fiber. The review highlights that basalt fibers deliver higher specific stiffness than glass fibers, meaning they provide greater rigidity per unit of weight, a critical advantage in any application where every kilogram matters, from aircraft interiors to logistics pallets and vehicle body panels.

What makes the thermoplastic angle particularly compelling is recyclability. Thermoset composites, which have historically dominated structural applications, cure permanently and are notoriously difficult to reprocess at end of life. Thermoplastic matrices, by contrast, soften when heated and can be remelted, reshaped, and in many cases welded, opening the door to genuine circular-economy manufacturing. The review emphasizes that combining basalt fiber with thermoplastics such as polypropylene, polyamide, polyetheretherketone, polylactic acid, and polyphenylene sulfide yields parts that are lighter, stiffer, more flame-resistant, and ultimately more sustainable than many incumbent solutions. The authors also point to cost reduction and improved interface durability as key performance advantages that could accelerate industrial adoption.

The chemistry of the fiber-matrix interface emerges as one of the central technical themes of the review. Basalt fibers, like all inorganic reinforcements, do not naturally bond strongly to polymer matrices, and weak interfaces translate directly into poor load transfer and disappointing mechanical performance. Researchers have therefore developed an extensive toolkit of surface treatments: silane coupling agents that chemically bridge fiber and matrix, polyurethane-based sizings tailored to elastomeric matrices, plasma treatments that activate the fiber surface, and even electrophoretic deposition of graphene onto individual filaments. Recent work reviewed by the authors shows that carefully engineered sizing agents, including hybrid flexible-rigid formulations and organic-inorganic networks built from carbon nanotubes, can dramatically improve interfacial shear strength and, with it, the tensile, flexural, and impact performance of the finished composite.

Processing technology forms another pillar of the review. Short-fiber composites, in which chopped basalt filaments are dispersed into the melt, can be processed on conventional extruders and injection molding machines, making them attractive for high-volume automotive parts. However, fiber breakage and orientation during processing strongly influence final properties, and the authors document how parameters such as melt temperature, screw speed, and mold design control fiber length distribution and therefore stiffness and strength. At the other end of the spectrum, continuous basalt fiber fabrics and tapes can be consolidated into high-performance laminates by compression molding, automated tape laying, or additive manufacturing. The review notes that 3D printing of continuous basalt fiber-reinforced polylactic acid has already demonstrated impressive mechanical properties, with raster orientation and fiber volume fraction emerging as decisive design variables.

The range of thermoplastic matrices explored with basalt fiber is striking. In commodity polypropylene, basalt fiber improves stiffness, thermal stability, and flame retardancy, with maleic-anhydride-grafted compatibilizers and elastomer toughening further refining the balance of properties. In engineering plastics such as polyamide 6 and 66, basalt reinforcement competes directly with glass fiber, and studies of in-situ polymerized polyamide 6 composites show that the fiber surface state strongly affects thermomechanical performance. At the high-performance end, basalt fiber has been combined with polyetheretherketone, polyetherimide, polyethersulfone, and polyphenylene sulfide to create laminates with exceptional wear resistance, flame retardancy, and even damage self-sensing capability, where the conductive network within the composite changes its electrical response as damage accumulates, effectively allowing the material to report its own condition.

Sustainability considerations run throughout the review, and the authors document a growing body of work on bio-based matrices. Basalt fiber has been successfully incorporated into polylactic acid, polybutylene succinate, polybutylene adipate-co-terephthalate, and bio-based polyamides, producing composites that pair a mineral reinforcement of volcanic origin with polymers derived from renewable resources. Studies of hybrid systems combining basalt with natural fibers such as flax, hemp, and wood fiber show synergistic effects, while research on recycled woven basalt fiber composites demonstrates that mechanical performance can be substantially retained through reprocessing cycles. The review also cites work on fire-retarded biobased polyamide 11 composites, which retained their structure, properties, and fire behavior after repeated reprocessing, an encouraging sign for closed-loop manufacturing.

Environmental durability, long a question mark for basalt composites, receives a balanced treatment. The authors summarize studies showing that basalt fibers and their composites can withstand corrosive environments, saltwater immersion, wet-dry cycling, ultraviolet weathering, and freeze-thaw exposure, with performance depending strongly on fiber chemistry and matrix selection. Marine applications feature prominently: basalt fiber-reinforced thermoplastic and epoxy composites have been evaluated for seawater ageing, marine decks, and structural components, and comparative studies suggest that thermoplastic matrices can offer advantages in long-term water resistance. In construction, basalt fiber-reinforced polymer bars and tendons have been investigated as concrete reinforcement, with research addressing creep, relaxation, and fatigue behavior under sustained and cyclic loads.

The application landscape described in the review is remarkably broad. In the automotive sector, basalt fiber-reinforced thermoplastics are positioned for interior and structural components where weight reduction drives fuel savings and emissions compliance. In aerospace, thermoplastic composites are gaining ground because they can be welded rather than adhesively bonded, speeding assembly, and basalt fiber offers a cost-effective alternative to carbon fiber for less critical structures. Hybrid armor systems combining basalt and Kevlar fabrics demonstrate ballistic performance benefits, while basalt composites are also being explored for wind turbine components, railway applications, sports equipment, thermal shielding, and even biomedical hard-tissue repair using polylactic acid matrices. The logistics sector, the authors note, stands to benefit from multifunctional, cost-effective parts that reduce vehicle weight across entire fleets.

Despite this momentum, the review is candid about the obstacles standing between laboratory promise and industrial standardization. Basalt fiber production still lacks the standardized grading and quality control enjoyed by the glass fiber industry, making it difficult for designers to specify materials with confidence. Variability in rock composition translates into variability in fiber properties, and the authors identify standardization as a key challenge that must be solved before widespread adoption can occur. Interface engineering remains part science and part craft, with sizing formulations often optimized empirically for specific fiber-matrix pairs. Yet the trajectory is clear: as the authors conclude, the combination of mechanical performance, thermal stability, flame resistance, recyclability, and raw material abundance positions basalt fiber-reinforced thermoplastic composites as one of the most promising material platforms of the coming decade, provided that industry and researchers can agree on the standards needed to trust rock spun from a volcano as a precision engineering material.

Subject of Research: Basalt fiber-reinforced thermoplastic composites: properties, processing, and applications

Article Title: Review: fundamentals, applications, and progress of basalt fiber-reinforced thermoplastic composites

Article References: Tayfun, Ü., Akar, A. Ö., & Yılmaz, V. M. (2026). Review: fundamentals, applications, and progress of basalt fiber-reinforced thermoplastic composites. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13835-0

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13835-0

Keywords: basalt fiber, thermoplastic composites, fiber-matrix interface, sizing agents, polypropylene, polyamide, recyclability, flame retardancy, automotive applications, aerospace materials, additive manufacturing, sustainable materials