A team of civil engineers has produced one of the most comprehensive head-to-head comparisons yet of sustainable cement-based mortars, testing twenty different mixtures that combine five aggregate types, two matrix systems, and flax fiber reinforcement under identical production and testing conditions. The study, published in Case Studies in Construction Materials, maps how density, porosity, strength, thermal conductivity, and sound absorption interact across a single experimental framework, and it identifies two standout formulations for non-load-bearing building applications: an ultra-light vermiculite foam for thermal insulation and a rubber-flax hybrid foam that absorbs more sound than anything else in the series.
The research was motivated by a persistent gap in the literature. Traditional dense mortars are mechanically stiff and thermally conductive but absorb almost no sound because of their compact microstructure, which drives up both energy demand and acoustic discomfort in buildings. Foamed mortars, whose densities can range from 400 to 1800 kilograms per cubic meter and whose thermal conductivities fall between 0.10 and 0.35 watts per meter-kelvin, offer a lighter and quieter alternative. Yet most previous studies examined a single aggregate within a single binder system, making it impossible to disentangle how aggregate shape, matrix architecture, and fiber inclusions jointly control multifunctional performance. The researchers, led by Elif Tuğçe Kocabeyoğlu and Fuat Köksal of Yozgat Bozok University together with Osman Gencel, set out to close that gap with a controlled benchmark of twenty mixtures.
The experimental design was deliberately systematic. Five fine aggregates in the 0–2 millimeter range were selected: crushed sand as the conventional reference, expanded clay, pumice, expanded vermiculite as lightweight mineral aggregates, and waste rubber granules as an eco-friendly damping additive. Each aggregate was combined with a normal mortar matrix and a foamed mortar matrix, and each of those ten combinations was produced with and without 15 kilograms per cubic meter of flax fiber. The flax fibers, used as received without surface treatment, had tensile strengths of 800 to 1500 megapascals, an elastic modulus of 50 to 70 gigapascals, and a density of just 1.54 grams per cubic centimeter. Foamed mixtures were produced by introducing preformed foam with a density of 80 grams per liter, generated from a plant-based foaming agent and compressed air. All specimens were water-cured for 28 days and then subjected to an extensive battery of tests covering physical, mechanical, thermal, acoustic, and microstructural behavior.
The physical results traced a clear hierarchy. Dry unit weights ranged from 2179 kilograms per cubic meter for the dense crushed-sand reference mortar down to just 562 kilograms per cubic meter for the flax-reinforced foamed vermiculite mixture, the lightest material in the entire series. Porosity followed the inverse pattern, climbing from 12.27 percent in the dense reference to 55.18 percent in the foamed vermiculite composite. Expanded vermiculite proved the most efficient single ingredient for reducing both density and thermal conductivity, a consequence of its lamellar, highly porous particle structure. Flax fiber addition consistently lowered unit weight and raised water absorption, capillary uptake, and porosity, because the fibers disrupted particle packing and introduced interconnected voids during mixing and drying.
Mechanical performance told a more sobering story. The crushed-sand reference mortar dominated every strength measure, achieving a compressive strength of 49.91 megapascals, a flexural strength of 10.6 megapascals, a splitting tensile strength of 5.32 megapascals, and an elastic modulus of 10.83 gigapascals. Foaming cut these values substantially; the foamed crushed-sand mixture reached only 21.17 megapascals in compression. Lightweight aggregates imposed further penalties, with vermiculite mixtures dropping to roughly 2.4 megapascals in compression. Intriguingly, the rubber mixtures recorded the lowest ultrasonic pulse velocities despite moderate porosity, because soft, hydrophobic rubber particles form weak interfacial transition zones that scatter and attenuate ultrasonic waves far more effectively than stiffer mineral pores. The study demonstrates that mechanical behavior cannot be predicted from porosity alone; the stiffness and interfacial compatibility of the aggregate skeleton matter just as much.
Thermal conductivity results delivered the study’s most dramatic numbers. The dense reference mortar conducted heat at 2.00 watts per meter-kelvin, but the foamed vermiculite composite with flax fiber achieved just 0.198 watts per meter-kelvin, a tenfold reduction and the lowest value of the entire series. This performance arises from three stacked mechanisms: the lamellar porosity of vermiculite particles, the air-void cellular structure of the foamed matrix, and the microstructural discontinuities introduced by fiber incorporation. Flax fiber alone reduced conductivity measurably even in dense mixes, dropping the crushed-sand mortar from 2.00 to 1.441 watts per meter-kelvin. Rubber aggregates also performed well thermally, at 0.371 watts per meter-kelvin in the best foamed variant, thanks to their low intrinsic conductivity and poor interfacial bonding with cement paste.
The acoustic findings overturned a common assumption. Noise reduction coefficients increased with specimen thickness across all mixtures, with 5-centimeter samples consistently absorbing the most sound, and foamed mortars outperforming their dense counterparts. But when the team moved from vermiculite to rubber mixtures, porosity dropped sharply while sound absorption rose. The flax-fiber-reinforced foamed rubber mixture, F-KAU-KL, achieved the highest noise reduction coefficient of the series at 0.5740 for 5-centimeter specimens, despite having lower porosity than the vermiculite foams. The researchers attribute this to the viscoelastic damping of rubber granules combined with the additional microstructural discontinuities created by flax fibers, which multiply internal friction and scattering pathways for sound energy. Porosity helps, but it is not the whole story.
Scanning electron microscopy and X-ray diffraction provided the microstructural evidence behind these macroscale trends. The dense reference mortar showed a compact, well-interlocked C–S–H gel network with minimal voids, explaining its strength and high conductivity. Fiber-reinforced samples revealed grooved flax fibers with partial debonding and interfacial gaps that created microchannels, reducing strength but opening pathways for thermal and acoustic insulation. The foamed rubber composite displayed large irregular voids between 50 and 390 micrometers, weak rubber-paste interfaces, and fragmented hydration products, a morphology that cripples load transfer but excels at dissipating sound and blocking heat. XRD confirmed the mineralogical signatures of each aggregate, from the quartz-rich expanded clay to the layered vermiculite and phlogopite of the GV particles and the amorphous rubber matrix with crystalline vulcanization residues.
The authors are careful about practical limits. The mechanically weak mixtures, particularly the foamed vermiculite and foamed rubber composites, are suitable only for protected non-load-bearing uses such as interior acoustic linings, insulation layers, lightweight infill, and sandwich-panel cores, and must never be used in load-bearing masonry, slabs, or impact-exposed elements. They also note that their comparisons are descriptive rather than statistically validated, since inferential analyses were not performed, and they recommend future work with confidence intervals, analysis of variance, dynamic mechanical analysis, and detailed pore evaluation. Within those constraints, the study delivers a rare integrated performance map: F-GV-KL is the material of choice when minimum density and maximum thermal insulation are the goals, while F-KAU-KL offers the best combination of low weight, high sound absorption, and moderate insulation for acoustic panels. By benchmarking five aggregates, two matrices, and a natural fiber in a single framework, the research gives designers of sustainable, acoustically enhanced buildings a quantitative basis for choosing the right mortar for the right job.
Subject of Research: Hybrid flax fiber reinforced normal and foamed mortars with five aggregate types for sustainable, acoustically enhanced non-load-bearing applications
Article Title: Comparative evaluation of hybrid flax fiber reinforced normal and foamed mortar using five aggregates for sustainable and acoustically enhanced non-load-bearing applications
Article References: Comparative evaluation of hybrid flax fiber reinforced normal and foamed mortar using five aggregates for sustainable and acoustically enhanced non-load-bearing applications. (n.d.). Original publication
Image Credits: AI Generated
DOI: Not provided
Keywords: foamed mortar, flax fiber, expanded vermiculite, waste rubber, sound absorption, thermal conductivity, lightweight aggregates, porosity, sustainable construction, non-load-bearing applications, cement composites, acoustic panels
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Denise Maddox. (September 23, 2026). Flax-Fiber Foamed Mortars Turn Waste Rubber and Lightweight Aggregates into Sound-Absorbing Building Materials. Scienmag. https://scienmag.com/flax-fiber-foamed-mortars-turn-waste-rubber-and-lightweight-aggregates-into-sound-absorbing-building-materials/
Denise Maddox. “Flax-Fiber Foamed Mortars Turn Waste Rubber and Lightweight Aggregates into Sound-Absorbing Building Materials.” Scienmag, 23 September 2026, https://scienmag.com/flax-fiber-foamed-mortars-turn-waste-rubber-and-lightweight-aggregates-into-sound-absorbing-building-materials/. Accessed 23 September 2026.
Denise Maddox. “Flax-Fiber Foamed Mortars Turn Waste Rubber and Lightweight Aggregates into Sound-Absorbing Building Materials.” Scienmag. September 23, 2026. https://scienmag.com/flax-fiber-foamed-mortars-turn-waste-rubber-and-lightweight-aggregates-into-sound-absorbing-building-materials/
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Tags: acoustic panelscement compositescomparison of foam mortars with different aggregatesexpanded vermiculiteflax fiberflax fiber reinforcement in construction materialsfoam insulation with lightweight aggregatesfoamed mortarimpact of aggregate type and microstructure on mortar propertiesinnovative use of waste rubber in constructionlightweight aggregateslightweight thermal and acoustic building materialsmultifunctional foam mortars for energy-efficient buildingsnon-load-bearing applicationsporosityporous microstructure for acoustic performancerubber-flax hybrid foamsound absorptionsound-absorbing building materialsSustainable cement-based mortarssustainable constructionthermal conductivitythermal insulation in non-load-bearing structureswaste rubber
