The review classifies the candidate materials into six families: bio-derived supplementary cementitious materials and ashes, bio-based lightweight aggregates, natural-fiber reinforcement, expanded-perlite aggregate, perlite powder and fines, and hybrid systems that combine several of these. Each family attacks the carbon problem from a different direction. Supplementary cementitious materials replace clinker directly, reducing the most carbon-intensive component of the binder. Lightweight aggregates substitute for quarried sand and gravel, easing pressure on primary resources. Natural fibers add tensile reinforcement without steel. Perlite, meanwhile, contributes something ordinary concrete cannot: trapped air. When the volcanic glass is heated, it expands like popcorn, forming a honeycomb of glassy pores that insulates far better than dense mineral aggregate.

The physics underlying all of these substitutions is captured in what the authors call the strength–insulation trade-off, and it recurs with remarkable consistency across the 139 studies. Introduce more porous, lightweight constituents into a mix and two things happen simultaneously: density falls and thermal conductivity falls with it, which is exactly what builders of energy-efficient envelopes want. But the same pores interrupt the continuous skeleton of hardened cement paste that carries compressive load. Matrix continuity weakens, moisture sensitivity rises, and compressive strength tends to decline. Across the reviewed datasets, compressive strength generally increased with density, while thermal conductivity declined as porosity rose. In other words, the two most desirable properties of a building material pull against each other, and every formulation sits somewhere on that spectrum.

What makes the review valuable is that it does not stop at the trade-off; it identifies the levers that shift where a formulation lands on it. Constituent type, replacement level, particle size distribution, binder chemistry, curing regime, moisture condition, and pore structure all emerged as decisive variables. A mortar that swaps in too much porous aggregate collapses mechanically, but a controlled substitution with well-graded particles and a binder designed to bond around the pores can retain most of its strength while gaining substantial insulating capacity. Curing matters too, particularly for bio-based systems, where fibers and ashes can interfere with hydration or release sugars that retard setting if the mix design is not adjusted to compensate.

The headline numbers from the review demonstrate how far the best-performing systems have come. A perlite-powder mortar with 15 percent cement replacement and a 2 percent dose of nanosilica reached a compressive strength of 68.3 megapascals, a figure that comfortably exceeds many structural-grade conventional mortars and shows that pozzolanic perlite fines, aided by nanosilica’s ability to densify the interfacial zones, need not sacrifice mechanical performance. On the bio-based side, a corn-stalk-ash mortar achieved 50.15 megapascals at just 5 percent cement replacement, turning an agricultural waste stream into a functional binder component. At the opposite end of the performance spectrum, lime–hemp composites reported thermal conductivity as low as 0.070 watts per meter-kelvin, roughly an order of magnitude below conventional concrete, illustrating the insulating potential of plant-aggregate systems even though they serve non-structural roles.

These figures matter because the building envelope is where the embodied carbon of construction meets the operational carbon of heating and cooling. A wall material that insulates well reduces energy demand for decades, and if that same material also displaces clinker or quarried aggregate, it cuts emissions at both ends of its life cycle. The review’s environmental analysis emphasizes that these benefits were most evident precisely when the alternative materials displaced clinker or primary aggregates. However, the authors caution that carbon outcomes depend heavily on assumptions about processing energy, transportation distances, service life, and end-of-life handling. A bio-ash that must be dried and calcined at high temperature, or a lightweight aggregate hauled across continents, can erode much of its theoretical advantage. Life-cycle thinking, not just lab performance, determines whether a formulation is genuinely low-carbon.

The circular-economy framing is central to the review’s argument. Bio-based residues such as corn stalk ash are waste products of agriculture, and valorizing them in construction diverts them from burning or landfill while substituting for manufactured inputs. Perlite, though a mined natural material, is abundant and requires relatively modest energy to expand compared with the 1,450-degree-Celsius clinker kiln. Hybrid systems that pair the two, using perlite for insulation and bio-ashes for pozzolanic reactivity, showed particular promise in the synthesis because each constituent compensates for the other’s weaknesses: the ash contributes strength-giving chemistry while the expanded glass supplies lightweight porosity, allowing formulators to balance insulation, mechanical adequacy, moisture resistance, and carbon reduction simultaneously.

Moisture emerges as a recurring caveat in the evidence base. Porous bio-aggregates and plant fibers can absorb water, swell, and degrade, and their presence can raise the moisture sensitivity of the composite, with consequences for durability, freeze–thaw resistance, and indoor air quality. The review indicates that binder design and curing protocols are the main defenses: binders that encapsulate fibers thoroughly, particle treatments that reduce water uptake, and curing regimes that ensure full hydration before exposure all improve long-term performance. This is where laboratory results diverge most sharply from field behavior, and the authors note that the most favorable systems are those engineered with moisture performance in mind from the outset rather than treated as an afterthought.

The practical implication is that the future of low-carbon construction may be less about a single replacement for Portland cement and more about a portfolio of tailored mixes matched to application. Structural elements can tolerate modest insulation penalties and should favor high-strength formulations like the nanosilica-modified perlite-powder mortar. Non-structural envelope infill, roof insulation, and partition walls can prioritize thermal performance with lime–hemp and other highly porous composites. Hybrid bio–perlite systems occupy the middle ground, offering balanced multifunctional performance for building-envelope applications where both load capacity and insulation matter. The review’s synthesis suggests that controlled substitution, careful particle grading, deliberate binder design, and appropriate curing are the tools that let engineers move deliberately along the strength–insulation spectrum rather than accepting whatever compromise a naive mix delivers.

For a sector under intense pressure to decarbonize, the message of this synthesis is quietly radical: the raw materials for a circular, low-carbon building industry may already be sitting in volcanic deposits and farm fields. Expanded perlite and bio-based residues cannot eliminate the cement industry’s carbon problem on their own, and the trade-offs they impose are real. But 139 studies of accumulating evidence show that with disciplined engineering, waste streams can become binders, quarried stone can become insulating pores, and the walls of future buildings might simultaneously lock up agricultural residue and cut heating bills. The trade-off between strength and insulation is not a barrier; it is a design space, and the map to navigate it has just become considerably more detailed.

Subject of Research: Expanded perlite and bio-based residues as low-carbon cementitious building materials

Article Title: Expanded perlite and bio-based residues for circular low-carbon cementitious materials: a systematic review

Article References: Ashraf, U., Khattab, T., & Abu-Rayash, A. (2026). Expanded perlite and bio-based residues for circular low-carbon cementitious materials: a systematic review. Discover Sustainability. https://doi.org/10.1007/s43621-026-04834-x

Image Credits: AI Generated

DOI: 10.1007/s43621-026-04834-x

Keywords: expanded perlite, bio-based residues, low-carbon cement, supplementary cementitious materials, compressive strength, thermal conductivity, circular economy, lightweight aggregates, hemp-lime composites, waste valorization, building envelope, sustainable construction

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