The study, published in Case Studies in Construction Materials, was led by Kari Steyn with Wibke De Villiers and Adewumi John Babafemi as supervisors. The motivation is urgent. Cement and steel production together account for roughly sixteen percent of global carbon dioxide emissions, and the United Nations Environment Programme has repeatedly warned that the building sector must transform rapidly to meet global climate targets. Bio-based materials such as hempcrete offer a rare opportunity to not merely reduce emissions but reverse them, because the hemp plant absorbs carbon dioxide as it grows and the lime binder continues to absorb the gas as it slowly carbonates over the life of the building.

The researchers prepared two hempcrete mixes with identical hemp-to-binder-to-water ratios of 1.0:2.8:2.8 by mass. The reference mix combined a dolomitic pressure-hydrated lime, a calcium hydrated lime and a highly reactive metakaolin in proportions of forty, twenty and forty percent of the binder respectively. The modified mix, designated M1, replaced three percent of that binder with potassium sulphate. The dosage was chosen deliberately, informed by prior studies showing that sulphate-based activators accelerate pozzolanic reactions in lime and slag systems. In related binders, sodium sulphate has produced strength gains of up to 274 percent at one day, and potassium sulphate has outperformed it in cement-slag mortars, lifting compressive strength by five to six megapascals at higher dosages.

The mechanism behind the enhancement is the accelerated precipitation of ettringite, a calcium aluminium sulphate hydrate mineral. Metakaolin supplies abundant reactive alumina, confirmed by X-ray fluorescence analysis showing 44.40 percent aluminium oxide in the material, while the limes supply calcium oxide at 35.94 and 71.06 percent respectively. The potassium sulphate delivers the sulphate ions needed for ettringite crystals to form rapidly during early curing. Scanning electron microscopy confirmed the prismatic and cuboidal crystal morphology of the activator and the angular, plate-like texture of the metakaolin, while energy-dispersive spectroscopy verified the elemental signatures of every constituent, including the hemp shives themselves, whose water-transporting vessels and tracheids were imaged in detail.

The mechanical results were striking. The activated mix achieved compressive strengths 50.47 percent higher than the reference at seven days, 99.73 percent higher at fourteen days, and 64.99 percent higher at twenty-eight days, reaching 0.681 megapascals compared with 0.413 megapascals for the control. Those figures may sound modest next to ordinary concrete, but for a material weighing roughly 548 kilograms per cubic metre, less than a quarter the density of conventional concrete, they represent a meaningful advance. The activated hempcrete outperformed comparable lime-metakaolin hempcretes reported in the literature, including mixes that incorporated Portland cement or were cured for ninety days, and it matched or exceeded sunflower bark and rice husk composites of similar density.

Equally important was what the additive did not do. Density, the property that underpins hempcrete’s thermal insulation, was essentially unchanged: ambient-cured specimens registered 547.27 kilograms per cubic metre for the reference mix and 548.59 for the activated mix at twenty-eight days. Oven-dried specimens were lighter still, at roughly 502 to 519 kilograms per cubic metre, placing the material in the same density class as autoclaved aerated concrete. The activated mix also retained its strength far better after oven drying, losing only 6.06 percent of its compressive strength compared with a 44.09 percent loss for the reference, suggesting that the sulphate activation produced a more stable internal microstructure.

Durability testing revealed further benefits. Using a digital image correlation technique with two cameras tracking microscopic displacements over six hundred hours, the researchers measured drying shrinkage, a property that has remained largely unexplored in hempcrete composites. The activated mix shrank by only 0.247 percent, against 0.344 percent for the reference, a clear sign of improved dimensional stability attributed to early ettringite formation. Capillary water absorption tests showed that the activated mix absorbed water significantly more slowly in the first minutes of exposure, with an initial rate of 4.290 kilograms per square metre per minute versus 5.840 for the control, though after twenty-four hours of immersion the two mixes converged and the activated mix absorbed slightly more water over the full 144-hour test.

The carbon accounting was the study’s most consequential finding. In a bubble column reactor through which a carbon dioxide-rich gas mixture was bubbled for sixty minutes, the reference binder sequestered 0.229 kilograms of carbon dioxide per kilogram of binder and the activated binder 0.207, with pH rising above thirteen within the first minute as calcium dissolved and then falling as carbonate precipitated. Scaling this up in a cradle-to-gate life-cycle assessment, each 290 by 140 by 90 millimetre specimen emitted about 0.75 kilograms of carbon dioxide equivalent during production, dominated almost entirely by lime calcination, but sequestered 0.920 kilograms through hemp growth plus roughly 0.3 kilograms through binder carbonation. The net result was negative: minus 0.491 kilograms of carbon dioxide equivalent per specimen for the reference mix and minus 0.470 for the activated mix.

That the potassium sulphate addition shifted the net footprint by only about four percent is the crux of the study’s argument. Previous attempts to strengthen hempcrete, whether by adding more binder or by densifying the mix, have typically raised embodied energy and degraded the hygrothermal performance that makes the material attractive in the first place. Here, a three percent chemical addition delivered near-doubled strength and measurably better dimensional stability at negligible environmental cost. The authors caution, however, that the proposed ettringite mechanism rests on compositional evidence from X-ray fluorescence and electron microscopy, and they recommend direct confirmation through X-ray diffraction and thermal analysis in future work.

The road to commercial viability still requires answers to harder questions. The researchers recommend expanding the dosage range to map the full response curve, and they stress that long-term performance, including freeze-thaw resistance, accelerated ageing and biodeterioration over multi-year cycles, remains untested. End-of-life pathways for hempcrete, and the degradation kinetics that would allow a true cradle-to-grave assessment, are also largely uncharted. But the direction of travel is clear. A material that grows in fields, absorbs carbon as it cures, and can now be made substantially stronger with a pinch of inexpensive fertiliser chemistry is edging closer to the mainstream of sustainable construction, where every kilogram of embodied carbon matters.

Subject of Research: Enhancing the mechanical and durability performance of hempcrete with potassium sulphate while maintaining carbon sequestration

Article Title: Performance of hempcrete for carbon-negative construction: Density, compressive strength, durability and carbon sequestration

Article References: Steyn, K., De Villiers, W., & Babafemi, A. J. (2026). Performance of hempcrete for carbon-negative construction: Density, compressive strength, durability and carbon sequestration. Case Studies in Construction Materials, 25, Article e06597. https://doi.org/10.1016/j.cscm.2026.e06597

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06597

Keywords: hempcrete, carbon-negative construction, potassium sulphate, compressive strength, ettringite, lime binder, metakaolin, drying shrinkage, carbon sequestration, life-cycle assessment, sustainable building materials, durability

The researchers built their investigation around cellulose nanofibers, or CNF, fibrils just 4 to 20 nanometers in diameter and 1 to 3 micrometers long, packed with hydroxyl and carboxyl surface groups. These renewable, high-aspect-ratio fibrils have attracted growing interest in cement science because they can nucleate hydration products, bridge nanoscale defects and, crucially, hold onto mixing water like tiny molecular sponges. But CNF comes with a notorious drawback: its enormous surface area and hydrogen bonding make the fibrils clump together, raising the water demand of a fresh mix and threatening the flowability that UHPC placement depends on. The team therefore faced a classic trade-off question. The issue was not whether fibers at different length scales can help control cracking, but how a nanocellulose-modified matrix interacts with micro-scale aramid fibers and macro-scale steel fibers when workability, strength, shrinkage and durability all impose competing demands.

The experimental system was built on a meticulously optimized UHPC matrix containing 1092 kilograms of cement per cubic meter, supplemented with silica fume and metakaolin in a ternary binder blend, plus high-purity quartz sand, at a water-to-binder ratio of just 0.16. All mixtures included 2.0 percent by volume of steel fibers, 12 to 13 millimeters long and about 0.20 millimeters in diameter, as the macro-scale reinforcement. Variable amounts of CNF, from 0.05 to 0.25 percent of binder mass, were dispersed using a clever metakaolin-assisted pre-dispersion procedure: the nanofibers and metakaolin were first co-dispersed in the mixing water to physically separate the fibrils before the suspension entered the highly ionic cementitious environment, limiting direct CNF-to-CNF contact and the agglomeration that would otherwise turn the nanofibers into defects. Aramid fibers, roughly 12 micrometers in diameter and about one millimeter long, were added at the final mixing stage in dosages of 0.2 to 0.6 percent by volume.

The results reveal a sharp optimum. As CNF dosage rose, flowability declined monotonically, from 232 millimeters on the flow table for the reference mix down to 205 millimeters at 0.25 percent CNF, an 11.6 percent loss. At the mechanically selected dosage of 0.15 percent, however, the workability penalty was modest, only 4.7 percent, while the mechanical rewards were substantial. The 28-day compressive strength reached 149.4 megapascals, an 11.4 percent gain over the control, and the mean flexural strength climbed from 30.9 to 51.6 megapascals, a striking 67.0 percent relative difference. The authors are careful to frame this correctly: because the control already contained the same steel fiber volume, the flexural jump reflects a system-level response, in which a tougher matrix cracks less readily and activates the existing steel fibers more efficiently, rather than a direct load-carrying contribution from the nanofibers themselves.

The mechanism behind these gains is a story of water management and nanoscale crack arrest. Well-dispersed CNF acts as nanoscale bridges that restrain the initiation and propagation of microcracks, while its surface functional groups provide nucleation sites for hydration products, densifying the microstructure. Most importantly for UHPC, the hydrophilic fibrils absorb and temporarily retain part of the scarce mixing water, releasing it gradually under the internal humidity gradient as hydration proceeds. This internal curing effect directly counteracts self-desiccation, the root cause of autogenous shrinkage. Beyond 0.15 percent, the benefits reversed: agglomerates formed, free water for lubrication and hydration dwindled, compaction quality suffered, and both strength and flowability dropped. The reinforcing effect of CNF, the study makes clear, depends entirely on dispersion state and dosage.

The shrinkage data are arguably the most consequential for practice. After seven days of sealed monitoring with laser displacement sensors, the autogenous shrinkage of the 0.15 percent CNF mixture fell to approximately 1190 microstrain, down from roughly 1600 microstrain for the control, a reduction of about 25.6 percent. At 0.20 percent CNF the reduction was smaller, around 11.3 percent, confirming that the optimum for volume stability coincided with the optimum for strength. In a separate drying-shrinkage series, the micro-scale aramid fibers stole the show: a mix containing 0.4 percent aramid fiber cut 28-day drying shrinkage from about 134 to 76 microstrain per meter, a 43.3 percent reduction. Because aramid fibers have far smaller diameters and much higher number densities than steel fibers at the same volume fraction, they bridge the distributed microcracks that form during moisture loss far more effectively, providing three-dimensional internal restraint precisely where drying damage begins.

The aramid fiber series exposed a striking loading-mode trade-off. At fixed 0.15 percent CNF and 2.0 percent steel fiber, raising aramid content from 0.4 to 0.6 percent by volume increased 28-day compressive strength from 154.7 to 159.4 megapascals, an 18.9 percent gain over the original control, but simultaneously reduced flexural strength from 54.0 to 48.4 megapascals, a 10.4 percent penalty. The explanation lies in how cracks travel. Compression damages a large distributed volume, where a denser population of fine fibers restrains lateral microcrack growth. Flexure, by contrast, is governed by a single localized critical section, where fiber crowding at high dosages can disturb orientation, reduce the effective number of well-bonded fibers crossing the crack plane and introduce local defects. The team therefore recommends 0.4 percent aramid fiber as the balanced choice, reserving 0.6 percent for cases where compressive strength alone governs.

Durability testing added a further dimension. Specimens cycled between immersion in 5 percent sodium sulfate solution and oven drying retained mass and compressive strength more effectively when CNF and the hybrid fibers were present. Sulfate attack exploits the same weaknesses that shrinkage does, namely connected capillary pores and microcracks that serve as preferential transport pathways, so a matrix densified by nanocellulose and stabilized against cracking by aramid and steel fibers resists ingress more effectively. Microstructural evidence supported the picture: X-ray diffraction showed the same principal crystalline phases, portlandite, calcite, residual alite and quartz, in control and nanocellulose mixtures, indicating no new crystalline products, while scanning electron microscopy revealed a markedly more compact matrix with fewer microcracks and microvoids in the 0.15 percent CNF specimen compared with the loosely packed, crack-riddled control region.

The study, published under the DOI 10.1016/j.cscm.2026.e06532, arrives with appropriately measured language: because the experimental program did not include every possible single- and dual-fiber control, the authors describe their finding as coordinated multi-scale enhancement rather than claiming rigorous proof of synergy. Even so, the demonstrated composition, 0.15 percent CNF plus 0.4 percent aramid fiber plus 2.0 percent steel fiber, delivered the highest measured flexural strength of 54.0 megapascals, an 11.4 percent compressive gain, roughly a quarter less autogenous shrinkage, over 40 percent less drying shrinkage in the aramid series, and improved sulfate resistance, all at a workability cost of under 5 percent. As the construction industry searches for materials that combine extreme performance with lower cracking risk and longer service life in bridges, marine works and protective structures, the idea that a dash of renewable plant nanofibers, working in concert with aramid and steel fibers, can simultaneously strengthen, stabilize and toughen the strongest concrete we know how to make is a genuinely compelling advance.

Subject of Research: Multi-scale hybrid fiber reinforcement of ultra-high performance concrete using cellulose nanofibers, aramid fibers and steel fibers

Article Title: Coordinated multi-scale enhancement of UHPC by a nanocellulose-based hybrid fiber system: Mechanical performance, shrinkage mitigation and microstructural evidence

Article References: Huang, J., Wang, S., Jiang, Y., Shi, R., & Yang, Y. (2026). Coordinated multi-scale enhancement of UHPC by a nanocellulose-based hybrid fiber system: Mechanical performance, shrinkage mitigation and microstructural evidence. Case Studies in Construction Materials, 25, Article e06532. https://doi.org/10.1016/j.cscm.2026.e06532

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06532

Keywords: ultra-high performance concrete, cellulose nanofibers, aramid fibers, steel fibers, autogenous shrinkage, drying shrinkage, flexural strength, compressive strength, sulfate resistance, internal curing, microstructure, cementitious materials

The team began at the source, manufacturing their own lightweight aggregates from a blend of clay and volcanic ash. By tuning the sintering regime—heating granules at 5 °C per minute, preheating at 400 °C for 20 minutes, then firing between 1100 and 1250 °C—they could dial in the pore structure, density, and strength of each batch. Raising the sintering temperature from 1100 to 1200 °C melted the glassy phase progressively, forming a continuous surface glaze and a uniform closed-pore structure that lowered both density and water absorption while boosting strength. Pushing the temperature to 1250 °C backfired: excessive softening of the glassy phase allowed bubbles to coalesce, enlarging pores and thinning their walls until the aggregates weakened. Holding time followed a similar pattern, with the best cylinder compressive strength of 10.7 MPa achieved after 20 minutes at temperature. Longer grinding of the raw materials made aggregates lighter but also more porous and fragile, since finer feedstocks foamed more vigorously during firing.

With a library of aggregates spanning density grades from 600 to 1200 kg/m³, the researchers turned to the fresh state. They developed a simple but powerful evaluation protocol: after vibrating a concrete specimen, partition plates divided it into upper, middle, and lower thirds, and the aggregates recovered from each layer were weighed. From these layer-by-layer mass fractions they computed a floating index, which measures the excess of aggregates in the top two layers relative to the bottom, and a coefficient of variation, which captures overall spatial non-uniformity. For a perfectly homogeneous mix, each layer would hold roughly one third of the aggregates; any deviation signals redistribution. Vibration duration proved critical—at 20 seconds of vibration both indices rose sharply as the paste’s flocculated structure broke down, so the team standardized on 10 seconds of vibration at 50 Hz to keep the comparisons meaningful.

The aggregate experiments delivered a clear hierarchy of influence. As density grade increased from 600 to 1200, both the floating index and the coefficient of variation fell substantially, confirming that the density difference between aggregate and paste is the primary driving force for upward migration. Particle size came next: larger aggregates, fewer in number at the same volume fraction and presenting less surface area to the paste, floated more readily. Morphology and gradation played secondary but measurable roles. Spherical aggregates, with their smooth surfaces and low rolling resistance, drifted upward more easily than irregular ones, while continuously graded mixes outperformed single-sized ones because smaller particles fill the gaps between larger ones, creating mechanical interlock that resists migration. The worst case—low-density, large, spherical, single-sized aggregates—produced floating indices as high as 47.9 percent.

Paste parameters told an equally instructive story. Increasing the water-to-binder ratio raised both segregation indices roughly linearly, because additional free water thins the paste and erodes its capacity to suspend particles. The researchers also introduced an elegant volumetric metric, the excess filling rate, defined as the volume of mortar beyond what is needed to fill the voids between packed aggregates, normalized by that void volume. At low filling rates, frequent aggregate-to-aggregate contacts provide mechanical restraint; as the rate climbs, those contacts thin out and floating intensifies. Most striking was the effect of supplementary cementitious materials. Fly ash, with its spherical ball-bearing particles, lubricated the paste and worsened segregation, while slag—with its high specific surface area and hydraulic activity—thickened the paste and suppressed floating. The binary fly ash–slag blend proved the champion, cutting the floating index to 5.68 percent and the coefficient of variation to 0.90 by balancing flowability with cohesiveness.

A crucial insight emerged from linking these results to concrete slump, the industry’s standard workability measure. Slump alone cannot predict segregation. Two mixes with identical slump can behave entirely differently depending on whether the flowability gain came from added water, extra paste volume, or mineral admixtures. Water addition destroys suspension capacity; extra paste mainly reduces particle contacts without sacrificing viscosity; slag raises flow while preserving cohesiveness. The fly ash–slag system achieved high slump with minimal floating, demonstrating that flowability and segregation resistance can coexist when the chemistry is right. This finding cautions against mix-design shortcuts that judge fresh concrete by a single workability number.

To explain these observations mechanistically, the team built a kinetic model based on the force balance on a rising aggregate particle: buoyancy drives it up, gravity pulls it down, and Stokes-type viscous drag resists motion, with an additional term accounting for interparticle restraint. In the steady state, the migration velocity scales with the square of particle radius and the density difference, divided by paste viscosity and modified by the restraint factor. This simple proportionality reproduced every major experimental trend—why bigger and lighter aggregates float fastest, why thinner pastes accelerate migration, and why gradation and shape act through particle contacts rather than viscosity. The model offers a predictive framework that can be calibrated for other lightweight aggregate systems, giving mix designers a quantitative tool rather than trial-and-error folklore.

The hardened concrete results tied distribution to performance in unexpected ways. Raising aggregate cylinder compressive strength from 2.4 to 18.7 MPa lifted 28-day compressive strength of the concrete from 23.0 to 59.5 MPa, because weak aggregates shift from the composite’s weakest phase to genuine load-bearers. Compressive strength correlated most strongly with the water-to-binder ratio (R² = 0.91) and aggregate strength (R² = 0.81), while flexural strength responded far more to paste parameters, with R² values of 0.98 for the water-to-binder ratio and 0.96 for the excess filling rate—consistent with flexural failure being governed by crack propagation through the matrix and interfacial transition zone rather than bulk crushing. Intriguingly, the floating index correlated only weakly with compressive strength (R² = 0.75) and barely at all with flexural strength (R² = 0.11), but strongly with specific strength—strength per unit density—with a correlation coefficient of −0.94. Segregation, in other words, is best detected not by strength loss alone but by the efficiency metric that couples load capacity to weight.

The practical payoff is substantial. The optimized mixes achieved specific strengths of 25 to 33 MPa·m³/t, roughly double the 8.5 to 16.5 typical of ordinary concrete, meaning structures can shed self-weight without proportional strength sacrifice. Drying shrinkage told a favorable story as well: all lightweight mixes shrank less than conventional counterparts, thanks to internal curing as porous aggregates release stored water into the hydrating paste, though very porous low-grade aggregates partially offset this benefit by restraining the skeleton less. Microstructural analysis of the best-performing fly ash–slag system showed pores below 20 nanometers accounting for about 95 percent of fine-pore volume at 28 days, with a continuous C-S-H gel network penetrating the open pores at aggregate surfaces and knitting paste to particle. For engineers racing to build lighter, taller, and more sustainable structures, the message is clear: controlling where lightweight aggregates sit in the fresh state—through density matching, gradation, paste viscosity, and blended binders—is as important as the aggregates themselves.

Subject of Research: Floating-driven segregation of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete

Article Title: Floating-driven spatial distribution of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete

Article References: Zhang, W., Qian, C., Wang, Z., Jia, F., & Zhao, S. (2026). Floating-driven spatial distribution of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete. Case Studies in Construction Materials, 25, Article e06533. https://doi.org/10.1016/j.cscm.2026.e06533

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06533

Keywords: lightweight aggregate concrete, aggregate floating, segregation, sintered aggregates, floating index, excess filling rate, specific strength, drying shrinkage, fly ash, slag, interfacial transition zone, internal curing