Red mud is chemically rich in silicon, aluminum, iron, and calcium, which in principle makes it an attractive candidate for cementitious materials. In practice, its high alkalinity, complex mineral assemblage, and low intrinsic reactivity have limited its usefulness. Researchers led by Jiawei Zhang and Guochao Yan tackled this problem by embedding red mud in a composite system containing coal gangue powder and ordinary Portland cement, then comparing three different activation pathways designed to wake up the material’s latent chemistry: alkali activation with sodium silicate solution, thermal activation by calcination at temperatures from 500 to 900 degrees Celsius, and sulfate activation using flue gas desulfurization gypsum, itself an industrial byproduct of coal-fired power plants.

The thermal route depended on a delicate balance. Thermogravimetric analysis showed that heating red mud drives off bound water and triggers the dehydroxylation of gibbsite and goethite, converting stable crystalline phases into a disordered, more reactive state. X-ray diffraction confirmed that gibbsite and goethite disappeared upon calcination while hematite reflections intensified, and infrared spectroscopy documented the loss of hydroxyl and carbonate bands. But calcination also coarsened the particles, with the mean size growing from 5.7 micrometers in raw red mud to 15.66 micrometers at 900 degrees Celsius as recrystallization and sintering took over. The result was a clear optimum: red mud calcined at 800 degrees Celsius produced the strongest thermal-activation samples, while the 900-degree material was weaker.

The mechanical results were striking. Unactivated control mixtures reached only 3.60 megapascals of compressive strength at 28 days, relying almost entirely on Portland cement hydration. Alkali activation with 4 percent sodium silicate lifted that figure to 5.56 megapascals, a 54 percent improvement, by raising pore-solution alkalinity and supplying soluble silicate that promoted the dissolution and repolymerization of aluminosilicate species. Thermal activation at 800 degrees Celsius delivered 6.08 megapascals, a 69 percent gain. But the champion was sulfate activation: mixtures containing 4 percent flue gas desulfurization gypsum achieved 7.33 megapascals, more than double the control, with early strength gains of up to 262 percent at three days.

Why did gypsum work so well? Microscopic and crystallographic evidence pointed to ettringite, a needle-like calcium sulfoaluminate mineral. X-ray diffraction of the sulfate-activated samples showed distinct ettringite reflections, and energy-dispersive spectroscopy revealed locally high calcium and sulfur contents. Scanning electron microscopy showed these sulfate-bearing crystals intergrowing into dense, honeycomb-like frameworks with few large voids, bridging gel phases into a connected load-bearing network. By contrast, the alkali-activated samples displayed spider-like networks of sodium-rich aluminosilicate gel around residual particles, while the thermally activated samples formed cotton-like flocculent products weaving a relatively continuous matrix.

The study’s most novel contribution came from applying fractal analysis to the pore structure. Conventional measures such as specific surface area, pore volume, and pore-size distribution describe how much void space exists and how large the pores are, but they say little about the geometric irregularity of pore surfaces. Using the Frenkel-Halsey-Hill model applied to nitrogen adsorption data, the researchers calculated a fractal dimension for each 28-day sample. All values fell between 2 and 3, as expected for rough surfaces, and increased in a precise order: untreated, then alkali-activated, then thermally activated, then sulfate-activated. That ranking exactly matched the compressive strength ranking, suggesting that the roughness and heterogeneity of pore surfaces are tightly linked to how well the material carries load.

The pore-volume data told a subtler story. Thermal activation produced the lowest total accessible pore volume, 0.1271 cubic centimeters per gram, yet did not yield the highest strength. The sulfate-activated material retained a slightly higher pore volume of 0.1324 but was substantially stronger. Total porosity alone, in other words, could not explain the mechanical differences; the geometric complexity captured by the fractal dimension could. This finding echoes earlier work showing that larger pores disproportionately harm strength, and adds a new layer: the texture of the remaining pore space matters as much as its quantity.

Energy dissipation broke the pattern entirely. By integrating stress-strain curves from uniaxial compression tests and partitioning the input energy into stored elastic strain energy and dissipated energy, the team tracked how each mixture absorbed and released mechanical work as damage accumulated. All samples followed the same broad sequence, with energy stored elastically before the peak and dissipated rapidly as microcracks initiated and coalesced. But the final dissipated energy densities ranked differently: alkali-activated samples led at 0.058 megajoules per cubic meter, followed by sulfate-activated at 0.052, thermally activated at 0.044, and untreated at 0.038. The strongest material was not the toughest one.

The explanation lay in how each material fails. Stress-strain curves showed that thermally and alkali-activated specimens softened gradually after the peak, retaining residual stress as damage progressed incrementally, while sulfate-activated specimens dropped steeply, failing more abruptly. The alkali-activated material’s connected gel network appears to promote progressive microcracking that soaks up energy, whereas the rigid crystalline ettringite framework that gives the sulfate system its strength also channels damage into a faster, more brittle failure. Ductility and strength, the results imply, are governed by partly independent microstructural features.

For a waste stream measured in billions of tonnes, these distinctions have practical weight. Choosing an activation strategy is no longer just a matter of maximizing compressive strength; engineers must weigh load-bearing requirements against energy absorption and damage tolerance, particularly for applications such as mine backfill where both properties matter. The work also demonstrates that fractal pore characterization can serve as a sensitive diagnostic tool, predicting strength trends that conventional porosity measurements miss. As the construction industry searches for low-carbon binders made from industrial residues, the humble geometry of red mud’s pores may prove to be one of the most informative fingerprints of performance, guiding the conversion of an environmental liability into infrastructure.

Subject of Research: Fractal pore structure and energy dissipation in activated red mud-based cementitious materials

Article Title: Pore structure fractality affects strength and energy dissipation behavior in red mud-based cementitious materials under different activation pathways

Article References: Zhang, J., Kong, S., Ren, K., Li, G., Chen, L., Zong, B., Wang, Y., Yan, G., & Liu, H. (2026). Pore structure fractality affects strength and energy dissipation behavior in red mud-based cementitious materials under different activation pathways. Case Studies in Construction Materials, 25, Article e06635. https://doi.org/10.1016/j.cscm.2026.e06635

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06635

Keywords: red mud, cementitious materials, fractal dimension, pore structure, alkali activation, thermal activation, sulfate activation, energy dissipation, compressive strength, coal gangue, ettringite, industrial waste

The team, led by Wenchang Feng and Jixiong Zhang with colleagues including Meng Li, Zhibo Cui, Zhangyu Li, and Yuzhe Zhang, set out to tackle two intertwined environmental burdens. Decades of high-intensity coal extraction have left vast stockpiles of coal gangue and fly ash that occupy land, risk spontaneous combustion and landslides, and can leach contaminants into soil and groundwater. Meanwhile, groundwater dissolving soluble minerals in coal-bearing strata produces mine water so laden with salts that discharging it untreated would salinize soils and pollute waterways. Desalination plants at the mines can strip the salts out, but the process concentrates them into a brine whose disposal is both difficult and expensive, with evaporative crystallization currently the dominant — and energy-hungry — fallback.

The researchers’ proposal was elegantly simple: let the brine do useful work. They prepared a high-salinity concentrated mine water-based coal-based solid waste cemented backfill, abbreviated HW-CB, using crushed coal gangue as coarse aggregate, fly ash as fine aggregate and supplementary cementitious material, and Portland cement as the binder. A parallel series made with tap water, TW-CB, served as the control. The brine itself came from the treatment plant at Jinqiao Coal Mine in Jining City, and its chemistry was striking: sulfate concentrations 313 times those of tap water, sodium 244 times higher, potassium nearly 40 times higher, and chloride about 17 times higher, all at a near-neutral pH of 7.52.

Flow behavior proved to be the first sensitive variable. As the solid content of the slurry rose from 74 to 82 percent by weight, slump declined gently at first and then collapsed — the material held a workable 278 to 282 millimeters up to 76 percent solids, then lost 28, 60, and 76 millimeters for each successive 2 percent increase. The team attributes the cliff-like drop to a threshold near 78 percent solids, beyond which free water can no longer coat every particle, friction and particle bridging take over, and flowability deteriorates drastically. Replacing fly ash with more cement improved slump in both water types, because the fly ash’s free calcium oxide slakes rapidly and consumes water within minutes, while cement’s calcium is locked in slow-hydrating silicate minerals. Yet at every cement dosage, the saline mix flowed slightly worse than its tap-water twin, since the dense ion soup suppresses electrostatic repulsion between particles and accelerates the formation of early hydration products that thicken the slurry.

Strength told a more surprising story. The 28-day unconfined compressive strength of the saline backfill rose linearly with solid content, gaining roughly 0.86 megapascals for every 1 percent increase in solids, with a coefficient of determination of 0.997 — climbing from 2.9 to 9.6 megapascals across the tested range. More intriguing was the interplay between cement dosage and salinity. At cement proportions of 6 percent or higher, the saline mix outperformed the tap-water mix at every age, with 3-day, 7-day, and 28-day strengths all higher. Below about 4 percent cement, the pattern inverted: early strengths were still boosted, but the 28-day strength fell below the tap-water control, with the 4-to-6 percent range acting as a transition interval.

Microscopy and diffraction explained why. X-ray diffraction revealed that the intensity of the ettringite peak tracked the strength results exactly. When calcium was plentiful, the sulfate, chloride, sodium, and potassium ions in the brine continuously drove ettringite formation while accelerating the dissolution of the aluminate phase, producing a dense microstructure in which well-developed, thick ettringite crystals interlocked into a three-dimensional skeleton and abundant calcium silicate hydrate gel filled the pores. Scanning electron microscopy showed the saline mix riddled with only small, disconnected pores, whereas the tap-water counterpart contained large connected voids and finer, shorter crystals. Thermogravimetric analysis sealed the case: mass loss between 100 and 200 degrees Celsius, which reflects bound water in the hydration products, ranked in precisely the same order as the measured strengths.

The mechanism behind the low-cement failure is a classic case of too much of a good thing. With insufficient calcium available, the brine’s acceleration caused hydration products to form so quickly that they encapsulated the unhydrated particles, sealing off the pathways later reactions needed. The result was short, sparse ettringite, unfilled voids, and a weaker final structure than the slower, steadier tap-water hydration achieved. In effect, the high-salinity environment acts as a throttle on the hydration clock — helpful when there is enough fuel to sustain the reaction, harmful when the fuel runs out early.

The economics may prove as compelling as the chemistry. At Jinqiao Coal Mine, the current zero-liquid-discharge scheme combining membrane concentration with evaporation and crystallization costs about 15.83 yuan per cubic meter of treated water, with throughput that can exceed 10,000 cubic meters per day during high-inflow periods. If the concentrated brine from the reverse-osmosis stage is diverted into backfill production instead, the downstream evaporation and mixed-salt disposal steps can be skipped, cutting the unit treatment cost by roughly 8 to 9 yuan per cubic meter — a reduction of more than 50 percent. On an annual treatment volume of 120,000 cubic meters, that translates into savings of roughly 960,000 to 1,080,000 yuan, before counting the simplified flowsheet and reduced capital costs.

The researchers are careful to note the boundaries of their findings. The backfill is designed for unreinforced goaf filling, so the brine’s chloride content poses no steel-corrosion concern in this application, but long-term strength evolution at 90 or 180 days, durability under sustained saline exposure, the effects of varying brine compositions, and full-scale field trials all remain open questions. Still, the core result stands: with the cement dosage tuned above the critical threshold, a waste stream that mines once paid to destroy becomes an active ingredient in the very material that stabilizes the voids coal mining leaves behind — turning two liabilities into one structural asset.

Subject of Research: Using high-salinity concentrated mine water as mixing water in coal gangue and fly ash cemented backfill and its effects on flow, strength, and hydration

Article Title: Performance and hydration mechanism of backfill materials prepared by coal-based solid waste and high-salinity concentrated mine water

Article References: Feng, W., Zhang, J., Li, M., Cui, Z., Li, Z., & Zhang, Y. (2026). Performance and hydration mechanism of backfill materials prepared by coal-based solid waste and high-salinity concentrated mine water. Case Studies in Construction Materials, 25, Article e06558. https://doi.org/10.1016/j.cscm.2026.e06558

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06558

Keywords: cemented backfill, coal gangue, fly ash, high-salinity mine water, ettringite, hydration mechanism, unconfined compressive strength, solid waste utilization, mine water treatment, zero-liquid discharge, Portland cement, X-ray diffraction

Sulfate-activated slag cements are environmentally attractive for several reasons. They require no high-temperature calcination, they can incorporate very high levels of industrial by-products, and their main hydration products, needle-like ettringite crystals and low-calcium aluminum-containing C-A-S-H gel, give the hardened paste excellent resistance to sulfate and chloride attack. Their modest hydration heat also helps avoid the thermal cracking that plagues massive concrete pours. These traits make them promising candidates for marine engineering, construction in saline-alkali environments, and subgrade stabilization. Yet one stubborn flaw has kept them from widespread adoption: at early ages, their hydration is sluggish. Slag barely reacts in pure water, and a dense protective layer of C-A-S-H rapidly forms on particle surfaces, choking off further reaction. The result is prolonged setting times and disappointing early strength.

Previous work showed that adding an alkaline boost, for instance quicklime or Portland cement clinker, can break the slag glass structure open and accelerate early strength gain. But the system is notoriously sensitive to alkali content and type, and too much alkali can actually reduce strength. What remained missing was a mechanistic, microscopic understanding of how different alkaline activators reshape slag dissolution kinetics, hydration product formation, and microstructural evolution. To close this gap, the team designed an elegant comparative experiment using three clinker-based activators arranged along a gradient of mineralogy and hydraulic reactivity. Ordinary Portland cement clinker, rich in highly reactive C3S and C3A phases, served as the fast, high-alkalinity benchmark. High-belite clinker, dominated by the slow-hydrating C2S phase, represented a lower-carbon, moderate-reactivity alternative. And fully hydrated cement paste powder, containing no unreacted clinker at all, acted as a pure chemical control that simply releases calcium hydroxide to supply alkalinity, cleanly separating chemical activation from any physical contribution of active clinker hydration.

The researchers blended each activator at 5 percent into a mix of 80 percent blast furnace slag and 5 percent gypsum, keeping a constant water-to-binder ratio of 0.40, and then deployed an impressive battery of techniques: isothermal calorimetry to track heat release over five days, in situ X-ray diffraction with Rietveld refinement to quantify evolving phases within the first twelve hours, pore solution extraction followed by ICP-OES analysis, thermodynamic modeling with PHREEQC, thermogravimetric analysis, scanning electron microscopy, low-field nuclear magnetic resonance for real-time pore structure monitoring, autogenous shrinkage measurements, and standard compressive strength testing. The reference mix without any activator remained unset even after three days of curing, underscoring how essential alkaline activation is to this system.

The strength results told a striking story. Even the weakest activated system, the one using fully hydrated paste powder, exceeded 8 megapascals at three days, while the reference only began to harden after a week. But the standout performer was the high-belite clinker system, which achieved the highest compressive strength at every curing age tested. This was puzzling at first, because the three activators should theoretically deliver comparable total alkalinity, roughly 1.65 percent calcium hydroxide from the ordinary clinker and 1.58 percent from the high-belite clinker upon complete hydration. Since the chemistry of supply was similar, the difference had to lie in the timing and kinetics of alkalinity release, driven by the intrinsic mineralogy of each clinker.

The calorimetry and in situ XRD data unraveled this paradox. In the ordinary clinker system, the heat flow curve showed four distinct stages, including a weak early peak from the very reactive C3A and a pronounced shoulder from rapid C3S hydration starting around six hours. Once C3S hydration began, slag consumption and C-A-S-H gel formation accelerated dramatically, confirming that a timely supply of hydroxide ions depolymerizes the slag glass network. In the high-belite system, the main reaction peak appeared roughly four hours later, but when it arrived it burned hotter, ultimately producing the greatest cumulative heat release and the highest degree of slag reaction by twelve hours. Pore solution analysis showed why: the ordinary clinker rapidly pushed the pH above 12, while the high-belite clinker raised alkalinity more gradually, maintaining a moderate, sustained supersaturation rather than a chemical jolt.

That difference in timing proved decisive for how ettringite, the system’s key crystalline hydration product, grew. Electron microscopy revealed that in the fast-release systems, the rapid pH spike created extreme local supersaturation, triggering burst-like homogeneous nucleation. Thousands of tiny ettringite crystals formed almost simultaneously, but with limited growth space and ionic supply they stayed small and poorly interlocked, producing a loose, porous microstructure. In the slow-release high-belite system, moderate and persistent supersaturation allowed thermodynamically controlled crystal growth instead. Ions deposited in an orderly fashion onto existing nuclei, developing large, well-defined needle-like and prismatic crystals that bridged pores and built a robust load-bearing skeleton. In other words, the strength advantage came not from making more hydration product, but from shaping its crystallization kinetics.

Low-field nuclear magnetic resonance captured the pore structure consequences in real time. In the ordinary clinker system, the dominant relaxation peak first shifted toward shorter times as pores refined, then reversed and coarsened dramatically, with the maximum relaxation time climbing from 2.76 milliseconds at twelve hours to 8.96 milliseconds at forty-eight hours, even higher than the fresh paste. This coarsening coincided with rapid ettringite precipitation driven by locally excessive alkalinity. The high-belite system, by contrast, showed sustained pore refinement throughout the observation window with essentially no coarsening, and the fully hydrated paste control showed pronounced coarsening at later ages. Autogenous shrinkage measurements reinforced the picture: all pastes expanded slightly, with the ordinary clinker system showing roughly 1.05 percent strain, about double that of the high-belite system, and the team concluded that excessive early ettringite-driven expansion generates internal microcracks that undermine strength development.

Synthesizing these results, the authors propose a clear regulatory mechanism. The hydrolysis of clinker-based activators provides the alkaline environment, around pH 12, needed for slag dissolution, but the rate and stability of alkalinity development play the decisive role. Highly reactive clinkers dump calcium hydroxide quickly, paradoxically suppressing early slag dissolution and promoting concentrated burst nucleation of fine ettringite, which yields a loose microstructure, elevated expansion, internal stresses, and restricted strength. Slow-release activators maintain a moderate, persistent supersaturation that favors oriented crystal growth, dense pore filling, and a homogeneous, stable matrix, while the activator’s own hydration still exceeds that of the slag and boosts early strength.

The implications reach well beyond the laboratory. Because sulfate-activated slag systems inherently expand slightly, precise microstructural control is especially critical for their engineering use, and this study demonstrates that selecting an activator with the right hydration reactivity can balance nucleation against growth, avoiding premature localized ettringite formation and detrimental expansion. High-belite clinker, itself a lower-carbon product with a dominant slow-hydrating phase, now emerges as a particularly attractive partner for slag-based binders. The researchers caution that their work covered a limited set of activators and testing durations, and they call for future studies to span broader activator systems, quantitative models linking hydroxide release kinetics to microstructural evolution, and long-term durability plus full life-cycle environmental assessment. Still, the core lesson is already compelling for anyone watching the cement industry’s decarbonization race: in green cement chemistry, as in many other fields, patience beats brute force, and a slow, steady supply of alkalinity builds a stronger material than a fast one ever can.

Subject of Research: The effect of clinker-based alkaline activators on hydration kinetics, ettringite crystallization, and hardening of sulfate-activated slag cementitious materials

Article Title: Effect of clinker-based activator on the hydration and hardening properties of sulfate-activated slag cementitious materials

Article References: Wang, J., SUN, N., Wang, D., & Liu, B. (2026). Effect of clinker-based activator on the hydration and hardening properties of sulfate-activated slag cementitious materials. Case Studies in Construction Materials, 25, Article e06551. https://doi.org/10.1016/j.cscm.2026.e06551

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06551

Keywords: sulfate-activated slag, cement clinker, ettringite, hydration kinetics, low-carbon cement, blast furnace slag, high-belite clinker, alkaline activator, microstructure, compressive strength, pore structure, autogenous shrinkage

The research team, led by Tongning Cao and colleagues including Han Wang, Qi Dong, Fengjuan Wang, Zhiyong Liu, Chenyun Yu, Bin Peng, and Jinyang Jiang, set out to close a persistent gap in concrete durability science. Most laboratory studies of sulfate attack have been performed under simple immersion, wet-dry cycling, or atmospheric pressure conditions. Those regimes capture shallow-marine and terrestrial exposure reasonably well, but they ignore the defining feature of the deep-sea environment: sustained hydrostatic pressure ranging from several to tens of megapascals. At such pressures, the state of pore water, the morphology of the pore network, and the very mechanisms of ion transport inside concrete may be fundamentally altered, making conventional durability predictions unreliable for deep-water structures.

To probe these coupled effects, the researchers built a custom hydrostatic pressure-sulfate erosion apparatus capable of simultaneously controlling pressure, temperature, and sulfate exposure. The system, rated to a maximum of 25 megapascals with a pressure resolution of 0.1 megapascals, centers on a one-liter high-pressure reaction vessel pressurized by nitrogen gas. Cubic cement paste specimens with a water-to-binder ratio of 0.4 were cast, cured for 28 days, and then sealed with epoxy resin on five faces, leaving a single 20 by 20 millimeter face exposed. This clever geometry forced sulfate ions to travel along one well-defined direction, simplifying the interpretation of the depth-resolved measurements that followed.

Specimens were exposed to a 5 percent sodium sulfate solution at three gauge pressures: 0, 1.2, and 2.4 megapascals, for a total of 150 days. The solution was refreshed every 15 days to maintain aggressive conditions. At 60, 90, 120, and 150 days, replicate specimens were retrieved, sectioned into 2-millimeter-thick slices along the exposure direction, and treated with isopropanol to halt hydration. All sample handling took place in a nitrogen-controlled glove box to prevent carbonation from contaminating the chemistry. One portion of each powdered slice was analyzed for sulfate ion content by gravimetric barium sulfate titration, while the other was subjected to quantitative X-ray diffraction with Rietveld refinement, using aluminum oxide as an internal standard, to measure the amount of ettringite, also known as AFt, formed at each depth.

The results reveal a striking pressure dependence. At a depth of 9 millimeters, the sulfate concentration after 150 days rose from 1.6 percent under atmospheric pressure to 2.7 percent at 2.4 megapascals. Deeper in the material the contrast became even more dramatic: at 19 millimeters, the sulfate content after 150 days was 0.5 percent at 0 megapascals, but 0.9 percent at 1.2 megapascals and 1.2 percent at 2.4 megapascals. In other words, moderate pressures that are routine in coastal and shallow offshore engineering already boost sulfate penetration by roughly 50 to 80 percent, and the enhancement grows with both depth and time. Ettringite formation followed the same pattern, accumulating preferentially near the exposed surface and decaying inward, with concentrations at 5 millimeters reaching 1.69 percent at 0 megapascals, 2.6 percent at 1.2 megapascals, and 3.1 percent at 2.4 megapascals after 120 days.

Microscopy and porosimetry explained why. Field-emission scanning electron microscopy showed the corrosion products as interwoven needle-like and rod-like crystals, the classic habit of ettringite, while energy-dispersive spectroscopy confirmed the enrichment of calcium, aluminum, sulfur, and oxygen in ratios matching the mineral’s stoichiometry. Notably, ettringite crystals clustered around portlandite, suggesting that as sulfate ions consume calcium from the pore solution, portlandite dissolves to replenish it, sustaining the reaction and progressively degrading the matrix. Mercury intrusion porosimetry then quantified the structural toll: total porosity climbed from 14.93 percent at 0 megapascals to 19.02 percent at 1.2 megapascals and 25.81 percent at 2.4 megapascals, with the pore-size distribution shifting toward larger, better-connected channels.

This pore-structure evolution sets up a vicious feedback loop. Pressure-enhanced sulfate transport delivers more reactant deeper into the material, accelerating ettringite formation, which coarsens and connects the pore network. The modified network in turn raises the effective diffusion coefficient, allowing still faster sulfate ingress. According to Fick’s law, this increase in effective transport capacity strengthens concentration-gradient-driven migration, producing deeper penetration and higher interior sulfate contents. The authors’ simulations capture this nonlinear amplification: after 150 days, predicted sulfate concentrations at 20 millimeters were 112 percent higher at 1.2 megapascals and 290 percent higher at 2.4 megapascals than under atmospheric pressure. From a crystallization-pressure perspective, the elevated sulfate supply also drives the pore solution to supersaturation sooner, triggering earlier ettringite crystallization and shortening the material’s latent expansion period.

To turn these observations into a predictive tool, the team constructed a coupled sulfate transport-reaction model grounded in Fick’s second law and mass conservation. The model treats sulfate ions as either free in the pore solution or consumed by reaction with calcium and aluminate-bearing hydration products to form ettringite, following the stoichiometry that one mole of AFt requires two moles of external sulfate. The effective diffusion coefficient is computed from porosity, tortuosity, and the Stokes-Einstein estimate of sulfate diffusivity in water, with a phenomenological pressure term linking hydrostatic stress to porosity change through the circumferential tensile stress around idealized cylindrical pores. The equations were solved numerically with the finite volume method on hexahedral grids, accelerated by a multigrid V-cycle scheme implemented in MATLAB.

Validation against the 1.2 megapascal experiments showed good agreement between predicted and measured sulfate and ettringite profiles across the full 150-day exposure and the entire penetration depth, with minor discrepancies attributed to the finite thickness of the sliced samples. The framework provides, for the first time, an experimentally supported quantitative link between hydrostatic pressure, sulfate transport, reaction kinetics, and pore-structure evolution in cementitious materials. The authors caution that the study used cement paste under controlled laboratory conditions rather than full concrete in service, and that further validation across broader materials, environments, and timescales is needed before deep-ocean application. Even so, the message for engineers is clear and urgent: durability assessments for submerged and deep-buried structures that ignore hydrostatic pressure may substantially underestimate how fast sulfate attack proceeds, and how soon the ocean’s quiet chemistry begins to dismantle the concrete we trust to hold it back.

Subject of Research: Effect of hydrostatic pressure on sulfate transport and ettringite formation in cement paste

Article Title: Effect of hydrostatic pressure on transport-reaction processes of cement paste under sulfate attack

Article References: Cao, T., Wang, H., Dong, Q., Wang, F., Liu, Z., Yu, C., Peng, B., & Jiang, J. (2026). Effect of hydrostatic pressure on transport-reaction processes of cement paste under sulfate attack. Case Studies in Construction Materials, 25, Article e06588. https://doi.org/10.1016/j.cscm.2026.e06588

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06588

Keywords: sulfate attack, hydrostatic pressure, cement paste, ettringite, concrete durability, deep-sea environment, ion transport, porosity, XRD-Rietveld, transport-reaction model, marine engineering, pore structure

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 research, conducted by Yaser Rashidi and Asghar Habibnejad Korayem, tackled a durability problem that has resisted conventional solutions for decades. When sulfate ions invade cementitious materials, two primary damaging products form. Ettringite arises when alumina-bearing hydrates such as monosulfate and calcium aluminate hydrates react with incoming sulfate in the presence of calcium hydroxide and water. Gypsum forms simultaneously through cation-exchange reactions between sulfates and calcium hydroxide. Both phases occupy greater solid volume than the phases they replace, so the matrix expands, cracks, and then admits even more sulfate in a self-accelerating feedback loop. Over time the cohesion of the hydration products collapses, and strength and mass drain away.

Engineers have traditionally fought back along three fronts: limiting the reactive tricalcium aluminate content of cement as in ASTM C150 Type II and Type V sulfate-resistant cements, blending in supplementary cementitious materials such as fly ash, slag, metakaolin, and silica fume to consume calcium hydroxide pozzolanically, and reducing permeability through lower water-to-binder ratios or fine mineral additives that tighten the pore network. Each strategy helps, but none fully solves the problem, because residual calcium hydroxide remains available for gypsum formation, and even well-refined matrices still transport ions. Nanoparticles promised a fourth front, yet materials such as nano-silica, carbon nanotubes, and graphene oxide carry significant production costs and environmental burdens, and their effectiveness depends critically on achieving good dispersion.

That is where halloysite nanotubes stand out. With outer diameters of roughly 30 to 70 nanometers and lengths of 1 to 3 micrometers, these natural tubes have enormous specific surface area, around 640,000 square centimeters per gram in this study, and an aluminosilicate composition of about 48 percent silica and 35 percent alumina. They act on cement through three complementary mechanisms: physical filling of the fine interstitial spaces between cement grains, heterogeneous nucleation that gives hydration products abundant surfaces on which to grow, and gradual pozzolanic reactions that consume calcium hydroxide and generate additional calcium silicate hydrate gel. Unlike carbon nanotubes or graphene oxide, they require no energy-intensive synthesis, chemical oxidation, or high-temperature functionalization; they are simply mined and processed.

A crucial subtlety the researchers confronted is that nanoparticles tend to clump in the brutal environment of cement pore solution. With a pH of 13.1 to 13.5 and a high concentration of dissolved ions, the synthetic pore solution used in the study compresses the electrical double layers around particles, collapsing the electrostatic repulsion that keeps them apart. Without help, ultrasonication only temporarily breaks agglomerates apart; they re-form within hours. The team’s answer was polycarboxylate ether, or PCE, the same high-range water-reducing admixture routinely used to fluidize concrete. PCE molecules adsorb onto halloysite surfaces through their anionic functional groups while their long polyethylene glycol side chains form a steric barrier, physically preventing neighboring tubes from touching.

The dispersion experiments were elegant in their design. Suspensions matching the dosages used in the mortars were mixed 1:1 with synthetic pore solution and tracked by visual sedimentation, ultraviolet-visible spectroscopy, and dynamic light scattering. The results were unambiguous. Without PCE, the suspension’s normalized UV-Vis absorbance collapsed from 1.0 to just 0.04 within six hours, and its average hydrodynamic particle diameter ballooned from 972 to 2,750 nanometers as particles aggregated and settled. With PCE, absorbance stayed high and particle growth was minimal, with the two lower-dose suspensions growing only from about 409 to 513 and 486 to 578 nanometers respectively. The highest-dose suspension, corresponding to 3 weight percent HNTs, showed slightly reduced stability, an early warning sign that would echo in the mortar performance.

The mortars themselves told the rest of the story. Replacing 1, 2, or 3 weight percent of ordinary Portland cement with PCE-stabilized HNTs, at a fixed water-to-binder ratio of 0.4 and identical workability, the researchers exposed mortar bars and cubes to a 5 percent sodium sulfate solution for 182 days following ASTM C1012. The plain reference mortar expanded by 0.084 percent over 26 weeks, above the 0.05 percent threshold that ASTM C1157 classifies as non-resistant to sulfate. The 1 percent HNT mixture fared little better at 0.080 percent. But the 2 and 3 percent mixtures stayed below the threshold at 0.032 and 0.043 percent respectively, a genuinely high level of sulfate resistance achieved purely by a clay additive.

The mechanical evidence was equally compelling. Before exposure, compressive strengths rose from 42.6 megapascals in the reference to 52.9 megapascals with 2 percent HNTs. After 182 days of sulfate immersion, the reference mortar had lost 18.8 percent of its compressive strength and a striking 29.9 percent of its flexural strength, while its ultrasonic pulse velocity, a proxy for internal integrity, dropped 9.6 percent. The 2 percent HNT mortar lost only 5.2 percent compressive and 13.1 percent flexural strength, and its pulse velocity actually increased by 3.3 percent, meaning sulfate products filling refined pores left it more acoustically dense than before. Scanning electron microscopy revealed denser, crack-poor microstructures before exposure and far fewer needle-like ettringite crystals afterward. X-ray diffraction showed progressively weaker ettringite and gypsum peaks with increasing HNT content, and Fourier-transform infrared spectra confirmed the pattern: weaker sulfate-related bands at 1110, 670, and 550 inverse centimeters, and a stronger calcium silicate hydrate band at 982.

Why does a humble clay tube outperform? The authors attribute it to synergy. Chemically, diluting the clinker reduces the reactive aluminate supply, while pozzolanic consumption of calcium hydroxide starves both the gypsum and ettringite reactions of raw material. Physically, the tubes pack into pores, nucleate hydration, and even act as internal water reservoirs whose hydrophilic bores release moisture gradually to sustain curing. The result is a matrix whose pore connectivity is so reduced that sulfate ions simply cannot reach their targets in damaging quantities. The one caveat is dosage discipline: at 3 weight percent, agglomeration created weak zones, and every measured property slipped slightly below the 2 percent optimum, a direct macroscopic echo of the dispersion tests.

Perhaps the most consequential finding is the sustainability comparison. Compiled literature data show halloysite carrying a global warming potential of roughly 5 to 6.6 kilograms of carbon dioxide equivalent per kilogram, versus 28.55 for carbon nanotubes produced by chemical vapor deposition and 8.55 for graphene oxide, and an embodied energy of about 21 megajoules per kilogram versus 480 for CNTs and 120 to 140 for graphene oxide. At around 0.7 dollars per kilogram, HNTs cost less than a tenth of nano-silica and a tiny fraction of carbon-based alternatives. For a construction industry desperately seeking durable, low-carbon materials, the message is remarkable: a naturally abundant clay mineral, stabilized by an admixture already in every ready-mix truck, can match the sulfate resistance of engineered nanomaterials while leaving a fraction of their environmental and economic footprint. The next generation of sulfate-proof infrastructure may literally be dug from the ground.

Subject of Research: Use of halloysite nanotubes to enhance the sulfate resistance and sustainability of cementitious composites

Article Title: Sustainable halloysite nanotubes for sulfate–resistant cementitious composites: Physicochemical mechanisms and environmental advantages

Article References: Rashidi, Y., & Korayem, A. H. (2026). Sustainable halloysite nanotubes for sulfate–resistant cementitious composites: Physicochemical mechanisms and environmental advantages. Case Studies in Construction Materials, 25, Article e06582. https://doi.org/10.1016/j.cscm.2026.e06582

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06582

Keywords: halloysite nanotubes, sulfate attack, cement mortar, durability, nanomaterials, ettringite, gypsum, pozzolanic reaction, polycarboxylate ether, sustainable construction, pore refinement, embodied energy

The threat itself operates through two intertwined mechanisms. The first is physical: sulfate solutions penetrate the porous network of cement through capillary action and osmosis, and as temperature and humidity fluctuate, dissolved sulfates reach supersaturation and crystallize inside capillary pores, voids, and micro-cracks. Growing crystals press relentlessly against pore walls, and once that pressure exceeds the internal cohesion of the matrix, pores expand and coalesce, the framework loosens, and surfaces begin to crack, peel, and powder. This damage opens fresh pathways for more sulfate ingress, creating a self-reinforcing cycle of erosion, deterioration, and accelerated erosion. The second mechanism is chemical and far more destructive. Sulfate ions react with calcium hydroxide to form gypsum and with calcium aluminates to generate ettringite—two expansive products that swell to roughly twice and one-and-a-half times their original volume, respectively. In the cramped interior of hardened cement, this expansion generates internal stresses that break chemical bonds between hydration products, raise porosity, propagate cracks, and ultimately strip away compressive strength, rupture strength, and interfacial bonding capacity.

Earlier work had already hinted that nanomaterials could help. Studies cited by the team showed that 1.5% nano-SiO₂ boosted the rupture strength of fly ash-cement mortar by 90%, while 0.5% raised compressive strength by 67.1% to 118.2%, and that 1% nano-CaCO₃ refined concrete’s microstructure enough to mitigate late-stage expansion damage. But nanomaterials carry a notorious drawback: their ultrafine particle size and enormous specific surface area make them prone to agglomeration during mixing, which raises water demand, degrades workability, and can undermine the very strength they are meant to enhance. The new research set out to resolve this tension by systematically comparing both nanoparticles across multiple dosages, combining macroscopic measurements of mass, compressive strength, and rupture strength with microscopic characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR).

The experimental design was rigorous and methodical. Seven mortar groups were prepared with a water-to-cement ratio of 0.55: an unmodified control, three groups with nano-SiO₂ at 1%, 3%, and 5% by mass of cement, and three groups with nano-CaCO₃ at the same dosage levels. The nano-SiO₂ particles measured about 20 nanometers with a specific surface area of 80 square meters per gram, while the nano-CaCO₃ particles were roughly 40 nanometers with 30 square meters per gram. After 28 days of standard curing and drying at 80 degrees Celsius, the 40 by 40 by 160 millimeter prisms were fully immersed in a 5% sodium sulfate solution at 20 degrees Celsius, with the solution renewed every 30 days to maintain stable concentration and pH. Specimens were retrieved at 30, 60, 90, 120, 150, and 180 days for testing, with three parallel specimens averaged for each measurement.

The mass evolution told a striking story. All groups first gained weight as invading sulfate reacted with hydration products to form pore-filling ettringite and gypsum, then lost weight as excessive expansion triggered micro-cracking, surface spalling, and detachment of erosion products. The nano-SiO₂ group at 3% dosage peaked at a 4.39% mass gain at 90 days and still retained a 3.70% mass change at 180 days, losing only 0.69% after its peak. By contrast, the 5% nano-CaCO₃ group reached a slightly higher peak of 4.55% but then shed a full 3.55% of its mass in the same window, ending at roughly 1%. After 180 days, the optimal nano-SiO₂ group’s mass change was 122.9% higher than that of the optimal nano-CaCO₃ group—a quantitative demonstration that the mass gained by nano-SiO₂ modification reflects a dense, penetration-resistant matrix, whereas the mass gained by heavily dosed nano-CaCO₃ stems from expansive ettringite that later causes severe spalling.

Compressive and rupture strength followed a three-stage trajectory of initial rise, mid-term plateau, and final decline, with peaks generally appearing between 30 and 90 days as ongoing hydration gave way to expansion damage. The 5% nano-SiO₂ group posted the highest peak compressive strength of 55.59 megapascals at just 30 days, while the 3% nano-SiO₂ group peaked at 51.47 megapascals. Crucially, strength retention revealed the deeper advantage of nano-SiO₂: at both the 1% and 3% dosage levels, nano-SiO₂-modified specimens retained more of their peak strength than their nano-CaCO₃ counterparts, with the 1% nano-SiO₂ group preserving 75% of its peak compressive strength after 180 days compared with 65.4% for the equivalent nano-CaCO₃ group. In rupture strength, the 3% nano-SiO₂ group held a retention rate of 44.4% versus 34.2% for 3% nano-CaCO₃, and its residual strength of 1.82 megapascals clearly exceeded the control group’s 1.65 megapascals. The team attributes this resilience to nano-SiO₂’s dual action: its pozzolanic reaction chemically consumes calcium hydroxide—the very ingredient sulfate ions need to form gypsum—while its micro-aggregate filling effect physically blocks ion transport channels.

The microscopic evidence sealed the argument. X-ray diffraction showed that in the unmodified control, ettringite and gypsum peaks surged between 90 and 120 days, reached maximum intensity by 150 to 180 days, and were joined by thaumasite, a notoriously destructive sulfate-bearing phase, marking the onset of accelerated deterioration. The 1% nano-SiO₂ group, by contrast, showed the lowest ettringite and gypsum peak intensities of all groups throughout the 90-to-180-day window and no obvious thaumasite signal at all, because the pozzolanic reaction had stripped the system of the calcium hydroxide needed to fuel attack reactions. Scanning electron microscopy at 10,000-fold magnification made the damage hierarchy visible: after 180 days, the control mortar displayed interconnected crack networks, spalling, and a failed hydration skeleton, while the 3% nano-SiO₂ specimen maintained a complete, continuous gel skeleton with only trace surface attack products. The overall degradation ranking ran from worst to best as control, 5% nano-CaCO₃, 3% nano-CaCO₃, 1% nano-CaCO₃, 1% nano-SiO₂, 5% nano-SiO₂, and 3% nano-SiO₂.

Nuclear magnetic resonance added a non-destructive, quantitative dimension by tracking pore size distribution through the relaxation behavior of hydrogen nuclei in pore water. At the outset, all nano-modified samples already showed lower initial porosity than the control’s 11.41%, thanks to nanoparticle filling. Porosity then fell during the first 90 days as expansive products temporarily plugged capillary channels—the 5% nano-SiO₂ group plummeting from 10.16% to a remarkable 3.97%—before rebounding after 120 days as expansion stress cracked the matrix. By 180 days, the control’s porosity had climbed back to 11.12%, nearly erasing its early densification, while the nano-modified specimens showed far smaller rebounds. Classifying pores by the widely used Wu Zhongwei system, the researchers found that the fractions of harmful pores (50 to 200 nanometers) and severely harmful pores (above 200 nanometers) grew most dramatically in the control group and most slowly in the 5% nano-SiO₂ group, whose T₂ spectra shifted least and whose harmless pore fraction remained the highest throughout.

The study’s practical verdict is nuanced: the optimal dosage depends on which performance indicator matters most. For nano-SiO₂, 3% proved best for mass stability while 5% excelled in compressive strength, rupture strength, and porosity, leading the team to recommend a 3% to 5% window. For nano-CaCO₃, 1% was optimal for mass and mechanical properties, and although 5% minimized porosity, SEM images showed clustered ettringite enrichment at that dose, with expansion stress initiating micro-cracks that no amount of physical filling could offset—so 1% stands as the overall recommendation. The fundamental difference lies in mechanism: nano-SiO₂ fights sulfate attack on two fronts, chemically through pozzolanic conversion of calcium hydroxide into dense calcium-silicate-hydrate gel and physically through pore refinement, while nano-CaCO₃ works mainly through nucleation and filling. For engineers designing structures destined for sulfate-laden coastlines, saline soils, or aggressive industrial environments, the message is clear: a few well-chosen nanometers of silica, added at just the right fraction, can mean the difference between a structure that crumbles within decades and one that endures for generations.

Subject of Research: Sulfate resistance of cement-based materials modified with nano-SiO₂ and nano-CaCO₃ nanoparticles

Article Title: Optimum dosage and long-term sulfate resistance of cement-based materials modified by nano-SiO₂ and nano-CaCO₃

Article References: Hao, E., Yuhang, L., Zhang, D., Wang, J., Ya, G., Wang, X., & Cao, Y. (2026). Optimum dosage and long-term sulfate resistance of cement-based materials modified by nano-SiO₂ and nano-CaCO₃. Case Studies in Construction Materials, 25, Article e06585. https://doi.org/10.1016/j.cscm.2026.e06585

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06585

Keywords: sulfate attack, nano-SiO2, nano-CaCO3, cement-based materials, concrete durability, ettringite, gypsum, pozzolanic reaction, pore structure, NMR, XRD, SEM

The mineral at the heart of the problem is pyrrhotite, a non-stoichiometric iron sulfide with the general formula Fe1−xS, where x can range from zero to 0.125. When even less than one weight percent of this mineral is present in concrete aggregates, its oxidation can trigger a chain of reactions known as internal sulfate attack, or ISA. As pyrrhotite reacts with oxygen and moisture, it releases sulfate ions and acidic species into the surrounding cement paste. These ions then react with hydration products in the cement to form gypsum and ettringite, minerals whose growth generates expansive pressures that crack the concrete matrix. The Canadian standard CSA A23.1:19, Appendix P formally recognizes this combined mechanism of iron-sulfide oxidation and subsequent sulfate reaction as the driver of ISA deterioration.

The damage is not hypothetical. Aggregate from the Maskimo quarry in St. Boniface, Quebec, has been linked to severe ISA affecting more than 1,700 housing foundations in Trois-Rivières, Quebec, many of which suffered serious damage after only five to ten years in service. Earlier work by Rodrigues and colleagues identified pyrrhotite as the principal reactive mineral phase in the damaged Trois-Rivières concrete. In the Maskimo rock, pyrrhotite and pyrite occur in an approximate ratio of one to two, with pentlandite and chalcopyrite frequently found as accessory inclusions within them. The quarry itself consists of a noritic microgabbro dominated by plagioclase feldspar, orthopyroxene, magnesiohornblende, and biotite, with total sulfur contents ranging from 0.85 to 1.8 weight percent, all bound in sulfides.

To capture the deterioration in action, a team led by Nikolas Å. Oberhardt of the Norwegian University of Science and Technology, together with colleagues including Klaartje De Weerdt, examined a sequence of samples spanning the entire life cycle of the reaction. They studied fresh Maskimo aggregate, mortar bars subjected to accelerated mortar bar testing for 12, 27, and 52 weeks, and a severely damaged concrete drill core taken from a Trois-Rivières foundation wall. The accelerated test, described in the Canadian standard, involves repeatedly immersing mortar prisms in sodium hypochlorite bleach, twice weekly for three hours, with interim storage first at 80 degrees Celsius and 75 percent relative humidity to accelerate sulfide oxidation, and later at 4 degrees Celsius and full humidity to promote thaumasite formation. The mortar bars expanded by 0.22 percent after 12 weeks, 0.27 percent after 27 weeks, and 0.47 percent after 52 weeks.

The analytical centerpiece of the study was scanning electron microscopy-based automated mineralogy, which uses quantitative energy-dispersive spectroscopy to classify every pixel of a polished thin section into a mineral phase. The researchers deployed several analytical modes: coarse field mapping at 10 and 20 micrometer step sizes for bulk mineralogy, line scanning at 1 micrometer for quantitative bulk composition, and high-resolution field mapping at 2 micrometers to resolve the fine-grained oxidation products and cement hydration phases. Complementing this, thermomagnetic analysis measured the magnetic susceptibility of powdered samples from room temperature to 650 degrees Celsius, allowing the team to detect and quantify two pyrrhotite superstructures, the hexagonal antiferromagnetic NC-type and the monoclinic ferrimagnetic 4C-type, using the characteristic lambda transition near 220 to 245 degrees Celsius and the Curie point at 315 degrees Celsius.

The results reveal a strikingly ordered oxidation sequence. In the pristine aggregate, pyrrhotite grains showed only incipient surface oxidation along mineral boundaries, with minor magnetite, goethite, and slightly oxidized pyrrhotite as the characteristic secondary phases. After 12 weeks of accelerated exposure, oxidation had penetrated the mineral interiors through etch pitting, and an oxidation rim of iron oxides, mainly hematite and magnetite, had grown around the grain peripheries. Ferrihydrite, an iron oxyhydroxide, appeared at around 1.4 weight percent, typically adjacent to the iron oxides rather than directly on the pyrrhotite. By 27 weeks, the grain interiors were severely altered and largely replaced by iron oxides and oxyhydroxides, leaving pyrrhotite only as relic-like structures. After 52 weeks, the crystals were almost entirely consumed, with ferrihydrite rising to 1.79 weight percent and ettringite increasing from 0.13 to 0.37 weight percent.

The element heat maps told an equally important story about where the atoms went. Iron remained largely immobile, locked in place within the relic zones of the oxidizing grains as iron oxides and oxyhydroxides replaced the original sulfide. Sulfur, by contrast, leached away as sulfate anions into the cement paste, and the sulfur maps showed a progressive increase in sulfur concentration within the cement matrix as exposure time lengthened. In the field sample, the cement paste adjacent to reacted aggregates contained as much as 10 to 12 weight percent sulfur. The team also identified a cryptocrystalline composite phase, labeled FeOSCa, averaging roughly 34 percent oxygen, 10.5 percent sulfur, 24.5 percent calcium, and 31 percent iron, which formed along cracks and near oxidizing pyrrhotite and may warrant further study with transmission electron microscopy or electron probe microanalysis.

Comparing the laboratory samples with the field concrete proved especially revealing. The field sample contained between 1.06 and 2.30 weight percent pyrrhotite, far more than the 0.19 to 0.37 weight percent measured in the mortar bars and the 0.50 weight percent in the fresh aggregate, underscoring the link between the amount of reactive pyrrhotite available and the magnitude of expansion. The field sections showed gypsum and the FeOSCa composite phase together reaching approximately 1.87 weight percent, against less than 0.05 weight percent gypsum in the laboratory samples, along with distinct cracking in the interfacial transition zone between aggregates and paste. The mortar bars, by contrast, showed little to no expansion cracking, suggesting that their measured expansion stemmed primarily from the formation of secondary iron oxides and hydroxides rather than from ettringite-driven paste expansion. Thermomagnetic analysis indicated that roughly two-thirds of the pyrrhotite in all samples was the hexagonal NC-type, a variant previous work suggests is more prone to oxidation than the monoclinic 4C form.

The study also carries a practical warning for laboratories running accelerated tests. Sulfur measurements by high-temperature combustion showed that the aggregate batches used in the Norwegian pilot testing contained only 0.44 to 0.51 weight percent sulfur after cement paste removal, considerably lower than the 0.85 to 1.08 weight percent reported for the reference Maskimo aggregate, which plausibly explains why the Norwegian mortar bars expanded less than Canadian counterparts tested under the same conditions. The authors recommend routinely determining total sulfur content in every test batch before accelerated mortar bar testing to ensure results are comparable and not misinterpreted. They further note that pre-oxidation of sulfide minerals, whether in the geological deposit, the quarry stockpile, or after concrete production, could accelerate sulfate attack, making the degree of pre-oxidation an important quality assurance consideration.

Beyond its immediate findings, the work demonstrates that automated mineralogy can serve as a diagnostic tool for assessing aggregate reactivity and predicting ISA-related deterioration in pyrrhotite-bearing concrete. By correlating a continuous transition from incipient surface alteration to complete pyrrhotite replacement with distinct secondary mineral assemblages, magnetite and goethite in fresh rock, ferrihydrite, gypsum, ettringite, and the FeOSCa phase in mortar and concrete, the researchers have established a mineralogical fingerprint of internal sulfate attack at each stage. Although the study drew on a single, well-characterized aggregate source, the authors expect the observed oxidation progression to be relevant to other ISA cases, noting that reactive aggregates from regions such as Connecticut exhibit comparable mineralogical characteristics. For the homeowners of Trois-Rivières and engineers elsewhere facing similar mysteries of crumbling concrete, the invisible enemy has finally been caught in the act.

Subject of Research: Pyrrhotite oxidation and internal sulfate attack in concrete aggregates and cement paste

Article Title: Pyrrhotite oxidation in aggregate and cement paste of accelerated laboratory and concrete field samples

Article References: Oberhardt, N. Å., Danner, T., Lode, S., Lindgård, J., Aasly, K., & De Weerdt, K. (2026). Pyrrhotite oxidation in aggregate and cement paste of accelerated laboratory and concrete field samples. Case Studies in Construction Materials, 25, Article e06544. https://doi.org/10.1016/j.cscm.2026.e06544

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06544

Keywords: pyrrhotite, internal sulfate attack, concrete deterioration, automated mineralogy, cement paste, Trois-Rivières, Maskimo quarry, thermomagnetic analysis, iron sulfide oxidation, ettringite, ferrihydrite, accelerated mortar bar test

The research team, led by Désiré Ndahirwa of UniLaSalle with colleagues including Hélène Lenormand, Hafida Zmamou and Nathalie Leblanc, focused on a question that has dogged the field for years. Wood ash contains reactive silica, alumina and calcium-bearing phases that can, in principle, behave like cement itself, reacting with water through hydraulic and pozzolanic pathways. In practice, however, pastes made from pure wood ash are weak and porous. Previous studies had shown that replacing moderate amounts of cement with wood ash often reduces compressive and flexural strength, and most work had examined substitution levels below 50 percent. Almost nothing was known about what happens at very high replacement levels, where wood ash dominates the mixture, and the stiffness of such materials, measured as Young’s modulus, had rarely been quantified despite its importance for structural design.

To close that gap, the researchers gathered four locally sourced wood ashes from the Normandy region of France. Two of them, designated WFA3 and WBA, came from the combustion of wood pellets and were delivered as wet sludge, requiring oven drying and crushing before use. The other two, WFA8 and WFA9, arrived as dry fine powders from a local heating plant operated by Coriance in Mont-Saint-Aignan, where boilers with power inputs of 6 and 8 megawatts burn forestry wood chips, bocage wood chips and pallet residues at temperatures between 900 and 1100 degrees Celsius. The team prepared seventeen paste formulations in total: four containing only wood ash, twelve blending 80 to 95 percent wood ash with 5 to 20 percent ordinary Portland cement, and a reference paste of pure cement, all compacted with a mini-Proctor device to boost density and cured for up to 28 days.

The chemical analysis alone revealed why wood ash is such a tricky raw material. The four ashes were dominated by calcium oxide, silica, potassium oxide and sulfur trioxide, but in wildly varying proportions. Sulfate contents reached 15.4 percent in WFA8 and 14 percent in WFA9, far above the 4 to 5 percent limit set by ASTM standards for pozzolans, and their combined pozzolanic oxides fell well below the required thresholds. In plain terms, these ashes do not qualify as conventional pozzolans, yet X-ray diffraction showed they carry crystalline phases such as portlandite, calcite, albite, alite, dolomite and sylvite that can still participate in binding reactions. The variability is a direct consequence of differing feedstocks, boiler designs and combustion temperatures, and it means each ash must be evaluated on its own terms rather than lumped into a single category.

When it came to mechanical performance, the effect of the small cement additions was unmistakable. Pure wood ash pastes managed compressive strengths of only 0.13 to 1.65 megapascals at 28 days, but raising the cement content to 20 percent lifted those values substantially. The best performer was the WFA8-based blend, which reached 5.49 megapascals, while WFA9 and WFA3 pastes achieved 4.01 and 3.87 megapascals respectively at the same dosage. The researchers attribute the gains to a richer supply of hydration products, including calcium silicate hydrate gel, portlandite and ettringite, formed as the cement’s tricalcium silicate reacts with water and progressively densifies the paste matrix. Stiffness told the same story: the modulus of elasticity, estimated from the linear portion of stress-strain curves, climbed with cement content, curing time and bulk density in three of the four ash families, with the highest values consistently recorded in mixtures containing 20 percent cement.

One ash refused to follow the script. The wood bottom ash, WBA, behaved atypically across every measurement. Its pastes lost compressive strength between 7 and 28 days at certain dosages, its modulus of elasticity peaked at 7 days and then declined, and thermogravimetric analysis found no detectable portlandite whatsoever. X-ray diffraction offered an explanation: the WBA pastes contained no alite, the calcium silicate phase that drives strength development in hydrating cement, and their dominant crystalline phases were calcite and quartz. Adding 20 percent cement pushed the estimated calcite content up from about 47 to 63.5 percent while quartz fell, a signature of carbonation of calcium silicate hydrate or the formation of complex, less efficient hydrate phases, both of which are associated with increased porosity and weaker binding.

Scanning electron microscopy added a visual dimension to the story. Under the microscope, pure cement paste appeared dense and well packed, bristling with the products of hydration, while pastes made from 100 percent wood ash showed loosely arranged particles riddled with interparticle voids, along with unreacted ash grains and dark fragments of unburnt wood. With 20 percent cement added, the microstructure tightened, porosity dropped and hydration products proliferated, with needle-like ettringite crystals, gel-like calcium silicate hydrates, plate-shaped portlandite and rhombohedral calcite all visible. The ashes from the heating plant, WFA8 and WFA9, consistently produced more compact matrices than the pellet-derived WFA3 and the bottom ash WBA, underlining how much origin and processing shape a material’s destiny.

The mineralogical detective work also turned up some genuinely unexpected chemistry. In the WFA8 blends, introducing cement promoted the formation of alunite, a potassium aluminum sulfate hydroxide phase not present in the unblended paste, alongside an array of compounds including syngenite, arcanite, serandite and harmotome. Thermogravimetric analysis at 7 and 28 days complemented the diffraction data, identifying calcium silicate hydrates and ettringite dehydrating between 50 and 200 degrees Celsius, AFm phases such as calcium monocarboaluminate and hemicarboaluminate decomposing between 200 and 300 degrees, portlandite dehydroxylating between 400 and 500 degrees, and carbonates releasing carbon dioxide from 500 to 800 degrees. The two techniques agreed closely, with the single discrepancy being ettringite in the bottom ash pastes, which thermal analysis detected but diffraction did not, likely because its concentration fell below the instrument’s detection limit.

What emerges from the study is a nuanced but practical message. Low doses of ordinary Portland cement, between 5 and 20 percent, can meaningfully upgrade pastes in which wood ash makes up as much as 95 percent of the solid content, provided the ash is fine and reasonably reactive. The improvements in stiffness, strength and microstructure are real, even if the resulting materials remain suited to low-strength applications such as lightweight binders rather than load-bearing concrete. The decisive variable, the authors conclude, is the variability of the ash itself: its origin, chemistry and mineralogy govern everything from phase development to porosity. That insight carries weight well beyond Normandy. With millions of tons of biomass ash generated annually and cement production responsible for a major share of global carbon dioxide emissions, even modest cement dosages that transform a landfill-bound waste into a functional building material represent a meaningful step toward circular, lower-carbon construction. The next challenge will be standardizing how ashes are characterized and selected, so that builders can trust what is in the bag before it ever reaches the site.

Subject of Research: The effect of low ordinary Portland cement content on the stiffness, microstructure and mineralogical composition of wood ash-based pastes

Article Title: Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes

Article References: Ndahirwa, D., Lenormand, H., Zmamou, H., Chenot, E., Potel, S., & Leblanc, N. (2026). Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes. Cleaner Engineering and Technology, 34, Article 101303. https://doi.org/10.1016/j.clet.2026.101303

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101303

Keywords: wood ash, ordinary Portland cement, Young's modulus, compressive strength, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, calcium silicate hydrate, ettringite, porosity, biomass ash, sustainable construction