Researchers compared untreated and cold-soaked samples using isothermal methane-adsorption experiments, swelling-deformation tests, Fourier transform infrared spectroscopy, nuclear magnetic resonance measurements and low-temperature nitrogen adsorption analysis. The three coal types were examined at different degrees of metamorphism. This experimental design linked the gas-adsorption and deformation responses to changes in pore structure and the chemical groups present on coal surfaces, rather than relying on adsorption measurements alone.

The pore-characterization results indicated that liquid-nitrogen cold soaking shifted the coals’ pore-size distributions toward larger pores and improved pore connectivity. At the same time, the treatment reduced the effective space available for methane adsorption. The study’s Pearson correlation analysis found that adsorption-pore specific surface area and BET specific surface area were strongly positively correlated with saturated adsorption capacity across coals of different metamorphic grades, with correlation coefficients greater than 0.71.

BJH total pore volume and average pore diameter were strongly negatively correlated with saturated adsorption capacity, with coefficients greater than 0.73 in magnitude. The authors describe the pore-network response as having two competing effects. Enhanced connectivity accelerated the initial migration of methane into the coal, whereas the smaller effective adsorption space slowed the subsequent deformation rate during gas uptake.

These reported mechanisms connect the larger, more connected pore network with the observed reduction in methane-induced swelling. The findings do not mean that all stages of methane migration were slower: the initial migration was accelerated, while the later deformation associated with continued uptake proceeded more slowly. Peak fitting of the Fourier transform infrared spectra showed that liquid-nitrogen treatment did not change the types of functional groups detected on the coal surfaces.

However, the contents of hydroxyl and carboxyl groups increased significantly. The researchers associated these groups with weaker coupling between coal and methane and report that they weakened the methane adsorption process. Together, the spectroscopic and pore-structure observations offer a microscopic explanation for the lower saturated adsorption capacities measured after cold soaking.

The size of the response differed among the coal types. Lignite showed the largest capacity reduction, while bituminous coal and anthracite experienced declines of similar, substantially smaller magnitudes. The reduction in swelling deformation also varied with coal type, although all three treated coals showed less swelling than their untreated counterparts.

The results therefore indicate that liquid-nitrogen cold soaking produced a consistent direction of change in the tested samples, but not an identical effect across coal ranks. The work is an experimental laboratory study, not a field trial, and its conclusions apply to the tested lignite, bituminous coal and anthracite samples under the reported conditions. The supplied findings do not establish whether repeated treatment would produce the same changes, how long the effects persist, or whether the process can reproduce these responses under underground temperature and pressure conditions.

The study also does not demonstrate consequences for methane recovery, mine safety, field-scale gas flow or operational performance in coal seams. Within those limits, the measurements connect surface chemistry, pore geometry, methane adsorption and mechanical deformation at the microscopic scale. The authors present the results as fundamental microscopic data for coal-reservoir research, providing a laboratory basis for examining how liquid-nitrogen treatment affects methane-adsorption behavior in different coal ranks.

Subject of Research: Effects of liquid-nitrogen cold soaking on coal pore structure, methane adsorption and swelling deformation

Article Title: Experimental study on microscopic mechanisms of coal deformation and methane adsorption under liquid nitrogen cold soaking

Article References: Liu, S., Wang, S., Li, X., Wang, D., & Fu, J. (2026). Experimental study on microscopic mechanisms of coal deformation and methane adsorption under liquid nitrogen cold soaking. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38314-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38314-6

Keywords: liquid nitrogen, cold soaking, coal deformation, methane adsorption, swelling strain, pore structure, pore connectivity, FTIR spectroscopy, NMR analysis, BET analysis, lignite, anthracite

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 research team, led by Xin Wang of the SINOPEC Petroleum Exploration and Production Research Institute together with colleagues from the China University of Petroleum in Beijing, focused on wells in the Changning, Luzhou, Da’an, Zigong, Yongchuan, Dazu and Hejiang areas of the Southern Sichuan Low-Steep Structural Belt. The Longmaxi Formation, deposited roughly 440 million years ago in a deep-water shelf to semi-enclosed stagnant marine basin, hosts 20 to 60 meters of organic-rich black shale. Anoxic bottom waters during deposition preserved abundant organic matter, and subsequent tectonic events including the Caledonian, Indosinian, Yanshanian and Himalayan orogenies subjected different parts of the basin to distinct burial and thermal histories. That spatial variation in heating, the study shows, is central to the electrical puzzle.

To classify the anomalies, the researchers introduced a dimensionless resistivity enhancement index, Zg, which compares the resistivity of gas-bearing shale intervals with that of adjacent non-gas-bearing dry intervals. Using this relative measure rather than absolute resistivity alone, they divided the shales into three categories: ultra-low-resistivity shale with Zg below 1, low-resistivity shale with Zg between 1 and 2, and conventional-resistivity shale with Zg above 2. The classification revealed striking regional patterns. Basin-margin wells in western Changning and Changning are overwhelmingly dominated by ultra-low-resistivity shale, with enhancement indices all below 0.5, while basin-interior wells in Luzhou, Yongchuan and Dazu show considerable divergence among individual stratigraphic sublayers, mixing conventional, low and ultra-low-resistivity intervals within single wells.

The link between resistivity and gas content proved equally revealing. When resistivity falls below 5 ohm-meters, most samples carry less than 2 cubic meters of gas per ton of rock, and gas content barely changes with resistivity. Between 5 and 15 ohm-meters, gas content rises rapidly with increasing resistivity, marking the most sensitive range for evaluating low-resistivity shales. Above 15 ohm-meters, most samples show medium to high gas contents, though the trend flattens. This means ultra-low-resistivity shales generally hold poorer gas-bearing capacity and higher exploration uncertainty, a practical warning for drilling programs that rely on resistivity logs alone.

Armed with an unusually broad experimental toolkit, the team then probed the physical basis of the conductivity. They combined well logging with geochemical analysis, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy, carbon dioxide and nitrogen adsorption, high-pressure mercury intrusion porosimetry, fractal analysis and laboratory electrical tests. Scanning electron microscopy revealed abundant organic pores ranging from 5 to 200 nanometers, including honeycomb-like pore networks tens of nanometers across, bubble pores up to 500 nanometers, and slit-like clay intercrystalline pores of roughly 0.5 to 2.0 micrometers. In some samples, carbonized organic matter had visibly bent and deformed, becoming filled with minerals or bitumen and losing most of its porosity.

Pore-size distributions told a consistent story. The shales show three dominant pore-size peaks, at 0.5 to 0.6 nanometers, 2 to 3 nanometers, and 10 to 10,000 nanometers. As resistivity decreases, total pore volume falls, driven mainly by the loss of mesopores and macropores. Ultra-low-resistivity shale has the dominant pore system shifted to 10 to 20 nanometers, low-resistivity shale to 10 to 50 nanometers, and conventional-resistivity shale to 1,000 to 10,000 nanometers. Micropores and small mesopores contribute more than 65 percent of the total specific surface area, and ultra-low-resistivity shale shows the least developed pore network of all, with its macropore surface area amounting to just 0.970 square meters per gram, or 2.899 percent of the total. The researchers interpret this meso-to-macropore loss primarily as a consequence of maturity-driven organic matter graphitization and the associated collapse or closure of organic pores.

Fractal analysis of nitrogen adsorption data from 16 samples added a geometric dimension to the picture. The fractal dimension D1, which describes pore-surface roughness, ranged from 2.5708 to 2.7200, while D2, which captures the structural complexity of the accessible pore network, ranged from 2.7685 to 2.8813, indicating strong heterogeneity. Resistivity increases with D1, because rougher surfaces impede the transport of charged particles, but decreases with D2, because more complex networks can enhance connectivity and open conductive pathways. Low-resistivity shales generally show higher D2 and lower D1 values than conventional shales, a combination that favors fluid retention and ionic conduction.

The most striking finding concerns the fate of organic matter itself. Because Lower Paleozoic marine shales lack indigenous vitrinite, the team measured solid-bitumen reflectance and converted it to an equivalent vitrinite reflectance, Ro,eq. When Ro,eq exceeds roughly 2.9 percent, shale resistivity drops sharply and inversely with total organic carbon content, following a negative logarithmic trend. Below that maturity level, the correlation between organic carbon and resistivity is weak or absent. Transmission electron microscopy made the transition visible at the atomic scale: the NX202 sample from western Changning displayed abundant ordered layered carbon structures with clear lattice fringes and ring diffraction patterns, evidence of advanced graphitization, while the Yongchuan sample showed only disordered carbon with no obvious lattice structure. XPS analysis quantified the trend, with the graphitic carbon proportion reaching a maximum of 24.68 percent in the NX202 kerogen and a minimum of 8.51 percent in a Zigong sample. Samples containing more than 15 percent graphitic carbon generally showed resistivity below 10 ohm-meters. The mechanism is physical: as aromatic carbon structures condense and order into graphite-like domains, delocalized pi-electron systems emerge that can carry current through the rock. The authors caution that the 2.9 percent value is an empirical division for their sample set, not a universal threshold.

At lower maturities, other conductors take over. Clay minerals enhance ionic conduction through cation exchange, electrical double layers and bound water, though correlations between clay content and resistivity proved weak, with determination coefficients of only 0.0713 to 0.3227. Pyrite showed stronger local effects, with determination coefficients reaching 0.4514 to 0.6754 in some wells, but the mineral’s influence depends on whether it forms interconnected framboidal aggregates, laminae or continuous bands rather than isolated grains. Given its low abundance, pyrite alone cannot span the rock; instead, connected pore water and clay-associated water films appear to bridge spatially separated pyrite grains, clay surfaces and graphitized organic matter into a composite conductive network. Formation water emerges as the primary ionic medium: average water saturation in low-resistivity intervals exceeds 40 percent in most wells, reaching 63.48 percent at NX202 and 51.19 percent at N211, compared with 33.06 percent at the conventional-resistivity well Ning-215. Laboratory electrical tests showed that resistivity rises steeply once water saturation drops below about 40 percent, as continuous water films break down, while above 50 percent saturation interconnected water films keep resistivity low.

Synthesizing these observations, the team proposes a maturity-dependent genetic model. Below Ro,eq of about 2.9 percent, low resistivity is governed mainly by ionic conduction through formation water and clay-mineral surface conductivity, with pyrite playing a secondary role and fractures promoting local water enrichment. Above that maturity, organic matter graphitization becomes increasingly important, and ordered graphitic carbon domains provide electron-conductive pathways that, together with clay-water ionic conduction and pore-structure evolution, drive resistivity down. The authors emphasize that the model is qualitative, since the conductivities of individual phases, formation-water chemistry and pressure effects were not independently quantified. Even so, the framework gives exploration geologists a practical rule of thumb: in highly to overmature basins, a low resistivity reading may signal not water but rock that has been cooked into a semiconductor, and reservoir evaluation methods must adapt accordingly to avoid writing off some of the richest shale gas intervals.

Subject of Research: Conductive mechanisms and genetic model of low-resistivity marine shales in the Longmaxi Formation, Sichuan Basin

Article Title: Influencing factors and genetic model of low-resistivity shales: a case study of the Longmaxi Formation, Southern Sichuan Basin, China

Article References: Wang, X., Cao, Z., Hu, Z., Jiang, Z., Hao, R., Shi, H., Gao, W., & Wang, D. (2026). Influencing factors and genetic model of low-resistivity shales: a case study of the Longmaxi Formation, Southern Sichuan Basin, China. Frontiers in Earth Science, 14, Article 1950939. https://doi.org/10.3389/feart.2026.1950939

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1950939

Keywords: low-resistivity shale, Longmaxi Formation, Sichuan Basin, organic matter graphitization, shale gas, pore structure, formation water, pyrite, clay minerals, thermal maturity, well logging, X-ray photoelectron spectroscopy

The research, led by Xiaoxia Dong and Nanxi Zhang of Sinopec Southwest Oil & Gas Company together with colleagues at Chengdu University of Technology, focused on the first member of the Lower Silurian Longmaxi Formation, one of China’s most important shale gas targets. The team collected 27 shale samples from seven core drilling wells that span a striking range of structural settings: quiet, gently folded synclines; fault-bounded uplifted blocks; clamped fault blocks squeezed between reverse faults; low-amplitude anticlines; and intensely deformed anticlines. Because the samples come from the same stratigraphic interval, differences in their microscopic fabric can be attributed largely to deformation intensity rather than to original depositional variation, making the dataset an unusually clean test of how tectonics sculpts a reservoir.

The analytical toolkit combined several complementary techniques. X-ray diffraction quantified the mineral composition of each sample, petrographic thin sections and field-emission scanning electron microscopy revealed pore types at scales down to a few nanometers, overburden porosity and permeability measurements characterized the bulk flow properties, and low-temperature nitrogen adsorption experiments mapped the distribution of pore sizes. On top of these measurements, the researchers applied the Frenkel-Halsey-Hill fractal model to the adsorption data, extracting two fractal dimensions that describe, respectively, the roughness of pore surfaces at low relative pressure and the complexity of the pore network at high relative pressure. Fractal dimensions near 2 indicate smooth, regular pore systems, while values approaching 3 signal rough, tortuous, highly heterogeneous networks.

The geochemical results confirm that the Yongchuan shale is a genuinely world-class source rock. Total organic carbon content ranges from 2.8 to 6.2 percent, averaging 4.9 percent, and vitrinite reflectance values between 2.65 and 3.02 percent place the organic matter firmly in the over-mature dry gas window. The rock is dominated by quartz, with contents spanning 29.0 to 72.8 percent, and contains relatively modest amounts of clay minerals, between 7.8 and 37.0 percent. Brittle minerals such as quartz and feldspar average 60.5 percent of the whole rock, yielding a brittleness index that ranges from 55.4 to 89.0 percent. In practical terms, this is a stiff, glassy shale that fractures readily under stress, which is precisely the property that allows tectonic forces to leave such a deep imprint on its internal architecture.

Under the scanning electron microscope, the contrast between weakly and strongly deformed areas is dramatic. In the syncline areas, where deformation is minimal, the pore system is dominated by honeycomb-like organic matter pores, typically a few nanometers to tens of nanometers across, with spherical and elliptical shapes and only sparse, small laminar cracks along bedding planes. In the anticlines, fault uplift areas, and clamped fault blocks, the picture changes completely. Organic pores are less abundant and often deformed or compacted, while inorganic pores, clay-mineral interlayer pores, dissolution pores, and a rich suite of microfractures take center stage. Three crack types appear: laminar cracks along bedding, abnormally high-pressure cracks opened by overpressured fluids, and tectonic cracks generated directly by structural stress. The abundance of tectonic cracks rises in step with deformation intensity, mirroring the fractures visible in the drill cores.

Quantitative measurements reinforce the visual story. Overall porosity ranges from 2.13 to 5.28 percent and permeability from 0.0002 to 0.4209 millidarcys, marking a classic low-porosity, low-permeability reservoir. Yet the ranking of structural units is revealing: the clamped fault block area shows the highest average porosity and by far the highest permeability, while the gently deformed synclines sit at the opposite end of the permeability spectrum. Nitrogen adsorption curves are type II with H3 and H4 hysteresis loops, indicating wedge-shaped, slit-like, and parallel-plate pores. Pore size distributions are double-peaked, with a dominant peak in the micropore range around 1 to 2 nanometers. In the synclines, micropores average 28.7 percent of the pore volume and macropores a mere 2.9 percent; in the anticlines, macropores balloon to an average of 17.4 percent while micropores shrink to 12.9 percent, clear evidence that compression has squeezed small pores into larger ones.

The fractal analysis delivers the study’s most elegant insight. Across all samples, the surface fractal dimension D1 ranges from 2.5528 to 2.7158 and the structural fractal dimension D2 from 2.8219 to 2.9285, with D2 consistently higher, meaning the larger seepage pores are more structurally complex than the tiny adsorption pores. The fault uplift area records the roughest surfaces and the most intricate pore networks, while the low-amplitude anticline area is the most homogeneous. Crucially, the factors controlling fractal dimension shift with deformation style. In synclines, pore complexity is largely decoupled from organic carbon and clay content, whereas in anticlines it correlates positively with both, and in fault blocks the relationships invert again. Structural deformation, in other words, does not just damage pores one by one; it rewrites the statistical rules governing the whole pore system.

These microscopic differences translate directly into gas behavior. Total gas content across the study area spans 2.57 to 7.86 cubic centimeters per gram, and the ranking by unit is striking: the low-amplitude anticline area averages 7.2 cubic centimeters per gram, more than double the syncline average of 3.51. Correlation analysis shows that gas content tracks organic carbon positively in the synclines, fault blocks, and low-amplitude anticlines, but negatively in the intense anticlines, where the authors infer that free gas escapes along open fractures. The balance between adsorbed gas held on pore surfaces and free gas stored in larger voids also flips with deformation: free gas fractions rise steadily from syncline to anticline as fractures and macropores proliferate, while adsorbed gas declines. Moderate deformation therefore boosts storage and flow, but excessive deformation opens escape routes that drain the reservoir, which the authors suggest may explain disappointing production from some anticlinal wells.

Synthesizing these observations, the team proposes a differential evolution model that reads almost like a metamorphic pathway for pore systems. As deformation intensity increases, the dominant pore type shifts from organic matter pores to an organic-clay composite pore-fracture system; macropore proportions climb, micropore proportions fall, and the gas phase migrates from adsorbed to free. The authors argue that this coupling between deformation intensity and pore evolution is not a local curiosity but a general principle, citing consistent patterns from the Qiongzhusi Formation in Yunnan, the Permian Basin in India, and the Newark Basin in the United States. For explorers, the practical message is to map deformation intensity as a primary reservoir variable and to target the sweet spot where deformation is strong enough to enhance permeability and free gas storage, but not so strong that the gas has already slipped away through fractures.

Subject of Research: Effects of differential structural deformation on pore structure, fractal characteristics, and gas enrichment in Longmaxi Formation shale reservoirs

Article Title: Influence of differential structural deformation on pore structure and fractal characteristics of shale reservoirs: a case study of the Longmaxi Formation in the Yongchuan area, South Sichuan Basin

Article References: Dong, X., Zhang, N., Wang, B., Wang, T., Fu, M., & Ouyang, J. (2026). Influence of differential structural deformation on pore structure and fractal characteristics of shale reservoirs: a case study of the Longmaxi Formation in the Yongchuan area, South Sichuan Basin. Frontiers in Earth Science, 14, Article 1901814. https://doi.org/10.3389/feart.2026.1901814

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1901814

Keywords: shale gas, Longmaxi Formation, Sichuan Basin, structural deformation, pore structure, fractal dimension, nitrogen adsorption, scanning electron microscopy, organic matter pores, microfractures, gas content, reservoir characterization

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 Borui Lu, Yue Wang, Peiyao Yu, Mingzhe An, Ziruo Yu, and Yu Zhang, investigated L-(+)-tartaric acid at dosages of 0, 0.10, 0.15, and 0.20 percent of cement mass, holding the water-to-cement ratio fixed at 0.40 and using a fixed polycarboxylate superplasticizer dosage of 0.02 percent to keep the admixture background constant. They paired two parallel series: aggregate-free cement pastes for setting time, thermogravimetric analysis, X-ray diffraction, and electron microscopy, and one-to-three cement-to-sand mortars for fluidity, mercury intrusion porosimetry, and mechanical testing. This paired design allowed the same chemical perturbation to be followed from paste-scale hydration chemistry through to mortar-scale pore structure and strength. Specimens were cured at 20 degrees Celsius and above 95 percent relative humidity and examined at 7, 28, and 56 days, giving a cross-age picture that most earlier tartaric acid studies, which focused on early hydration alone, did not attempt.

The fresh-state results show a textbook dosage response. Both initial and final setting times increased continuously as the tartaric acid dose rose, with the delay becoming steep at the higher dosages. Mortar fluidity followed the same pattern: the flow-table spread grew from 214 millimeters for the control mix to 242 millimeters at 0.20 percent, an increase of about 13.1 percent under the fixed superplasticizer dosage. Because cement setting occurs when hydration products connect neighboring particles into a load-bearing network, the measured delay points directly to a longer period before that network reaches percolation. The authors interpret the combined setting and flow data as progressively stronger early-age retardation, consistent with a molecular picture in which tartrate ions coordinate calcium in solution and adsorb on clinker and hydrate surfaces, modifying dissolution and the conditions required for calcium silicate hydrate nucleation.

Quantitative X-ray diffraction with Rietveld refinement, using a 50 percent corundum internal standard and a common constrained five-phase protocol across all twelve datasets, provided an independent marker of the same trend. At 7 days, residual C3S, the reactive alite phase that drives early strength, rose systematically from 13.45 percent in the control paste to 17.33, 19.53, and 20.35 percent at the 0.10, 0.15, and 0.20 percent dosages respectively. Residual beta-C2S showed a much weaker dosage trend, indicating that the retardation acts primarily on alite. By later ages, residual C3S declined in all mixtures, demonstrating that silicate consumption continued after the strongly retarded early stage rather than being permanently suppressed. This distinction between delayed and diminished hydration is central to the study’s argument that retardation can be a benefit rather than a penalty.

Portlandite, the crystalline calcium hydroxide that cement hydration produces in abundance, served as a second chemical record. Thermogravimetric analysis, quantifying portlandite from the dehydroxylation mass loss between 400 and 550 degrees Celsius, showed contents falling monotonically at 7 days from 18.75 percent in the control to 18.01, 16.26, and 14.43 percent with increasing dosage. Yet by 56 days the picture had inverted: the 0.10 and 0.15 percent pastes reached 22.58 and 23.12 percent portlandite, exceeding the control’s 21.12 percent. The quantitative XRD data mirrored this early suppression and later recovery, and the agreement between the two independent techniques at 7 days provides a cross-method consistency check. The divergence in absolute values at later ages reflects the different analytical signals involved, mass loss versus diffraction intensity, and the authors interpret each dataset within its own measurement basis.

Scanning electron microscopy with energy-dispersive X-ray spectroscopy added local chemical evidence for the microstructural story. A representative elongated plate-like crystal in the 0.15 percent paste at 7 days gave a calcium-rich composition with a calcium-to-silicon atomic ratio of about 3.55 and no detectable aluminum or sulfur, a signature compatible with portlandite once the silicon contribution from adjacent silicate hydrate within the interaction volume is considered. Across all dosages and ages, similar plate-like crystals appeared with dosage- and age-dependent changes in size, abundance, and spatial arrangement. This matters because portlandite habit is known to be sensitive to the chemical environment of growth: organic molecules can alter crystal number, size, and growth pattern, and dissolved ions can shift the relative stability of specific crystal faces. The tartaric acid environment thus appears to leave a legible fingerprint on the crystal architecture of the hardened paste.

The pore network of the corresponding mortars evolved in step with the phase history. Mercury intrusion porosimetry, interpreted in terms of pore-entry or threshold dimensions rather than true pore sizes, showed that for the 0.15 percent mortar, cumulative intrusion at the smallest displayed entry diameter of about 11 nanometers fell from 0.0395 milliliters per gram at 7 days to 0.0314 milliliters per gram at 56 days, a deficit relative to the control that widened from 9.6 percent to 22.2 percent. The principal differential peaks stayed within a narrow 32 to 40 nanometer band throughout curing, meaning the characteristic entry scale was stable while the accessible volume changed most. By 56 days the 0.15 percent mixture had the lowest cumulative intrusion of all four mortars, linking the sustained post-retardation hydration documented in the pastes to a progressively denser, less accessible pore network in the mortars.

Strength development crowned the sequence. Both flexural and compressive strengths increased as the dosage rose from zero to 0.15 percent and then declined at 0.20 percent, making the 0.15 percent mix the best performer at every age. Relative to the control, its flexural strength was higher by 20.0, 14.6, and 9.6 percent at 7, 28, and 56 days, while compressive strength exceeded the control by 33.7, 16.0, and 11.7 percent at the same ages. The advantage was largest at 7 days and narrowed as the control caught up in hydration, consistent with the established understanding that strength in hydrated cement is governed jointly by porosity, pore structure, and degree of hydration. Notably, the 0.20 percent mixture, despite showing the lowest early cumulative intrusion at 7 and 28 days, did not match the 0.15 percent strength, suggesting that excessive retardation leaves the hydration degree too far behind for pore refinement alone to compensate.

The study also raises an intriguing stereochemical question. Previous work on brushite bioceramics and calcium sulfate cements found that L-(+)-tartaric acid and other L-enantiomeric alpha-hydroxycarboxylic acids regulated crystal growth and enhanced mechanical performance more strongly than their D counterparts, identifying molecular handedness as a possible control variable in calcium-mineral crystallization. The present dataset characterizes only the L stereoisomer in ordinary Portland cement, so matched comparisons of L-, D-, meso-, and racemic tartaric acid systems remain an open experimental frontier. The authors likewise note that the portlandite observations connect plausibly to carbonation behavior, since both the amount and spatial form of calcium hydroxide influence how carbonation progresses, motivating future measurements of carbonation depth and dimensional change.

For now, the practical takeaway is a carefully bounded one. At a water-to-cement ratio of 0.40, 0.15 percent L-(+)-tartaric acid best balanced early hydration control with later hardened-state development, combining pronounced setting delay and improved flow with continued silicate consumption, the lowest late-age accessible pore volume, and the highest measured strengths. Because tartaric acid’s retarding action is strongly concentration-dependent, and because dosage expressed per cement mass translates into different effective concentrations in the mixing water at different water-to-cement ratios, the optimum window is expected to shift with mixture design. The authors caution that verification across other ratios and cement chemistries is needed before broader dosage recommendations can be made. Even so, the work demonstrates that a modest, wine-cellar-simple molecule, dosed with precision, can orchestrate the entire arc of cement hydration from first stiffening to final strength.

Subject of Research: Effects of L-(+)-tartaric acid dosage on hydration, microstructure, and strength development of Portland cementitious materials

Article Title: Hydration regulation, microstructural evolution and strength development of cementitious materials modified by L-(+)-tartaric acid

Article References: Lu, B., Wang, Y., Yu, P., An, M., Yu, Z., & Zhang, Y. (2026). Hydration regulation, microstructural evolution and strength development of cementitious materials modified by L-(+)-tartaric acid. Case Studies in Construction Materials, 25, Article e06609. https://doi.org/10.1016/j.cscm.2026.e06609

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06609

Keywords: cement hydration, tartaric acid, retarding admixture, portlandite, Rietveld QXRD, thermogravimetric analysis, mercury intrusion porosimetry, compressive strength, microstructure, C3S, pore structure, construction materials

The research team, led by Qiaoyun Wu, Zhifeng Xu, Xuyong Chen, and Shukai Cheng, set out to resolve a long-standing trade-off. Lightweight aggregate concrete reduces structural dead load, improves thermal insulation, and lowers seismic forces, which is why it appeals to builders of high-rises, long-span bridges, and coastal infrastructure. But lightweight aggregates are inherently weaker and more porous than natural sand and stone, so piling them into a mix eventually erodes mechanical performance. Earlier work on ultra-high-performance concrete showed that strength first rises and then falls as lightweight aggregate content increases, peaking at moderate replacement levels. The question was whether the superior chemistry of LC3 could compensate for higher doses of a weak, porous aggregate, and whether pre-soaking that aggregate could tip the balance further.

LC3 itself is one of the most promising low-carbon binders in cement science. It blends ordinary Portland cement with calcined clay and limestone powder, typically replacing 30 to 50 percent of the clinker. That substitution cuts carbon dioxide emissions by roughly 30 percent, from about 834 kilograms of CO2 per tonne of cement down to about 556 kilograms, while delivering 28-day strengths comparable to ordinary Portland cement. The magic lies in a synergistic reaction: aluminate species released by the calcined clay react with carbonate ions from the limestone to form carboaluminate hydrates, known as hemicarbonate and monocarbonate phases, alongside additional calcium-aluminosilicate gel. These phases fill pores, consume portlandite, and refine the microscopic architecture of the hardened paste.

Shale ceramsite, the second ingredient, is an expanded, sintered clay aggregate with an apparent density of 1720 kilograms per cubic meter and a water absorption capacity of 5.3 percent. Its porous interior can soak up water and release it later, a property the researchers exploited through internal curing. In the experiments, the ceramsite was either used dry or fully pre-wetted to its saturated surface-dry condition. The pre-wetting water was carefully subtracted from the mixing water so that every mixture shared the same effective water-to-binder ratio of 0.4, ensuring that any performance differences could be attributed to the internal reservoir effect rather than to extra free water. The sand-to-binder ratio was varied from 1.1 to 1.5, and the binder was held constant at 420 parts cement, 120 parts metakaolin, and 60 parts limestone powder per cubic meter.

The mechanical results were striking. All mixtures weighed in below 1700 kilograms per cubic meter when oven-dried, comfortably satisfying the lightweight mortar criterion of 2000 kilograms per cubic meter under EN 206-1. As expected, increasing the aggregate content reduced both density and strength. But the pre-wetted series consistently outperformed its dry-aggregate twin at 7 and 28 days, with the best formulation showing a 10.8 percent gain in 28-day compressive strength at the lowest aggregate ratio. Flexural strength told a subtler story: it fell by 25 to 35 percent as ceramsite content rose, a steeper decline than the 17 to 18 percent seen in compression. The porous particles act as preferential crack initiation sites under tension, while under compression the matrix can redistribute load around the softer inclusions. Pre-wetting strengthens the interfacial transition zone through enhanced local hydration but cannot fully erase the aggregate’s intrinsic weakness.

Perhaps the most commercially compelling number is specific strength, the ratio of compressive strength to density. Across a 36 percent increase in ceramsite dosage, the specific strength of both series remained remarkably stable, and the optimal pre-wetted mixes exceeded 50 megapascals with specific strengths above 0.03 megapascals per unit density. Conventional C50 concrete manages only 0.017 to 0.021, meaning the new material delivers roughly 1.7 to 2.0 times the strength per unit weight. That efficiency translates directly into slimmer columns, lighter foundations, and smaller seismic inertia forces, savings that compound across an entire structure.

Durability results were even more dramatic. In capillary water absorption tests, the pre-wetted mixes absorbed 64 to 67 percent less water over 150 hours than their dry-aggregate counterparts at identical aggregate ratios. Electric flux measurements, a standard proxy for chloride penetrability, told the same story: the pre-wetted mortars showed 59 to 80 percent lower flux, and the best mix registered below 100 coulombs, placing it in the very low permeability class, a level normally associated with high-performance concrete. For marine and coastal construction, where chloride-induced rebar corrosion is the dominant failure mode, that classification carries real practical weight.

The microstructural evidence explains why. X-ray diffraction revealed stronger hemicarbonate and monocarbonate peaks in the pre-wetted samples, confirming that the sustained internal water supply accelerates the signature LC3 reaction between metakaolin-derived aluminate and limestone carbonate. Thermogravimetric analysis quantified the shift: the best pre-wetted mix contained more carboaluminate hydrates and more calcium-aluminosilicate gel while consuming portlandite, converting a relatively inert crystalline byproduct into pore-filling binding phases. Non-evaporable water content, a proxy for hydration degree, was consistently higher in the pre-wetted series, and scanning electron microscopy showed fewer unhydrated clinker grains, fewer portlandite crystals, and a denser, crack-free matrix.

Mercury intrusion porosimetry completed the picture. The pre-wetted mortar with the lowest aggregate ratio achieved the highest gel pore fraction, 47.2 percent, and the lowest macroporosity, just 3.1 percent, with the pore size distribution shifting decisively toward fine capillary and gel pores. Because capillary water uptake is governed by large capillary pores while chloride migration depends on the connectivity of the whole network, this refinement suppresses both transport mechanisms simultaneously. The authors are careful to note that electric flux reflects pore solution chemistry as well as pore structure, and they recommend confirmation through long-term chloride migration and diffusion testing, along with studies of carbonation, freeze-thaw, and sulfate resistance.

The sustainability ledger is nuanced but favorable. Sintering ceramsite consumes about 0.534 gigajoules per tonne, nearly eight times the energy of natural sand, because the process requires kilns running at 1100 to 1250 degrees Celsius. Yet the aggregate can be made from industrial wastes such as fly ash and coal gangue, its lightness shrinks foundation and reinforcement demands, its insulating blocks cut building heating and cooling loads, and its low bulk density reduces transport emissions per cubic meter. Weighed over a full life cycle, the authors argue, these operational and structural savings outweigh the production penalty. Combined with LC3’s 30 percent carbon advantage at the binder stage, the system points toward a generation of lightweight, high-strength, chloride-resistant concrete suited to the decarbonized, resilient infrastructure the coming decades demand.

Subject of Research: Internal curing and phase assembly in limestone calcined clay cement mortar with pre-wetted shale ceramsite

Article Title: Synergistic internal curing and phase assembly in limestone calcined clay cement mortar incorporating pre-wetted shale ceramsite: Mechanical properties, chloride resistance, and pore structure refinement

Article References: Wu, Q., Xu, Z., Chen, X., & Cheng, S. (2026). Synergistic internal curing and phase assembly in limestone calcined clay cement mortar incorporating pre-wetted shale ceramsite: Mechanical properties, chloride resistance, and pore structure refinement. Case Studies in Construction Materials, 25, Article e06607. https://doi.org/10.1016/j.cscm.2026.e06607

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06607

Keywords: limestone calcined clay cement, LC3, shale ceramsite, lightweight aggregate concrete, internal curing, chloride resistance, pore structure, carboaluminate hydrates, sustainable construction, compressive strength, capillary absorption, low-carbon cement

The material at the center of the study is called self-protected underwater mortar, or SPUM. Rather than loading the mortar with viscosity-modifying chemicals, the method relies on an underwater protective agent, a water-soluble polymer dissolved in the surrounding water at a concentration of 0.5 percent by mass. The polymer, a linear ionic macromolecule with a weight-average molecular weight of at least 7.5 million, accumulates at the boundary between the fresh paste and the water, forming a stable protective film. This layer suppresses the dispersion of cementitious material into the water without increasing the viscosity of the mortar itself, which means the mix can still flow freely into place. Earlier work has shown that this approach can retain around 90 percent of its compressive strength when cast in seawater compared with dry-land casting, an impressive figure for underwater construction.

The motivation for the new study comes from one of the most demanding environments on Earth: the Qinghai-Tibet Plateau. Hydraulic structures there are often cast underwater and then spend their service lives in water-level fluctuation zones, where seasonal freezing and thawing batter the material year after year. To make matters worse, the plateau is dotted with saline lakes whose surface and groundwater carry high concentrations of sulfate ions. Sulfate attack is a notorious durability problem because sulfate ions react with cement hydration products to form expansive compounds such as ettringite and gypsum, generating internal stresses that crack the material from within. When sulfate attack and freeze-thaw cycles act together, previous research has shown they can accelerate damage far faster than either agent alone.

To probe these effects, the team prepared three versions of the same mortar mix, which used ordinary Portland cement partially replaced with 10 percent fly ash and 30 percent tuff powder, a locally available mineral admixture, along with a polycarboxylate superplasticizer. The first batch, designated SPUM-LE, was cast and cured in a conventional land environment. The second, SPUM-TW, was cast and cured in water containing 0.5 percent of the protective polymer. The third, SPUM-SS, was cast and cured in water containing both the polymer and 5 percent sodium sulfate, and was later exposed to freeze-thaw cycling in the same sulfate solution. The researchers measured slump flow, compressive and flexural strength at 3, 7 and 28 days, and then subjected all specimens to 100 rapid freeze-thaw cycles spanning temperatures from minus 18 to 5 degrees Celsius, tracking mass loss and relative dynamic elastic modulus along the way.

The rheology results showed immediately how much the surrounding medium matters. The land-cast mortar spread the farthest and fastest, while the version cast in the polymer solution lost 11.6 percent of its slump flow and needed 66.8 percent longer to reach a 200-millimeter spread, largely because the viscous solution and added buoyancy resist the flow of fresh mortar. The sulfate-polymer bath was harsher still: slump flow dropped by 33.8 percent and the spread time ballooned by 772.5 percent, a consequence of the denser liquid further suppressing the driving force for spreading. These numbers matter because flowability is the entire selling point of the self-protection concept, and the study shows that the chemistry of the water being poured into can quietly erode that advantage.

Strength development told a more subtle story. At 3 days, the underwater specimens were dramatically weaker than their land-cast counterpart, with the polymer-only batch reaching just 8.73 megapascals against 19.22 megapascals on land. The sulfate batch fared slightly better early on, apparently because sulfate ions initially accelerated hydration. But that early boost did not last. By 28 days, the land mortar reached 34.50 megapascals, the polymer-only batch 27.00 megapascals, and the sulfate batch only 23.40 megapascals, giving underwater-to-land compressive strength ratios of 0.80 and 0.60 respectively. The explanation lies at the interface: the polymer film that protects the mortar from washout also slows the transport of water to cement particles, retarding hydration, while accumulating gypsum and other sulfate products coat unhydrated cement and further impede the reaction. Flexural strength proved even more sensitive, with the sulfate-exposed batch actually declining from 6.29 to 5.55 megapascals between 7 and 28 days.

Freeze-thaw performance split the specimens in an unexpected way. The land-cast mortar held up best, losing only 1.29 percent of its mass and retaining 84.61 percent of its dynamic elastic modulus after 100 cycles. The polymer-only underwater mortar suffered the worst surface damage, shedding 5.30 percent of its mass as scaling and particle detachment, while still retaining 76.09 percent of its modulus. The sulfate-exposed batch showed the opposite pattern: modest mass loss of 3.99 percent, partly masked by white sulfate deposits accumulating on the surface, but the steepest internal degradation, with its dynamic elastic modulus falling to just 67.92 percent. In other words, one material fell apart from the outside in, while the other crumbled from the inside out. Sulfate ions penetrating the specimen during cycling formed expansive products that drove internal microcracking even as deposited crystals compensated for surface mass loss.

Microscopy and porosity measurements explained why. Scanning electron microscopy revealed dense, continuous matrices in the land-cured specimens but looser structures with visible micropores in the underwater batches, and abundant needle-like sulfate products in the sulfate-exposed material at 28 days. Mercury intrusion porosimetry added a twist: after 100 freeze-thaw cycles, the underwater specimens actually showed lower total porosity and fewer large harmful pores than the land-cast material, because continued hydration filled their more open pore systems. Yet they still performed worse. The lesson is that pore structure measured after the fact can be misleading; what governs freeze-thaw durability is the initial pore structure before cycling and the cumulative damage that accumulates within it. A loose starting framework gives hydration products plenty of room to fill, but that same looseness lets ice crystals and expansive reactions inflict damage that no amount of later pore-filling can undo.

To turn these observations into something engineers can use, the team fitted their damage data to a Weibull statistical model, using both mass loss and dynamic elastic modulus as damage indicators. The fits were strong, with coefficients of determination reaching 0.99 for the modulus-based model. Converting laboratory cycles into field years using an acceleration factor of 12 and an estimated 100 natural freeze-thaw cycles per year in the Lhasa region, the model predicted service lives of roughly 23.7 years for the land-cast mortar, 10.9 years for the polymer-only underwater mortar and 13.3 years for the sulfate-exposed batch under the more conservative mass-loss criterion. The authors caution that these figures are engineering estimates, since laboratory acceleration can never perfectly reproduce field conditions, and they note that the coupled nature of casting, curing and freezing makes it difficult to isolate the contribution of each stage.

The broader significance of the work is twofold. For engineers planning hydropower and hydraulic projects in cold, chemically aggressive regions, it provides hard numbers on a construction method that could otherwise be adopted on faith, showing that self-protected underwater mortar can genuinely work but that its durability ceiling depends heavily on the water it is born into. For materials science more broadly, it is a vivid demonstration that durability cannot be judged from a single snapshot of microstructure. A material that looks denser after testing may in fact be more damaged than one that looks coarser, because the true story lives in the sequence of events, the initial defects, the migrating ions and the accumulating cracks, rather than in the final photograph.

Subject of Research: Freeze-thaw durability and sulfate resistance of self-protected underwater mortar cast with an underwater protective agent

Article Title: Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar

Article References: Zhang, J., Liu, Z., Shen, D., Lang, T., Li, Y., Sui, H., & An, X. (2026). Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar. Case Studies in Construction Materials, 25, Article e06601. https://doi.org/10.1016/j.cscm.2026.e06601

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06601

Keywords: self-protected underwater mortar, underwater protective agent, freeze-thaw cycles, sulfate attack, concrete durability, Qinghai-Tibet Plateau, cement hydration, pore structure, Weibull damage model, hydraulic engineering, anti-washout, service life prediction

The researchers, led by Li Gong and Xiaoxiao Liu, prepared a family of C50-grade concretes in which desert sand replaced river sand at levels of 0, 20, 40, 60, 80, and 100 percent by mass. Rather than Portland cement, the binder consisted of ground granulated blast-furnace slag and fly ash—industrial by-products activated by a sodium hydroxide and sodium silicate solution. This alkali-activated chemistry sidesteps the carbon-intensive clinker production of ordinary cement while valorizing waste streams that would otherwise be landfilled. The water-to-binder ratio, binder composition, and activator dosage were held constant, making desert sand content the sole experimental variable.

The desert sand itself is a peculiar ingredient. Its particles are smaller, smoother, and more rounded than river sand, with a fineness modulus of just 0.34 compared with 2.78 for the river sand—a sign of extremely fine, poorly graded grains. It also absorbs nearly twice as much water. On paper, these traits look like liabilities, and at high replacement levels they are. But at moderate doses, the fine desert grains slip into the gaps between coarser river sand particles, tightening the packing of the aggregate skeleton in much the same way a well-shuffled mixture of ball bearings and marbles leaves less void space than either alone.

The sweet spot turned out to be 40 percent. The DS40 mix, as the team labeled it, achieved a 28-day compressive strength of 58.77 megapascals and a 91-day strength of 71.25 megapascals—the highest of all six formulations at every age tested. Specimens from this mix also failed more gracefully under compression, developing inclined cracks rather than shattering into fragments, a signature of a dense matrix with strong aggregate-paste bonding and uniform stress transfer. Mixes with 80 or 100 percent desert sand, by contrast, showed irregular failure modes and large-area spalling, evidence that too many fine particles inflate water demand, spawn extra pores, and weaken the interfaces between paste and aggregate.

Workability told the same story. Slump rose steadily from 164 millimeters in the all-river-sand mix to 182 millimeters at 40 percent replacement, then collapsed to just 87 millimeters when desert sand reached 100 percent. The filling effect of the fine grains initially lubricates and homogenizes the fresh mixture, but beyond the optimum the exploding surface area of fine particles soaks up mixing water and starves the paste. Notably, no mix showed bleeding, confirming that the chosen water-to-binder ratio of 0.42 was adequate across the entire replacement range.

The durability trials were the heart of the study. Prismatic specimens were saturated in water and subjected to up to 200 accelerated freeze-thaw cycles, each consisting of 2.5 hours at minus 18 degrees Celsius followed by 1.5 hours at 5 degrees. After 200 cycles, the DS40 mix had lost only 0.23 percent of its mass and retained 94.56 percent of its relative dynamic elastic modulus, the smallest stiffness decline of any formulation. The fully desert-sand mix fared worst, losing 0.38 percent of its mass and nearly 9 percent of its stiffness. Surface damage followed the same nonlinear pattern: the 40 percent blend kept relatively intact faces while the high-desert mixes developed flaky spalling and exposed aggregate.

Peering inside with nuclear magnetic resonance, scanning electron microscopy, and X-ray diffraction, the researchers traced the damage mechanism in detail. Freeze-thaw cycling coarsened the pore network, shifting pores from tiny gel pores toward capillary pores and macropores, and gradually connecting isolated voids into crack networks. Total porosity rose only modestly—from roughly 1.16 to 1.40 percent in the best mix—but the redistribution toward larger, connected pores proved far more consequential than the raw volume change, because it opened highways for moisture migration and concentrated frost-heave stresses at weak interfaces. Statistical analysis reinforced the link: after 200 cycles, porosity correlated strongly with mass loss and inversely with late-age compressive strength, while mass loss and stiffness decay were almost perfectly coupled.

To translate laboratory cycling into real-world longevity, the team fitted a Weibull reliability model to the stiffness-decay data, defining failure as the point where dynamic modulus falls below 60 percent of its initial value. Converting predicted failure cycles to years using an empirical coefficient of 12 and an annual freeze-thaw count of 118 cycles—typical of the alpine cold regions of Northwest China—the DS40 mix yielded an equivalent service life of 113.3 years, with a 95 percent confidence interval stretching from 89.5 to 141.5 years. The all-river-sand control predicted 99.1 years, while the fully desert-sand mix trailed at 75.4 years. The authors are careful to stress that these figures are model-dependent extrapolations beyond the tested 200-cycle range, intended for comparing mixes rather than certifying actual structures.

That caution is well placed. Sensitivity analyses showed the predictions swing substantially with the assumed conversion coefficient and failure threshold: raising the coefficient from 8 to 16 stretched the DS40 estimate from 75.5 to 151.1 years, while tightening the failure criterion shaved its predicted life from 130.6 to 97.2 years. Crucially, however, the ranking of the mixes held steady under every scenario, with the 40 percent blend always on top and the fully desert-sand mix always at the bottom. The framework, in other words, is far more trustworthy as a comparative tool than as a crystal ball.

The broader implications are striking. Deserts hold theoretical sand reserves of roughly six million cubic kilometers, an essentially untapped aggregate bank, and many countries have already restricted river sand mining to protect ecosystems. This study suggests that desert sand need not be an all-or-nothing proposition: blending it at around 40 percent into alkali-activated slag-fly ash concrete simultaneously improves particle packing, boosts strength, and extends frost durability, all while cutting reliance on both virgin cement and river sand. The authors acknowledge that salt freezing, carbonation, drying shrinkage, and sustained loads—none captured by clean-water freeze-thaw testing—still need investigation, and that the gel chemistry of the alkali-activated matrix warrants verification with additional spectroscopic techniques. But as a proof of concept, the work transforms a ubiquitous desert nuisance into a measured, modeled, and surprisingly durable building material.

Subject of Research: Freeze-thaw durability and service-life prediction of alkali-activated concrete made with desert sand

Article Title: Study on freeze–thaw resistance and equivalent service-life prediction of alkali-activated desert sand concrete

Article References: Gong, L., Liu, X., Bu, Y., Yang, T., Hu, H., & Zhang, H. (2026). Study on freeze–thaw resistance and equivalent service-life prediction of alkali-activated desert sand concrete. Case Studies in Construction Materials, 25, Article e06606. https://doi.org/10.1016/j.cscm.2026.e06606

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06606

Keywords: desert sand concrete, alkali-activated materials, freeze-thaw resistance, Weibull service-life prediction, relative dynamic elastic modulus, pore structure, ground granulated blast-furnace slag, fly ash, sustainable construction, river sand substitution, cold-region durability, Tengger Desert

LECA is a manufactured aggregate produced by expanding clay at high temperature, yielding hard, spherical granules riddled with internal voids. Because it weighs far less than natural sand and gravel, it is attractive for lightweight concrete, which reduces structural dead load and improves thermal and acoustic insulation while lowering cost. The catch is well documented: previous studies found that replacing natural aggregate with LECA can significantly weaken concrete. Corinaldesi and Moriconi reported compressive strength losses of roughly 18 to 28 percent when natural sand was replaced with LECA in self-compacting concrete, and Youm and colleagues found that coarse LECA reduced splitting tensile strength, compressive strength, and elastic modulus by up to nearly 48 percent. Polypropylene fibers, meanwhile, are known to stitch microcracks together and improve toughness, but above roughly 0.3 percent by weight of cement they tend to clump and ball during mixing, degrading the mix rather than improving it.

The Iranian research team set out to explore the underexplored middle ground: what happens when a modest 10 percent of sand is replaced by volume with LECA and a modest 0.2 percent by volume of polypropylene fibers is added, individually and in combination? They prepared four mortar mixes with a constant cement content of 670 kilograms per cubic meter and a water content of 302 kilograms per cubic meter: a plain control mix (P0L0), a mix with 10 percent LECA (P0L10), a mix with 0.2 percent fibers (P0.2L0), and a hybrid containing both (P0.2L10). The LECA had dry and saturated surface-dry densities of 1542 and 1704 kilograms per cubic meter with 5.0 percent water absorption, while the fibers were 12 millimeters long, 19 micrometers in diameter, with a tensile strength of 400 megapascals. Fibers were added gradually in several steps to ensure uniform dispersion, and cube and prismatic specimens were water-cured and tested at 3, 7, 14, and 28 days.

The compressive strength results reveal a striking early-age reversal. Replacing sand with LECA alone cut the 3-day strength from 19.20 to 15.87 megapascals, a drop of about 17.4 percent, as expected from the porous, weaker aggregate. Yet by 14 and 28 days the LECA mix had overtaken the control, with gains of 23.8 and 18.2 percent respectively. The authors attribute this to LECA’s internal water reservoir effect: the porous aggregate absorbs mixing water early and gradually releases it during hydration, densifying the cement matrix and strengthening the bond between paste and aggregate over time. Fibers alone had little effect at 3 days but raised 7-day strength by about 26.9 percent, from 24.27 to 30.80 megapascals, by bridging and restraining early microcracks during setting.

The hybrid mix delivered the best of both worlds early on. Combining LECA and fibers lifted 3-day compressive strength by roughly 23 percent, to 23.60 megapascals, and 7-day strength by about 25 percent, to 30.27 megapascals, compared with the control. At 28 days, however, the benefit largely evaporated: the hybrid reached only 32.80 megapascals, a marginal 2 percent gain over the control and slightly below the LECA-only mix. The researchers explain that by later ages the fiber-induced voids and weak fiber–matrix interfaces begin to offset LECA’s hydration benefits. Interestingly, the standard logarithmic strength-versus-time model fit the fiber-containing mixes poorly, with a bi-linear relationship describing their development better.

Flexural strength told an even more compelling story. LECA alone reduced early flexural strength, dropping the 3-day value from 6.43 to 4.70 megapascals and the 7-day value by about 16.5 percent, because its porous structure weakens the aggregate–paste interface. Fibers alone slightly hurt 3-day flexural strength but boosted it substantially from 7 to 28 days, reaching 11.69 megapascals at 28 days, thanks to their crack-bridging action under bending stresses. The hybrid mix, however, produced the highest flexural strength at every single age tested: 6.74, 7.95, 9.12, and 10.75 megapascals at 3, 7, 14, and 28 days respectively. Continued hydration refined the matrix while the fibers delayed crack propagation, a synergy that proved most valuable precisely where concrete is most vulnerable—under tension at early ages.

Durability was probed with two complementary techniques. In the Rapid Chloride Migration Test, performed according to AASHTO TP64 on 28-day cylinders, a 60-volt direct field drives chloride ions from a sodium chloride solution into the specimen for about 18 hours, after which silver nitrate spraying reveals the penetration depth. The control mix showed a migration rate of 0.0123 millimeters per volt-hour. Adding 10 percent LECA reduced this by about 4.9 percent to 0.0117, an improvement the authors link to increased tortuosity of transport pathways. Fibers alone left migration essentially unchanged, and the hybrid mix measured 0.0125, only marginally above the control. Electrical conductivity, measured with an alternating-current system to avoid polarization errors, dropped noticeably in the LECA mixes, from 0.0370 to 0.0339 per ohm-meter, confirming that the lightweight aggregate interrupts the continuous capillary pore network that normally serves as the highway for ion movement.

Scanning electron microscopy helped explain why apparent porosity and actual transport do not always go hand in hand. Images of LECA-containing specimens revealed abundant spherical voids up to 188 micrometers across, and in the hybrid mix up to 395 micrometers, yet these belong largely to the sealed interior of the LECA particles themselves, whose impervious outer shell and dense interfacial transition zone keep them out of the connected pore network. Fluid and ion transport occurs mainly through the fine capillary pores of the cement paste, and LECA particles interrupt that network, forcing ions onto longer, more winding paths. The microscopy also exposed a subtle trade-off with the fibers: hydrophobic polypropylene bonds poorly to the hydrophilic cement matrix, creating micro-gaps of roughly 15 to 30 micrometers around the filaments and at the LECA interface. These gaps do not connect into continuous channels, but under the sustained 60-volt field of the migration test they can act as local weak zones, which is why the hybrid mix showed slightly higher chloride migration despite lower overall conductivity.

The broader significance lies in the balance achieved. Previous studies in which large amounts of lightweight aggregate weakened the matrix, or in which high fiber dosages caused clustering and non-uniform transport, suggest that more is not better. By capping LECA at 10 percent sand replacement and fibers at 0.2 percent by volume, the researchers produced a composite that reaches 32.80 megapascals in compression and 10.75 megapascals in flexure at 28 days, resists bulk ion transport by disrupting continuous capillary pathways, and weighs measurably less than the control, at 2.03 versus 1.92 grams per cubic centimeter for the LECA mixes. For infrastructure exposed to chlorides—marine structures, bridge decks, and pavements subjected to de-icing salts—such a material could extend service life while reducing structural weight. The study also delivers a methodological caution: mechanical strength alone does not capture durability, and pairing migration testing with conductivity measurements, as this team did, reveals transport behavior that strength data alone would hide. As lightweight and fiber-reinforced concretes move toward wider structural use, this kind of coupled mechanical–transport–microstructural framework may become the standard by which new mixes are judged.

Subject of Research: Mechanical and durability characterization of polypropylene fiber-reinforced mortar containing lightweight expanded clay aggregate

Article Title: RCMT, electrical conductivity, and mechanical characterization of polypropylene fiber-reinforced LECA mortar

Article References: Taghvaei, D., Behzad, P., Ghaderan, S., & Dastan Diznab, M. A. (2026). RCMT, electrical conductivity, and mechanical characterization of polypropylene fiber-reinforced LECA mortar. Results in Engineering, 32, Article 113256. https://doi.org/10.1016/j.rineng.2026.113256

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113256

Keywords: LECA, polypropylene fibers, lightweight concrete, chloride migration, electrical conductivity, compressive strength, flexural strength, durability, pore structure, SEM, cement mortar, RCMT