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: Not provided

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

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

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

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

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

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

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

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

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

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

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

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06532

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