@article{YANG2026, 
author = {Qingchun YANG and Jiaming WU and Libo YU and Zhaolin Gao and Jun JIANG and Zhengmao YE},
title = {Formation of C2S-α'L in High-Potassium Portland Cement Clinker and Its Influence on Hydration Performance},
year = {2026},
journal = {Journal of the Chinese Ceramic Society},
volume = {54},
number = {5},
pages = {1564-1575},
keywords = {high potassium silicate cement clinker, C3S/C2S-α'L ratio, mineral composition, hydration behavior},
url = {https://www.sciopen.com/article/10.14062/j.issn.0454-5648.20250335},
doi = {10.14062/j.issn.0454-5648.20250335},
abstract = {IntroductionConventional modification of high-potassium cement clinker relies on adding SO3 to reduce K2O solid solution and boost C3S content. However, SO3 causes operational issues and stabilizes low-reactive C2S-β, compromising strength. In this study, SO3 was used to stabilize highly reactive C2S-α'L as a strength-enhancing phase, while K+ could suppress C3S formation. To harness this "contradictory duality", an optimal C3S/C2S-α'L ratio that could maximize the strength contribution of C2S-α'L without significantly sacrificing C3S was established. This strategy could overcome the limitations of SO3-based methods and enable an efficient utilization of high-potassium limestone.MethodsRaw meals were formulated from analytical-grade reagents (i.e., CaCO3, SiO2, Al2O3, Fe2O3, K2CO3, CaSO4·2H2O ≥99.0%) with fixed silica modulus (SM=2.2), alumina modulus (IM=1.5), and K2O content (2%, in mass). The lime saturation factor (KH) was adjusted from 0.84 to 0.92 to modulate C3S/C2S ratios. Raw materials were wet ground in a ball mill at 600 r/min for 1.5 h, dried at 105 ℃, pelletized under 100 kN, and then step-calcined (i.e., 950 ℃/1 h → 1450 ℃/30 min). The materials were followed by rapid air-quenching to inhibit C2S-γ formation. The resulting clinker was dry ground to a fine powder with the sizes of ≤200 mesh, blended with gypsum of 3%, and cast into paste specimens (20 mm×20 mm×20 mm, w/c=0.3) for standard curing at 20 ℃ and 95% RH for 3-d/28-d ages.The chemical composition was analyzed on borate-pressed pellets with a starch binder of 10% by X-ray fluorescence spectrometry (XRF). The phase quantification was determined by X-ray diffraction (XRD)-Rietveld refinement (Bruker D8 Advance, 5°–80°, 40 kV/20 mA) via a software named TOPAS V4. 2 with March-Dollase texture correction for the key phases. The microstructural evolution was characterized by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) (gold-sputtered polished sections, 10-point EDS averaging) and optical microscopy (NH4Cl-etched surfaces). The hydration kinetics were monitored by isothermal calorimetry (TAM Air, at 20 ℃ for 72 h), while the thermal decomposition behavior was assessed by thermogravimetric analysis (TGA) (RT→550 ℃ at 10 ℃/min under N2). The amorphous content in hydration products was quantified using corundum internal standard (20%) after ethanol immersion and vacuum drying.Results and discussionThe results demonstrate that elevating the C3S/C2S-α'L ratio fundamentally restructures phase equilibria and ion distribution. As the ratio increases from 0.78 to 1.80, K+ solid solubility in C2S is decreased by ~42% (quantified via EDS point analysis), destabilizing the metastable α'L phase and triggering its transformation to C2S-β, which is evidenced by α'L content reducti on from 39.27% to 25.08% and intensified twinning striation. Concurrently, K+ migration toward interstitial phases increases the conversion from C3A-o to C3A-c, correlating with increasing C3A content (i.e., from 9.97% to 1.02%) and declining C4AF (i.e., from 8.94% to 7.39%). Based on the microstructural analysis, enlarged C3S crystals with prominent etch pits emerge at high ratios due to K+-enriched liquid phases suppressing nucleation, while promoting disordered growth. This is compounded by elevated C3A/C4AF ratios increasing melt viscosity, hindering ion diffusion. The elemental mapping confirms that K+ predominantly partitions into C2S and interstitial phases, with minimal C3S incorporation (i.e., <0.3%, in mole).The mechanical performance exhibits dual-phase characteristics. The 3-d strength is increased by ~18 MPa across the ratio gradient, driven by three synergistic factors, i.e., increased C3A hydration activity, accelerated reaction kinetics from free CaO, and enhanced C3S surface defects. The 28-d strength is declined by ~12 MPa, mechanistically linking to hydration restructuring. The result of calorimetry reveals sulfate depletion peaks advancing for 1.7 hours at higher ratios due to a rapid gypsum consumption by abundant C3A. This shifts hydration products from AFt to Hc phases, which is evidenced by the XRD quantification, where AFt content decreases from 5.44% to 1.19% at 3 d, while Hc increases from 3.94% to 6.14%. At 28 d, Hc dominates (i.e., 7.55% at the highest ratio), with AFt nearly depleted (i.e., 0.27%).The C-S-H gel morphology transitions from needle-like aggregates (i.e., Ca/Si=1.99 at a low ratio) to flaky networks (i.e., Ca/Si=1.26 at a high ratio). This structural shift reduces K+ adsorption capacity. The data establishe a Ca/Si ratio as a critical variable governing gel-K+ interactions, resolving prior discrepancies.ConclusionsAs the C3S/C2S-α'L ratio decreased, the solid solubility of K+ and Al3+ in C2S declined, whereas K+ incorporation in interstitial phases increased. This ultimately reduced the C2S-α'L/C2S-β ratio in clinker, while elevating the C3A-o/C3A-c ratio. Decreasing the ratio could significantly enlarge C3S particle size in high-potassium clinker, resulting in less distinct edges and increased etch pits. Although the fundamental morphology of C2S remained largely unchanged, its surface became rougher with pronounced twinning striations.The 3-day compressive strength of high-potassium clinker diminished at a lower C3S/C2S-α'L ratio, whereas the 28-day strength exhibited an inverse trend. As C2S-α'L competitiveness weakened, the peak heat flow and cumulative heat release in hydration curves both intensified. However, the sulfate depletion peak could emerge earlier, accompanied by increased hydrocalumite (Hc) phase content, further amplified when C3S dominated over C2S-α'L.The C-S-H gel morphology transitioned from aggregated needle-like fibers at low C3S/C2S-α'L ratios to flake-assembled network structures at high C3S/C2S-α'L ratios.}
}