Formation of C₂S-α'_L in High-Potassium Portland Cement Clinker and Its Influence on Hydration Performance

Conventional 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.

Raw 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.

The 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.

As 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.