高钾硅酸盐水泥熟料中C2S-α'L的形成及其对水化性能的影响OA

Introduction 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. Methods 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. Results and discussion 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. Conclusions 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.