Study on the composition and hydration properties at early stages of high-sodium Portland cement clinker under the synergistic effects of SO3

Cement kiln co-processing is a common carbon reduction strategy that has become increasingly popular in recent years. However, the disposal of Na2O-containing solid wastes and fuels has resulted in elevated Na2O content in clinker, leading to the generation of high-sodium Portland cement clinker. To address this issue, the addition of SO3 was investigated in this study to improve the performance of high-alkali clinker. The synergistic effects of SO3 and Na2O on the mineral composition and hydration properties of high-sodium Portland cement clinker were studied. Five clinkers with different SO3 concentrations were synthesized and characterized using various techniques, including X-ray diffraction (XRD) combined with the Rietveld method, X-ray fluorescence spectrometry (XRF), backscattered electrons-energy-dispersive spectrometry (BSE-EDS), isothermal calorimetry, laser particle size analyzer, and mechanical properties testing. The results indicate that Na2O cannot be trapped by SO3 to generate Na2SO4, and instead dissolves in clinker phases to stabilize Ca2SiO4-low temperature form of α polymorph (C2S-α’L) and Ca3Al2O6-orthorhombic (C3A-o). As the SO3 concentration increases, the content of Ca3SiO5 (C3S) decreases, while the proportion of Ca3SiO5-monoclinic-type1 (C3S-M1) increases. The amount of C3A-o remains in a constant range, with the Ca3Al2O6 (C3A) content exhibiting a declining trend. Striations in belite can be observed when the SO3 concentration reaches 0.719%. The hydration rate of high-sodium Portland cement is accelerated with the increase of SO3 content at the ages of 3 and 7 days, leading to an improvement in compressive strengths.