Influences of Crystal Seed on Formation of Calcium Sulfoaluminate Modified Low-Calcium Portland Cement Clinker
Introduction The existing cement clinker systems have some challenges.The production process of calcium sulfoaluminate(CSA)clinker,which primarily contains mineral C4A3$,requires a significant amount of high-quality bauxite resources,thus leading to a higher production cost.Compared to CSA,a high belite calcium sulfoaluminate(BCSA)cement reduces the demand for high-grade bauxite during production,but has a slower strength development over time.To ensure the synergistic development of both early age performance and strength development in clinker systems,calcium sulfoaluminate-modified Portland cement(designated as S.M.P.)is further developed.However,alite(C3S)remains a dominant mineral,resulting in relatively high carbon emissions.For the compositional and performance characteristics of S.M.P.and BCSA clinker,this study proposed a new system for preparing a low-calcium calcium sulfoaluminate-modified Portland cement clinker with belite(C2S)as a dominant mineral(i.e.,C3S/C2S-C4A3S-C4AF-CaSO4).This study also utilized minerals such as C3S and C2S from Portland cement clinker as crystal seed additives.The effect of adding Portland cement clinker on the sintering process and the mineral composition structure of the low-calcium calcium sulfoaluminate-modified Portland cement clinker was investigated.In addition,the underlying mechanisms were explored to provide a theoretical and technical guidance for the design and development of low-calcium cement clinkers.Methods The designed mineral composition of the calcium sulfoaluminate-modified low-calcium Portland cement clinker was 45％-50％(in mass,the same below)of C2S,5％-10％of C3S,25％of C4A3$,10％of C4AF,and 10％of f-CaSO4.Calcium sulfoaluminate-modified low-calcium Portland raw meals were prepared with industrial raw materials.Portland cement clinker was used as a crystal seed at different incorporation contents of 0％,1％,3％,5％,10％,and 12％.The mineral composition and microstructure of calcium sulfoaluminate-modified low-calcium Portland cement clinker were determined by in-situ high-temperature X-ray diffraction(XRD),linear shrinkage measurement,free lime titration,scanning electron microscopy(SEM)and thermal analysis.Results and Discussion The linear shrinkage of the samples generally increases gradually with the increase in clinker addition percentage from 0％to 10％.The linear shrinkage remains relatively unchanged at＞12％of clinker additions.The XRD patterns of the synthesized clinker samples within the 20° to 51° range reveal that the diffraction peak intensity of β-C2S gradually increases,while the intensity of α′-C2S and C4A3$ peaks decreases,and no significant diffraction peaks for C3S appear as the clinker crystal seed content increases.The analysis by rietveld whole-pattern fitting indicates that the addition of Portland cement clinker seeds does not lead to a significant increase in C3S content.There is a notable increase in C2S content and a gradual decrease in C4A3$ content,accompanied by a relative increase in the iron phase content.The high-temperature in-situ XRD patterns indicate that at 950℃,an original C3S diffraction peak disappears,instead of relatively weak C2S diffraction peaks at approximately 27.5° and 38.0°.This indicates that C3S decomposes into C2S and CaO at 950℃.According to the analysis by backscattered electron-energy dispersive spectroscopy(BSE-EDS),the solid solubility of aluminum,sulfur and iron in C2S increases with increasing the addition of clinker crystal seeds,thus favoring the stability of β-C2S.Furthermore,the presence of sulfur impedes a reaction between C2S and CaO,thus inhibiting the formation of C3S.An increase in the iron phase content and the Al/Fe ratio within this phase,coupled with enhanced Al solubility in C2S,reduces the amount of Al available for sulfate reaction.Conclusions The incorporation of Portand cement clinker seeds facilitated the formation of liquid phases,favoring the liquid-phase sintering of clinker and enhancing the densification of the microstructure of the synthesized clinker samples.This incorporation also increased the grain sizes of C2S and C4A3$.The introduction of clinker seeds lowered the decomposition temperature of CaCO3,promoting the formation of C2S and C4AF.Consequently,the contents of C2S and the iron phase increased sulfur-aluminous modified low-calcium Portand cement clinker.However,the decomposition of C3S in clinker crystal seeds at lower temperatures prevented an effective promotion of C3S formation alongside C4A3$.The content of the iron phase and the Al/Fe ratio increased with increasing the addition of clinker crystal seeds.Increased Al solubility in C2S led to a reduction in the Al available for sulfate reactions,thereby decreasing the formation of C4A3$.This effect was particularly pronounced when the clinker crystal seed content reached 10％,resulting in a significant reduction in C4A3$ content.The inclusion of Portland cement clinker seeds led to a decrease in the content of highly reactive α′-C2S and c-C4A3$ in the clinker.As the clinker crystal seed content increased,this reduction becomes dominant.This was primarily due to an increase in the calcium-to-silicon ratio in C2S and an increased dissolution of Al,Fe,and S in C2S,stabilizing β-C2S but destabilizing α′-C2S,and was not conducive to C3S formation.Concurrently,a decreased Fe dissolution in C4A3$ favored the stabilization of orthorhombic C4A3$ rather than cubic C4A3$.
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