Preparation Of New Low-carbon Clinker Based On The Structure And Activity Regulation Of Ternesite

As a major area of carbon emission,it is very important for the cement industry to promote its carbon emission reduction to achieve the target of carbon peak and carbon neutrality over the world.As one of the technical ways to reduce CO₂emission in the process of clinker production and cement preparation,low-carbon cement development has great potential to further reduce CO₂emission in the cement industry,and is a key technical field to achieve carbon emission reduction in the cement industry.Belite-Ye’elemite-Ternesite（BYT）cement system is regarded as new type cement.Not only does BYT cement take the advantages of both Portland cement and sulfoaluminate cement on properties,but also shows great excellence on less energy consumption and lower carbon emission during manufacturing process.However,at present,the scientific theoretical research on this series of cement is still limited.In particular,there is a lack of detailed and in-depth research on the evolution mechanism of thermal stability and hydration activity of the important clinker mineral ternesite（C₅S₂$）,which greatly limits the further improvement of the preparation and performance of this cement.In this work,the formation mechanism of C₅S₂$and the influence of external conditions such as sintering procedure and ion doping on the structure and thermal stability of C₅S₂$were studied by using a multi-technique approach.The hydration process of C₅S₂$and the temporal distribution of the products were clarified.The effects of sintering procedure and ion doping on the hydration activity,product structure and mechanical properties of C₅S₂$were described.On the basis of the above research results about C₅S₂$,the single-stage sintering preparation of BYT clinker under the influence of mineral composition and ion doping was studied.The hydration process and property development of BYT cement were clarified.The specific work can be drawn as follows:（1）Analytical reagents were used as raw materials.TG-DTA,XRD,SEM/EDS,HRTEM,FTIR and Rietveld structure refinement methods were used to investigate the formation mechanism,crystal growth and structure evolution behavior of C₅S₂$.The essential relationship between the above behavior of C₅S₂$and its thermal stability was constructed.The influence mechanism of external conditions such as sintering procedure and ion doping on the thermal stability of C₅S₂$was discussed.The results show that the formation of C₅S₂$benefited from the preformation of the intermediate silicate phase,and its formation mechanism showed a“sunflower”core-shell structure.During the growth of C₅S₂$,the crystal structure was gradually developed and improved,and its thermal stability was gradually improved.Among them,the appropriate increase of sintering temperature,the prolonging of duration,the increase of repetition,the decrease of cooling rate,the doping of P₂O₅and Mg O were beneficial to further optimize the development degree and crystal structure of C₅S₂$,so as to further improved its thermal stability.（2）The synthetically pure C₅S₂$was studied.XRD,NMR,TG-DTG,SEM/EDS and hydration calorimetry were used to study the hydration process of C₅S₂$and the spatial and temporal distribution of products.The effects of sintering procedure and ion doping on hydration activity,product structure,microstructure and mechanical properties were investigated.The constitutive relation between the hydration activity of C₅S₂$and its structural modification was constructed.The results show that the hydration products of C₅S₂$mainly included C$H₂,CH and C-S-H.Among them,C$H₂was preferentially produced.C₅S₂$obtained by a single-stage sintering process and air quenching has excellent early hydration activity.The doping of Li₂O,Na₂O,K₂O and Al₂O₃reduced the structure order and crystallinity of C₅S₂$,significantly improved the early hydration activity of C₅S₂$,accelerated its early hydration process and the development of mechanical properties.Ion doping induced the growth of different forms of C-S-H gel and significantly affected the degree of polymerization.（3）Analytical reagents were used as raw materials.XRD,SEM/EDS and Rietveld structure refinement methods were used to investigate the single-stage sintering preparation of BYT clinker under the influence of different mineral composition and ion doping.The proper sintering procedure,microstructure and abrasive property of cement clinker were emphasized.The influence of impurity ions on the above properties of clinker was investigated.The results show that the clinker with different mineral compositions based on w（C₄A₃$）:w（C₅S₂$）≈0.63,had a uniform optimum sintering procedure:sintering at 1200℃for 1 h.Appropriate amount of alkali metal oxide doping reduced the sintering temperature of clinker to1100℃,the doping of 1.0 wt%P₂O₅increased the sintering temperature range of clinker to 1250℃.Mineral particle aggregates were existed in prepared clinker.Among them,the high content of C₄A₃$and C₅S₂$and the doping of P₂O₅and Mg O contributed to the existence of larger size aggregates,which reduced the abrasive property of clinker.Na₂O and K₂O doping improved the abrasive property of clinker by reducing the size of aggregate.（4）XRD,TG-DTG,SEM and hydration calorimetry were used to study the hydration process and properties development of BYT cement.The constitutive relationship between mineral composition,mineral activity and cementitious property of cement was systematically expounded from the aspects of hydration heat release,hydration product composition,microstructure and strength development.The results showed that the hydration products of BYT cement mainly included AFt,AFm,AH₃,CH,C₂ASH₈and C-S-H.Among them,the generation of C₂ASH₈product was mainly attributed to the co-hydration of C₄A₃$and C₅S₂$.The doping of impurity ions in the clinker had different effects on the formation time and mutual conversion of hydration products.The high mineral composition of C₄A₃$and C₅S₂$contributed to the cement’s low water demand for normal consistency and excellent development performance.The doping of Li₂O promoted the rapid hydration of C₄A₃$mineral,shortened the setting time of cement and contributed a higher hourly strength.Both Na₂O and K₂O activated the hydration activity of the C₅S₂$mineral and enhanced the hydration synergistic effect between C₄A₃$and C₅S₂$,thus giving the cement excellent development properties during the early,middle and late stage of hydration.