Study On The Transport And Binding Mechanism Of Chloride Ions In Limestone-calcined Claycementitious Materials

Concrete is treated as the largest and most widely used building material in the world.Cement,one of the key materials needed in the production process of concrete,is a high energy consumption product.Every ton of cement produced will release about a ton of CO₂,which will cause serious pressure on human living environment.An effective way to reduce CO₂ emissions is replacing cement by traditional supplementary cementitious materials,such as fly ash,slag and silica fume.One problem is that their storage and production is difficult to meet the increasing demand for cementitious material.Therefore,a new type of supplementary cementitious materials-calcined clay and limestone powder composite,becomes a kind of promising supplementary cementitious material.It has been found that suitable proportional of calcined clay and limestone powder combination can effectively refine the pore size and improve a series of properties,including compressive strength and durability.At present,more in-depth studies are needed on the durability impact of this materials,especially on chloride ions transport mechanism.It is of great significance to improve the performance of cementitious materials by exploring the chloride transport mechanism,microstructure evolution,regulation of bound chloride of limestone-calcined clay-cementitious materials.First of all,considering the simple system of Portland cement-limestone powder,the influence of limestone powder on chloride resistance of cement-based materials was analyzed.Combined with the quantitative analysis of multi-scale pores,chemical and physical chloride ions,the effect of limestone powder was explored from the microscopic point of view.Secondly,this study further extended to the pozzolanic supplementary cementitious materials,including bianry or ternary systems of calciend clay,slag or fly ash with or without limestone powder.Chloride resistance of limestone-calcined clay-cementitious system and other systems was compared,which led to the investigation of the pore structure,binding capacity and pore solution influence on chloride ions transport.Thirdly,from the static study to the dynamic study,the change of pore structure and phase assemblage of different systems during chloride ion transport and their influence on later chloride ion transport were studied.The evolution of AFm phase during chloride ion transport in limestone-calcined clay-cementitious materials was analyzed.Last but not least,the effect of calcined clay on chemically and physically bound chloride ions in C₃S system were preliminarily explored.The main conclusions of this study are as follows:(1)In Portland cement-limestone powder system,when the content of limestone powder is less than 15%,the chloride resistance of the systems stay constant,which comes from the combined effect of filler effect,dilution effect and chemical reaction of limestone powder on the pore structure and chloride binding capacity.It is found that capillary porosity is an important factor affecting chloride ion transport in Portland cement-limestone powder system.For chemically bound chloride,the presence of Friedel's salt solid solution needs to be considered in systems with limestone powder.However,the physical bound chloride mainly changes with the content of C-S-H.(2)In blended systems,chloride resistance of limestone-calcined clay-system is better than that of binary systems and other slag and fly ash systems.In addition to the critical pore sizes and the binding capacity,alkaline ions in pore solutions is also a major factor affecting chloride ion transport.In general,the low content of alkaline ions contributes to the low diffusion coefficient of chloride ions.(3)When chloride ions transport in cement-based materials,the microstructure of the system is also changing.The hydration of cement clinker,the secondary hydration of pozzolans and the reaction between chloride ions and hydration phase will lead to the gradual decrease of chloride ion diffusion coefficient.In limestone-calcined clay-cementitious materials system,the change of the chemically bound chloride during the invasion is as follows:in OPC and binary systems,the Monosulfoaluminate is first converted to Kuzel's salt,and then to Friedel's salt.In ternary system,the Monocarboaluminate is transformed into Hemicarboaluminate,and the Hemicarboaluminate is transformed into Friedel's salt or its solid solution.(4)In C₃S system,the addition of calcined clay is conducive to the formation of chemically bound chloride ions.However,its effect on physically bound chloride ions is more complicated.The reason is that calcined clay not only changes the amount and Ca/Si of C-S-H,but also reduces the pH value of the pore solution in the system.In addition,the total specific surface area of the system is also changed.