Evolution Characteristics And Mechanism Of Microstructure And Properties Of High Early Strength Cement Paste

High early strength cementitious materials are widely desired in precast concrete,additive manufacturing and low-temperature construction,which plays an important role in the development of construction industrialization and green buildings in China.However,the rapid increase in the early strength of cementitious materials is usually detrimental to the development of later mechanical properties and durability,and its mechanisms involved is significantly lacking.Therefore,it is important to further understand the evolution characteristics of the microstructure and properties of high early strength cementitious materials and their mechanisms for performance improvement and application promotion of high early strength cementitious materials.In view of the above,according to the latest research progress on high early strength cementitious materials,this dissertation carries out research on the evolution characteristics and mechanisms of hydration process,microstructure,as well as mechanical and shrinkage properties of early strength cement pastes according to the common accelerated curing techniques of steam curing and accelerators,and explores the hydration enhancement techniques for cement-fly ash systems,with a view to providing theoretical and technical support for the development of high quality,green and low carbon high early strength cementitious materials and products.The main innovative work achievements are summarized as follows:(1)Combining the Arrhenius equation and the equivalent age function,an iterative calculation model for the hydration heat release of cement during the unsteady steam curing is developed to determine the hydration characteristics of cement paste under steam curing.And the difference of hydration characteristics between steam curing cement paste and cement paste with accelerators is clarified.(2)The influence of accelerators on the characteristics of hydration and microstructure and its mechanisms are revealed.Results show that CS-H seeds not only promote the early hydration of silicate phase in cement,but also promote the formation and transformation of ettringite.The different inferences of accelerators and steam curing on the microtopography of C-S-H gels are observed.Calcium formate significantly increases the specific surface area of the hydration products,while steam curing obviously decreases the specific surface area of the hydration products.(3)The mechanical properties,chemical shrinkage,self-desiccation shrinkage and drying shrinkage characteristics of cement pastes with steam curing and accelerators are obtained.A method for evaluating the evolution of mechanical properties and shrinkage of high early strength cement pastes as functions of hydration degree is proposed.The different growth characteristics of mechanical properties and shrinkage of high early strength cement pastes with hydration degree are found,and the adverse effects of shrinkage deformation on mechanical properties of high early strength cement pastes are explained.(4)The three stages effect of fly ash on the early hydration of cement is determined.An effective technical measure to enhance the hydration process of cement-fly ash system based on the synergistic effect of C-S-H seeds and triethanolamine is proposed to realize promotion of both early hydration of cement and subsequent hydration of fly ash.