Microstructure Evolution Characteristics And Mechanism Of Fresh Cement Paste

Civil engineering construction techniques stay focused on efficiency and quality,developing toward advanced construction techniques,which are greener,more industrialized,and more intellectualized.However,advanced construction techniques（e.g.,3D printing building and Smart dynamic casting）have put forward higher requirements on the performance of cement-based materials.Cement-based materials are required to have good fluidity in the processes of mixing,transportation,and pumping,but can quickly obtain a certain shape retention ability after pouring.The essence of this requirement is the precise regulation of the microstructural development of cement-based materials.The systematic study of the formation and development mechanism of the microstructure of fresh cement-based materials is the premise of revealing the mechanism of concrete setting and hardening,and also the premise of realizing the accurate control of the microstructure development of fresh cement-based materials,which is of theoretical and engineering significance.This paper focuses on the characterization method,formation mechanism,and regulation principle of microstructure evolution of fresh cement-based materials before the final set.Through theoretical analysis and tests,the main innovative work achievements are summarized as follows:（1）According to the hydration reaction characteristics of Portland cement in the setting period,the complex hydration reaction of Portland cement is simplified into three reaction systems,including C₃S+water,C₃A+gypsum+water,and C₃S+gypsum+water.The microstructure evolution characteristics of C₃S paste,C₃A-gypsum paste,and C₃S-gypsum paste during setting were analyzed by using small amplitude oscillation shear（SAOS）and ¹H NMR methods.Both the roles of C₃S hydration and C₃A hydration on the microstructure development of fresh cement paste are clarified.（2）Isothermal calorimetry,XRD,SEM,Zeta potential,and ICP tests were performed to investigate the variation characteristics of solid and liquid phase composition,the time-space distribution law of particles-hydration products-water,and the dynamic evolution of the electric double-layer repulsion during the microstructure evolution of fresh single clinker pastes and Portland cement paste.The contributions of the nucleation and growth of ettringite and C-S-H on the microstructure development were quantified.（3）The regulation mechanisms of gypsum,curing temperature and alkali content on the microstructure development of Portland cement paste were revealed from the perspectives of hydration kinetics,colloidal interaction,and C-S-H cohesion.The effects of these three parameters on the cement hydration kinetics,the phase compositions,the growth characteristics of hydration products,and the C-S-H morphology were explored.The new insights of accelerating and retarding effects mechanisms on cement setting were put forward.（4）Due to the difficulty of using SAOS and ¹H NMR at the construction site,AC impedance spectroscopy（ACIS）was developed for the real-time characterization of fresh cement-based materials.The feasibility and validity of the ACIS method were demonstrated.The viscoelasticity parameters,magnetic resonance signal of free water,and electrical resistivity were correlated with the microstructure characteristics of fresh cement paste.The non-destructive microstructure characterization system for fresh cement-based materials was enriched.