Water State Evolution And Pore Structure Development During Hydration Of Cement-based Materials

Concrete is currently the most widely used building material in the world due to its good working performance,excellent durability and the most competitive economy.One of the key components that determine these properties of concrete is cement.Cement hydration is the root cause affecting the physical properties,mechanical properties,volume stability and durability of cement-based materials.As hydration proceeds,the state of water changes from bulk water partially to chemically bound water,physically confined water and free water.Macroscopically,it is reflected in the gradually formed pore structure during the hydration process.Both physical and chemical processes affecting durability of cement-based materials involve the migration and distribution of moisture in the pores.Therefore,quantitative research on the evolution of water state and the formation and development of pore structures during the hydration process is helpful for in-depth understanding of the cement hydration mechanism,which is of great scientific significance and engineering value.The topic of this thesis focuses on the key scientific issue of the quantitative characterization and evaluation of the hydration process after adding fly ash,silica fume,marble powder and other cementitious materials and silane to the cement system.The basic process of hydration reaction of cement-based materials was quantitatively studied,and the morphology evolution of hydration products was quantitatively characterized.Then nuclear magnetic resonance technology（NMR）was applied to analyze the evolution of different water states（capillary water,flocculated water,and wrapped water）during the hydration process.Meanwhile,the development process of the pore structure in the hydration process of cement-based materials was explored,the pore structure development model in the early and middle stages of hydration was established,and the effects of water removal methods and lightweight aggregates on the pore structure were analyzed.Finally,the relationship between macroscopic properties and microscopic indicators such as water state evolution and pore structure development was clarified.The main findings are as follows:（1）The morphological evolution of hydration products was quantified:C-S-H length was increased during the hydration process,and it was basically increased linearly within the initial 10 hours.After 24 hours,the surface of cement particles was almost completely covered by the foil form C-S-H whose length was not easy to measure.In the main hydration stage,C-S-H mainly nucleates on the surface of cement particles and grows in needle-like shapes.The transition from the acceleration phase to the deceleration phase occurs when the surface is completely covered with needle-like hydration products.Although C-S-H length increases in the deceleration stage,the increasement is very small.（2）The evolution of water state was characterized:The signal quantity of flocculated water gradually decreases until it disappears;The formation of wrapped water is related to the condensation process of silane,and its appearance time is affected by silane content;Capillary water is continuously consumed,and its differential curve is consistent with the three known hydration stages,including the initial stage（induction stage）,the main hydration peak stage（including the acceleration stage and the deceleration stage）and the slow reaction stage.（3）The development model of small pores was established:The exponential model was able to characterize the development of small pores in the early hydration stage for cement-based materials with different water to cement ratios,and the power model was able to characterize the development of the small pores in the later hydration stages,which is consistent with the development of the small pores when silane was incorporated.The small pore development characteristics in the fly ash-mixed systems can be expressed as the exponential model（when the amount of fly ash is small）,the Dose Resp model（when the amount of fly ash is large）in the early hydration stage,and the power model in the later hydration stages.When silica fume was incorporated,the development of small pores can be characterized by the Dose Resp model in the early hydration stage,and by the Bradely model in the later hydration stage.（4）The effects of water removal methods and lightweight aggregates on the pore structure were explored:The oven drying method led to an increase in the signal intensity of the small pores and a deterioration of the pore structure.IPA replacement method revealed the presence of large capillary pores not detected by the oven drying method.The pore structure can be improved to a certain extent when lightweight aggregates were added.When the water to cement ratio is large,the effect of dry spherical ceramsite on the improvement of the pore structure is more significant;when the water to cement ratio is small,the pore structure can be improved by adding an appropriate amount of water-saturated spherical ceramsite and shale,and the improvement is more pronounced for the spherical ceramsite regulated samples.（5）The relationship between macroscopic properties and microscopic indicators was established:There is a good linear correlation between fluidity,T_2w value of capillary water,T_2w value of flocculated water and a certain relationship between fluidity and C-S-H length;The setting time is not directly related to the C-S-H length and T_2w value of wrapped water,but it is linearly related to the T_2w value of capillary water,and the T_2w value of flocculated water for 1～3 h;the compressive strength was nonlinearly correlated to the T_2w value of capillary water,negatively correlated with the NMR small pore signal,positively correlated with the gel space ratio,negatively correlated with the saturated water content,and negatively correlated with the Hymo Struc simulated capillary porosity;the capillary water absorption coefficient was positively correlated with the NMR small pore signal.Until now,cement-based materials are still widely used in infrastructure construction in the fields of hydraulic engineering,civil engineering and national defence.Focusing on the hydration process of cement-based materials,this work reveals the water state evolution and pore structure development law and clarifies the dynamic correlation law of macroscopic properties and microstructure during the hydration process of cement-based materials,which promotes the development of accurate description and quantitative prediction of cement scientific research,and provides basic theory and technical support for in-depth research related to cement and scientific guidance in the field of production.