Study On Hydration And Microstructure Evolution Of Nanosilica-Slag-Cement Composite Cementitious Materials

With the steady implementation of the Maritime Power Strategy and Western Development Strategy,infrastructure construction in the marine and salt lake environment has been rapidly built.However,chloride and sulfate erosion in marine and salt lake environments seriously affects the durability of cement-based materials,leading to premature destruction of cement-based materials and serious economic losses.High-volume slag can improve the microstructure of cement-based materials and significantly improve the chloride and sulfate resistance of cement-based materials.Therefore,the use of high-volume slag is a common technical means to prepare high-performance marine concrete.The use of high-volume slag can also reduce the consumption of cement and energy,decrease the environmental load.However,when the content of slag is high（more than 50%）,the early performance of cement-based materials decreases significantly,which becomes the bottleneck restricting the development of cement-based materials with high-volume slag.In this paper,nanosilica-slag-cement composite cementitious material is constructed to solve the problem of insufficient early performance of cement-based materials with high-volume slag.The core scientific problem is to study the rheology,hydration behavior and microstructure evolution of composite cementitious materials under the synergistic effect of nanosilica-slag-cement system.The rheological evolution of composite cementitious materials is studied and the corresponding rheological models are established.The setting time,hydration heat,the content of hydration product and hydration kinetics of composite cementitious materials are studied to figure out the hydration mechanism of composite cementitious materials.The mechanical properties,pore structure,phase distribution and C-（A）-S-H gel microstructure of the hardened paste of composite cementitious materials are studied to clarify the microstructure enhancement mechanism of hardened paste.The orientation of Ca（OH）₂,micro-morphology,phase distribution and porosity in the interface transition zone（ITZ）between hardened paste of composite cementitious materials and aggregate are studied to reveal the strengthening mechanism of the ITZ.This study provides a theoretical basis and technical support for further improvement of the properties of cement-based materials with high-volume slag,and has important theoretical significance and engineering application value.The main conclusions are as follows:（1）Rheological properties of nanosilica-slag-cement composite cementitious materialsNanosilica has obvious flocculation promoting effect,which can increase the rheological parameters of fresh paste of composite cementitious materials at different resting time,and the more nano-silica is added,the more obvious the incremental effect will be.Slag has a certain deflocculation effect,which can reduce the rheological parameters of fresh paste of composite cementitious materials at different resting time,and the greater the content of slag,the more obvious the reduction effect.In this study,the microstructure parameter-flocculation rate（λ）is introduced,and the initial rheological model of composite cementitious materials paste is established according to the micro-rheological theory of suspension dispersion system.Based on the time-varying rheological data of composite cementitious materials,a time-varying rheological model of nanosilica-slag-cement composite cementitious materials is established by introducing the relative hydration degree（α’）.The above model can accurately predict the static yield stress of composite cementitious materials with different nanosilica and slag.（2）Hydration and kinetics of nanosilica-slag-cement composite cementitious materialsHigh-volume slag can significantly reduce the hydration rate of the composite cementitious materials,reduce the content of Ca（OH）₂ and chemically bound water,and generate new hydration products-monocarboaluminate and monosulfate.Nanosilica can promote the hydration of C₃S in composite cementitious materials,accelerate the transition from AFt to AFm,and promote the early hydration of slag.Therefore,nanosilica can accurately control the setting time of blended cement with high-vloume slag,which makes it possible for the rapid construction of concrete structures with high-vloume slag.Nanosilica has a weak effect on the late hydration of cement in composite cementitious materials,but it can always increase the content of chemically bound water in composite cementitious materials.The hydration kinetics study shows that the hydration kinetics process of composite cementitious materials has not been changed by high-volume slag and nanosilica,and both have gone through the NG-I-D process.However,high-volume slag weakened the process of crystal nucleation and crystal growth in composite cementitious materials,and increased the difficulty of phase boundary reaction and ion diffusion.Nanosilica promotes the process of crystal nucleation,crystal growth and phase boundary reaction of composite cementitious materials,but increases the difficulty of ion diffusion process.（3）Microstructure of hardened paste of nanosilica-slag-cement composite cementitious materialsComposite cementitious material can reduce the total porosity and average pore size,improve the pore structure of hardened paste.However,with the increase of slag content,the ability of nanosilica and slag to synergistically optimize the pore structure gradually weakens.BSE image analysis also shows that nanosilica can always promote the reaction of slag（28 d）.In addition,the microstructure of C-（A）-S-H gel in hardened cement paste can be improved by high-volume slag and nanosilica.Under the synergistic effect of nanosilica-slag-cement,the chain length of C-（A）-S-H in the hardened paste of composite cementitious materials further increased,the substitution ratio of Al/Si further increased,and the Ca/Si ratio of C-（A）-S-H further decreased,which improved the microstructure of C-（A）-S-H gel in the hardened paste.Finally,the mechanical properties of the hardened paste of composite cementitious materials are improved,and the greater the content of nanosilica,the more significant the improvement effect.In addition,with the increase of slag content,the improvement effect of nanosilica on the mechanical properties of composite cementitious materials is gradually weakened.（4）Microstructure of ITZ between hardened paste of composite cementitious materials and aggregateThe preferential orientation of Ca（OH）₂ in ITZ can be weakened by adding high-volume slag and nanosilica.The preferential orientation of Ca（OH）₂ in ITZ further decreases under the synergistic effect of nanosilica-slag-cement.SEM-EDS and BSE show that the nucleation effect,pozzolanic effect and filling effect of nanosilica can cooperate with the pozzolanic effect of slag to increase the content of C-（A）-S-H gel in ITZ,reduce the Ca/Si ratio of C-（A）-S-H gel,decrease the pores,which can improve the microstructure of ITZ.Ultimately,ITZ between hardened paste and aggregate is improved under the synergetic effect of nanosilica-slag-cement,and the tensile bond strength is also improved.