Study On Composition Coordination And Microstructure Optimization Of Ultra-high Performance Concrete

Ultra-high performance concrete（UHPC）is a new type of cementitious composite material with higher compressive strength,flexural strength,and durability than the widely used high-performance concrete.Its ultra-high performance and environmentally friendly,low-carbon properties make it a sustainable construction material with significant development potential for major infrastructure projects.However,UHPC requires a large amount of cement,which leads to high heat generation during hydration and significant shrinkage.Additionally,due to a low water-to-cement ratio（0.15～0.24）and low hydration degree（30%～40%）,the effective utilization of cement is low,resulting in waste and high production costs.Therefore,exploring the replacement of some cement with supplementary cementitious materials（SCMs）,while maintaining the desired performance of UHPC,and maximizing the usage of these materials,is an important measure for reducing production costs and total heat generation of UHPC.The main mechanisms of SCMs include the filling effect and pozzolanic effect,with the latter playing a significant role in performance.The pozzolanic activity of SCMs is influenced by their composition,particle size,reactive silica content,and the hydration process of cement,particularly the activity of newly formed calcium hydroxide.Therefore,investigating the synergy effect between cement and SCMs,as well as the hardening mechanisms between hydration products and SCMs,and regulating the hardened structure at a more microscopic level,are crucial for enhancing the performance of UHPC.In this study,comprehensive research on the synergistic effect of UHPC components and optimization of its microstructure has been systematically conducted,and the main research findings are as follows:（1）The pozzolanic effect and filling effect of different reactive fly ashes in UHPC were studied to elucidate the mechanisms by which high-activity and low-activity fly ashes influence the hydration and microstructure of UHPC.Compared to low-activity fly ash,high-activity fly ash（HFA）releases more amorphous silica and alumina during pozzolanic reaction at 28 d.This leads to a higher substitution of Al for Si at bridging sites in the C-S-H gel,resulting in a significant increase in the average chain length（MCL）of C-S-H gel and the volume fraction of gel pores.（2）The influence of hydrophobic silica fume（HPS）on the rheological properties and microstructure of UHPC was studied.Compared to silica fume,HPS reduced the viscosity and yield stress of UHPC better,improving the flowability of the mixture.HPS promotes the elongation of the MCL and the degree of polymerization of the C-S-H gel.Moreover,the pozzolanic effect of HPS is stronger,consuming more calcium hydroxide（CH）and increasing the content of C-S-H gel,thereby refining the pore size of the paste and enhancing the strength of UHPC.In the UHPC ternary system of cement-fly ash-silica fume,the synergistic effect of pozzolanic effect of HPS and HFA further consume CH increases the content of C-S-H gel,reduces the total porosity of the hardened material,and improves the microstructure,leading to further strength enhancement at 28 d.（3）The influence of the synergistic effect between ultrafine slag-silica fume and ultrafine slag-limestone powder on the hydration and microstructure of UHPC was studied,and the correlation between the strength,porosity,and pore complexity（fractal value,Ds）of the hardened material under the synergistic effect was established.The synergistic effect between ultrafine slag and silica fume promotes the pozzolanic reaction of both materials,increasing the amount and degree of polymerization of the C-S-H gel,reducing the porosity,improving the microstructure,and enhancing the strength of UHPC.A positive correlation between the synergistic effect and Ds was also confirmed.The synergistic effect between ultrafine slag and limestone powder promotes the hydration of cement and increases the chain length of C-S-H gel,and the volume fraction of gel pores.Additionally,the reaction between limestone powder and aluminum in ultrafine slag forms monocarboaluminate,stabilizing ettringite and further refining the pore structure,which enhances the strength of UHPC.The correlation between Ds and the synergy effect of UHPC was verified.（4）The influence of the synergistic effect between C-S-H seeding and triethanolamine（TEA）on the hydration and microstructure of high-alite UHPC was studied.TEA accelerates the hydration of C₃A but delays the hydration of C₃S,prolonging the induction period of cement hydration.On the other hand,C-S-H seeding provide nucleation sites and promote the hydration of C₃S.When both are co-blended,C-S-H seeding overcome the delay effect of TEA on C₃S hydration while maintaining the accelerating effect on C₃A hydration.The synergistic effect of both further promotes the hydration of cement.（5）The influence of the synergistic effect between C-S-H seeding,nano-CaCO₃,and TEA on the performance of high-belite UHPC was studied.In the early stage,C-S-H seeding,nano-CaCO₃,and TEA accelerate the hydration of C₃S and C₃A,causing the hydration heat peak to occur earlier and increasing the heat release.In the later stage,the synergistic effect of the three promotes the hydration of C₂S,increases the degree of polymerization of C-S-H gel and the Al/Si ratio,thereby reducing the porosity at 28 d and improving the strength.