Study On The Effects Of Nano-Silica And Curing System On Mechanical Properties Of Ultra-high Performance Concrete

In recent years,ultra-high performance concrete（UHPC）has become the research focus of most scientific researchers and has gradually been applied to engineering practice,due to its ultra-high strength and excellent durability.In order to improve the performance of UHPC,improving raw materials and curing systems are two effective methods.Studies have shown that the addition of Nano-material can significantly improve the mechanical properties of UHPC.The combined curing of UHPC has also received more and more attention due to its efficiency in the development of UHPC strength compared to single curing.The impact of the curing systems on the mechanical properties of UHPC is mainly reflected in the types and quantities of hydration products and other microscopic issues.However,the current research on combined curing is still immature,and the interaction mechanism between the curing systems and Nano-material is not clear.This thesis mainly investigated chooses Nano-SiO₂and four curing systems（20℃standard curing,90℃steam curing,185℃dry heat curing and steam-dry heat combined curing）as the research objects,focuses on comparing the content of Nano-SiO₂（0%,1%,3%,5%,7%）and the influence of the maintenance system to prepare UHPC,using nonlinear curve fitting and two-factor multi-level analysis of variance as auxiliary analysis methods.In order to reveal the mechanism of action,this study also used scanning electron microscopy to micro-characterize the hydration products,fiber bonding properties,and stability of the interface transition zone of UHPC from a microscopic point of view,to further clarify the role of Nano-SiO₂and curing systems on UHPC mechanism.The main conclusions are as follows:（1）The flexural strength and compressive strength of UHPC increase with the Nano-SiO₂content.When the content reaches 3%,the two strength indexes of UHPC both reach the highest.（2）Compared with standard curing,the three types of thermal curing can significantly improve the mechanical properties of UHPC,of which steam-dry heat combined curing has the best effect.Due to the influence of high temperature and high humidity environment,Nano-SiO₂can fully participate in the hydration reaction,which makes the improvement of the early strength of UHPC particularly obvious.But compared with another two curing,dry heat curing is not conducive to the development of UHPC flexural strength.The flexural strength of 3%UHPC is 33.73MPa,14.5%and 20.3%lower than steam curing and combined curing.This is mainly related to a large amount of free water inside the concrete that evaporates due to dry heat curing at a high temperature of 180℃,which reduces the bonding performance between the fiber and the matrix.（3）For different curing systems,there has a big difference in the optimal content range of Nano-SiO₂.The content range of Nano-SiO₂when the mechanical properties of steam curing and dry heat curing UHPC reach the highest is less than 3%,while the standard curing and combined curing are more than 3%.It shows that combined curing is not a simple superposition of two single thermal curing.The change of the curing environment has a complex impact on the strength development of UHPC.And compared with the content of Nano-SiO₂,the curing systems have a more noticeable improvement effect on the mechanical properties of UHPC.（4）There have significant differences in the microstructure of UHPC under different curing systems,which are mainly reflected in the morphology and distribution of hydration products,the bonding properties of fibers and the matrix,and the stability of the interface transition zone.Among them,in the steam curing stage of the combined curing stage,the high temperature and high humidity environment make the gelling material fully participate in the hydration reaction,generating C-S-H gels,CH and other hydration products;In contrast in the dry heat curing stage,the overall development of the C-S-H gels is denser.CH and other substances which are not conducive to the improvement of the strength of UHPC are further involved in the secondary hydration and consumed,generating stronger hydration products such as C-S-H gels and xonotlite,which fill the pores of UHPC and optimize the microstructure.