Research On The Influence Of Nano-Silica On The Mechanical Properties And Microstructure Of Rubber-Recycled Concrete

In recent years,whether developing or developed countries,the continuous growth of economy and population has led to an increasing demand for infrastructure construction in these countries.The sharp increase in infrastructure construction has led to a large consumption of natural resources.At the same time,in the process of industrialization and urbanization,a large number of waste facilities need to be upgraded,and the output of waste tires and construction waste is increasing.The construction industry is facing two major problems: the lack of natural resources and the sharp increase of construction waste,and the reuse of construction waste and waste tires to prepare sand and gravel aggregate has become one of the keys to solve these two problems.However,the current research on recycled aggregate concrete(RAC)mostly focuses on recycled coarse aggregate,resulting in the insufficient application of recycled fine aggregate produced in the process of preparing recycled coarse aggregate and rubber particles prepared from waste tires in the engineering field.In addition,in view of the current lack of natural sand and gravel resources in China,the use of recycled coarse and fine aggregate instead of natural aggregate to prepare concrete can not meet the needs of the current national conditions.At the same time,adding two kinds of recycled aggregate and rubber particles is the direction that needs further research.Therefore,in this paper,recycled coarse aggregate,rubber particles and recycled fine aggregate are used to replace natural sand and gravel to prepare rubber recycled concrete.The performance of rubber recycled concrete is improved by adding auxiliary cementitious material nano silica instead of cement.The influence of nano silica on the hydration characteristics of cement-based composites and the change law of flow,mechanics and impermeability of nano silica rubber recycled concrete are studied.At the same time,the microstructure of nano silica rubber recycled concrete was analyzed by Mercury intrusion porosimeter(MIP),Thermo gravimetry and Derivative thermal gravimetry(TG-DTG)analyses,X-ray diffraction analysis(XRD)and Field emission scanning electron microscope(FESEM),and the improvement mechanism of nano silica on its properties was discussed.The main research conclusions of this topic are as follows:(1)By analyzing the properties of nano silica cement composite cementitious materials,it can be seen that nano silica accelerates the initial and final setting time of cement and improves the compressive strength of cement mortar,but it will reduce the fluidity of cement-based materials;Nano-silica increases the rate of hydration reaction in the early stage of cement,resulting in the third exothermic peak in the deceleration period,which increases the total exothermic heat of hydration reaction in the early stage of cement;Nano-silica accelerates the nucleation and crystal formation,limits the size and multidirectionality of the crystal,and speeds up the reaction process in the phase boundary(I)stage and diffusion(D)stage.(2)By analyzing the flow,mechanical and impermeability of nano silica rubber recycled concrete,it can be seen that the recycled coarse and fine aggregates after pre wetting have less negative impact on the flow performance of rubber recycled concrete,but nano silica has a greater negative impact on its flow performance;Nano silica can effectively improve the mechanical and impermeability of rubber recycled concrete.When the content of nano silica replacing cement is 3%,the improvement effect is relatively better.At the same time,due to the agglomeration phenomenon,when the nano silica exceeds a certain amount,the performance improvement decreases.(3)By analyzing the microstructure of Nano-silica rubber recycled concrete,it can be seen that the addition of recycled coarse and fine aggregates and rubber particles increases the porosity in the concrete,increases the content of harmful and multi harmful pores,inhibits the development of hydration products,and increases the number of weak interface transition zone(ITZ),resulting in loose microstructure;Nano-silica can refine and fill the pores of rubber recycled concrete,and promote harmful and harmful pores to harmless and less harmful pores,promote the formation and development of hydration products,react with Calcium hydroxide(CH)crystal to form C-S-H gel,reduce the content of CH crystal and enhance the density of microstructure.At the same time,it acts as a “bridge” to guide C-S-H gel to repair the weak ITZ inside the rubber recycled concrete and improve the strength of ITZ.(4)The analysis of the agglomeration phenomenon of nano silica shows that nano silica has high specific surface energy and is very easy to produce agglomeration phenomenon.The agglomerated nano silica adsorbs a large number of hydration products and nucleates on its surface,which increases the density of hydration products around the aggregate,enhances the ion barrier and greatly reduces the hydration reaction rate,resulting in the poor development of hydration products around the aggregate,Finally,the performance of cement mortar is reduced.Therefore,in the absence of special dispersion means,the content of nano silica instead of cement should not be greater than 3%.(5)According to the performance indexes of nano silica rubber recycled concrete,when the replacement rates of nano silica,recycled coarse aggregate,rubber particles and recycled fine aggregate are 3%,60%,3% and 70%,it has great engineering potential.