Research On The Inhibition Of Efflorescence And Mechanical Properties Of Concrete Of Slab Ballastless Track

The research background of this paper is the efflorescence disease of the concrete of ballastless track of Chengdu-Mianyang-Leshan high speed railway.Because alkali substances can flow through the pore network in the concrete,especially in the high humidity external environment,it is easier for the track slab and the surrounding environment to exchange water.The migration of water can be used as the carrier for the leaching of alkali from the inner pores,resulting in alkali overflow and the gradual formation of white flocculent substances deposited on the surface of the track slab.Under the influence of physical and chemical factors such as dry-wet alternation and freeze-thaw cycle,the track slab is accompanied by fatigue load and continuous load of mechanical action under the complex engineering operation condition of acid rain and sulfate attack and alkali aggregate reaction.The long-term severe efflorescence will lead to the decomposition of gel C-S-H,resulting in the reduction of strength and the safety of ballastless track system.The appearance quality is a part of the construction quality.It has the effect of having the best of both aspects for the practical application of ballastless track concrete by restraining the efflorescence phenomenon of concrete and improving its mechanical properties at the same time.Compared with other admixtures,nano silica has larger surface area,finer particle size and higher volcano ash activity than other admixtures,which can promote early hydration of cement and consume excess alkali substances in concrete pore solution to form gel.The formation of gel reduces porosity of concrete,and pore structure improvement can block free water and free alkali transport path.At the same time,it improves the compactness of concrete and enhances the bonding effect between cement particles,which has a positive contribution to the mechanical properties of concrete.Nano silica has many advantages.Because of its very high specific surface area,it can be used as filler more effectively.Based on this point of view,nano silica is used in this paper to inhibit the efflorescence of C60concrete,the raw material of CRTS I and CRTS III slab ballastless track slab of Chengdu-Mianyang-Leshan passenger dedicated line,and improve its mechanical properties.The specific research contents and conclusions are as follows:（1）The main component of efflorescence in slab ballastless track concrete is calcium hydroxide produced by cement hydration.The reaction between nano silica and cement is essentially a single chemical reaction between nano silica and calcium hydroxide.However,no scholar has systematically studied the single chemical reaction process mechanism between nano silica and calcium hydroxide,In this paper,the changes of crystal morphology after nano silica acted on calcium hydroxide within 28 days were observed by SEM;EDS and XRD experiments were carried out respectively for the element analysis and phase analysis of NS-Ca（OH）₂slurry;The chemical reaction conversion of single chemical experiment of nano silica and calcium hydroxide was calculated by TG analysis,and the basis of adding nano silica content to cement was put forward.（2）The efflorescence of slab ballastless track concrete is essentially the efflorescence of cement paste.A pre-test of efflorescence is carried out in this paper to explore the changes of efflorescence degree and mechanical properties of cement paste without sand and aggregate under the mix proportion of different admixture combinations.The reaction mechanism of NS,PC and FA was analyzed.The influence mechanism of PC,FA and NS on the microstructure and performance enhancement was discussed.The SEM micro morphology analysis experiment of cement paste powder was carried out,and it was found that the paste with the joint action of NS,PC and FA exhibited the best dense morphology;The XRD experiment is carried out.It is concluded that the diffraction peak intensity of tricalcium silicate can be better reduced when NS,PC and FA work together.According to the semi quantitative method,it is concluded that the cement slurry of J3 group can be hydrated better;The TG experiment was carried out.Through the calculation of the segmented temperature loss content at the age of 28 days,it was found that the content of Ca（OH）₂produced by hydration of hardened cement paste was the highest when NS,PC and FA acted at the same time,indicating that when the three acted at the same time,and because the water cement ratio of each sample is the same,they could better promote the hydration reaction of cement.（3）In the ballastless track of Chengdu-Mianyang-Leshan high speed railway,the raw material of CRTS I or CRTS III slab ballastless track slab is C60 high-strength concrete with low water cement ratio.NS exists as nano particles in the early stage,which makes an obvious contribution to the early strength of concrete.NS is easy to agglomerate,which is not conducive to the later strength of concrete,while the addition of FA is conducive to the secondary hydration reaction in the later stage and enhances the later strength of concrete,the coupling effect of NS and PC can effectively avoid the flocculation and agglomeration of nano particles and increase the fluidity of concrete.This paper effectively makes use of the coupling effect of three admixtures.（4）Through the innovative"disease amplification method"by this paper,the Y1 group was added with excessive calcium hydroxide（calcium hydroxide plus 5%）,and the"semi immersion method"was used to simulate the efflorescence humid environment,resulting in a 3.9%decrease in the compressive strength compared with the benchmark control group.It was verified that the serious efflorescence had a negative impact on the compressive strength of C60 concrete.（5）The compressive strength of Y2（2%NS）and Y3（3%NS）increased by 1.8%and4.6%respectively compared with Y0 in the control group;Compared with the control group Z0,the flexural strength of Z2（2%NS）and Z3（3%NS）increased by 2.7%and 4.0%respectively.It is confirmed that when the content of NS is less than 3%,it has a positive contribution to the compressive strength and flexural strength of C60 concrete.When the content of NS is more than 3%,the compressive strength and flexural strength of the specimen are lower than that of the reference control group.（6）When the content of NS is less than 3%,it has obvious inhibition on efflorescence.However,the optimum content of NS is greater than 4%for the inhibition of its efflorescence,but because of its strong agglomeration effect,when NS is greater than 4%,the gain rate of its efflorescence inhibition effect decreases with the increase of its content.Considering the inhibition effect of efflorescence and the improvement of mechanical properties,this paper suggests that the optimal content of nano silica in C60 concrete is less than 3%,which provides a reference value for the content of NS in CRTS I or CRTS III slab ballastless track slab.