Study On The Effect Of Mineral Admixtures On The Electrochemical Chloride Extraction Effect And Performance Of Concrete

Reinforced concrete structures in coastal and saline areas are subject to long-term sulphate and chloride attack,resulting in corrosion of reinforcing steel and reduced durability and service life of concrete structures.Among the many repair techniques,electrochemical chloride extraction（ECE）is widely used for engineering structures in chloride and salt environments because of its rapid,non-destructive and low-cost advantages.The effectiveness of ECE for ordinary concrete and the effect of ECE on the performance of ordinary concrete have also changed with the addition of mineral admixtures.This paper first analyses the effects of anode material,electrolyte solution type and concentration,and reinforcement type on the electrochemical chloride extraction effect,and obtains the best electrochemical chloride extraction device for the chloride extraction effect.This paper provides experimental data and theoretical guidance for the application of electrochemical chloride extraction technology to concrete with mineral admixtures,as well as for its durability modification.The main findings and conclusions of this paper are as follows.（1）Using different anode materials,electrolyte solutions and steel types for ECE treatment of plain concrete,the chlorine removal effect of three electrode materials was ranked as follows:ruthenium coated titanium electrode>Ti metal mesh>stainless steel mesh;the chlorine removal effect of six electrolyte solutions was ranked as follows:1 mol/L Na₂CO₃solution>saturated Ca（OH）₂solution>0.5 mol/L Na₂CO₃solution>0.1 mol/L Na₂CO₃solution>0.1 mol/L K₂CO₃solution>stainless steel;the chlorine removal effect of two steel types was ranked as follows:plain carbon steel>stainless steel.0.1 mol/L Na₂CO₃solution>1 mol/L K₂CO₃solution>tap water;two steel types in order of chlorine removal:plain carbon steel>stainless steel.（2）Electrochemical chloride extraction（3A/m²,28d）of concrete mixed with different types and amounts of mineral admixtures revealed that the addition of mineral admixtures reduced the ECE efficiency of concrete,and the chloride extraction efficiency in descending order was:OPC>15%SM>25%SM>20%SL>40%FA>30%FA>20%FA>10%FA/10%SL>5%SF>8%SF>10%SF>15%FA/15%SL>55%SL>40%SL>20%FA/20%SL>5%SM;fly ash dosing（20%～40%）and ECE efficiency are positively correlated;ECE efficiency decreases and then increases with the increase of slag dosing（20%～55%）,and the lowest ECE efficiency（37%）is achieved when the slag dosing is 40%.The ECE efficiency increases with the amount of stone dust（5%-25%）and then decreases,and the highest ECE efficiency（62%）is achieved at 15%of stone dust;the ECE efficiency decreases as the proportion of cement replaced by fly ash/slag compound increases;the initial chloride curing ability of mineral admixture and the porosity of concrete itself determine the ECE efficiency;the initial chloride curing ability of mineral admixture and the porosity of concrete itself determine the ECE efficiency.The initial chloride curing capacity of the mineral admixture and the porosity of the concrete itself determine the ECE efficiency;the Cl^-concentration in the vicinity of the reinforcement can still be reduced to below the critical chloride ion concentration（1.4 kg/m³）after the ECE treatment of concrete with mineral admixture.（3）The results of pull-out tests on reinforced concrete before and after ECE treatment show that there is a loss of bond strength in all groups of reinforced concrete specimens after ECE treatment,and the loss rates in descending order are:OPC>10%FA/10%SL>15%FA/15%SL>20%FA/20%SL>20%FA>40%FA>20%SL>30%FA>40%SL>55%SL,the addition of mineral admixtures such as fly ash and slag can reduce the loss of concrete bond strength caused by ECE treatment.The rate of concrete bond strength loss decreases and then increases with the increase of fly ash admixture（20%～40%）,and the lowest rate of bond strength loss（8.2%）is found at 30%fly ash admixture;the rate of slag admixture（20%～55%）is negatively correlated with the rate of concrete bond strength loss,and the rate of loss decreases from 8.5%to 5.3%;as the proportion of fly ash/slag compound replacing cement increases,the rate of concrete bond strength The loss rate of concrete bond strength gradually decreased from 11.7%to 9.7%as the proportion of fly ash/slag compound replaced cement increased.（4）XRD tests showed that no new phases were formed after the ECE treatment of concrete with single fly ash,slag and compound fly ash/slag,and the changes of the same phases were the same.gradually weakened;through nitrogen adsorption and mercury compression tests,it was found that the total porosity near the reinforcement of each group of specimens increased after ECE treatment,and the magnitude of the effect was in the following order:OPC>slag>fly ash>compound fly ash/slag concrete,where the cumulative porosity of 0～100nm decreased and the coarse porosity greater than 100nm increased;using ultra-view depth and SEM tests,it was found that the incorporation of fly ash and slag could weaken the damage caused by ECE treatment to the concrete interface,reduced the occurrence of cracks and improved the rusting condition;ECE treatment made the morphology near the reinforcement rougher,increased the pores and reduced the amount of calcium alumina and C-S-H in each group of specimens,and the addition of fly ash and slag weakened the negative effect of ECE treatment on the microscopic morphology of concrete.（5）The electric flux test and the freeze-thaw cycle test showed that ECE treatment reduced the chloride ion permeation resistance of concrete with mineral admixture;the order of influence was:OPC>slag>fly ash>compound fly ash/slag concrete,the addition of mineral admixture helped to reduce the influence of ECE treatment on the chloride ion permeation resistance of concrete,after ECE treatment only OPC concrete,20%SL concrete,and the other groups remained in the low range.The ECE treatment reduced the mass loss of concrete due to freeze-thaw cycles and accelerated the rate of decrease of dynamic modulus of elasticity,but did not change the mass loss and the trend of dynamic modulus of elasticity with mineral admixtures,with 30%FA alone,20%SL and 20%FA/20%SL concrete having the lowest mass loss compared to the other groups.The mass loss of SL concrete is the lowest and the relative dynamic modulus of elasticity decreases the slowest,and each mineral admixture still has a certain degree of frost resistance after the ECE treatment.