The Design And Preparation Of Lightweight Functional Aggregate And Its Application In The External Anode Mortar

Impressed current cathodic protection(ICCP)is an effective corrosion protection and repair technique for the chloride-contaminated reinforced concrete,and can improve the durability and service life of reinforced concrete structures.The external anode mortar is the most significant component in ICCP system,uniformly distributing the protection current to different reinforcements embedded in reinforced concrete structures,thus its performance is of great importance,guaranteeing the protection efficiency of cathodic protection.However,the anodic reactions happened during ICCP treatment will result in the acidification damage in the external anode system,not only reducing the stability and protection efficiency,but also shortened the service life of ICCP system.At present,the investigations on the cement-based secondary anode materials are mainly focused on the optimization of its conductivity by using different types of conductive enhancement materials(e.g.carbon fibers,graphite and etc.),however the investigations on the acidification damage mechanisms in the external anode system and related mitigation methods are very limited.In this study,a novel type of lightweight functional aggregate(LFA)presenting high ionic conductivity and ability to maintain the high alkalinity in cement-based materials was designed and prepared;based the above prepared LFA,the high performance external anode mortar was prepred,aiming at solving the acidification damage problem and improving the conductivity and conductivity uniformity of the external anode mortar.Further,the mechanisms related to the effect of LFA on the conductivity and conductivity uniformity and acidification damage of the external anode mortar was extensively investigated in this study.The main research contents and conclusions of this study can be summarized as follows:(1)A novel type of modified agar gel presenting high conductivity and ability to maintain the alkalinity was designed and prepared.Lightweight functional aggregates(LFA)were prepared by loading the above modified agar gel into porous ceramsites via high temperature vacuum impregnation treatment.The prepared LFA on one hand exhibited low volume weight(0.93-1.12 g/cm~3)and excellent conductivity(with electrical resistivity in the range of 0.4~0.6 ?·m),on the other hand possessed good alkalinity maintaining capacity,which could maintain the high alkalinity of concrete pore solution.(2)The influence of LFA on the conductivity of the external anode mortar was investigated.The results indicated that an efficient conductive network was established among the lightweight functional aggregates in the external anode mortar,thus improving its conductivity.The effect of LFA on the conductivity of cement-based composite materials and related mechanisms were further investigated by general effective media model and microstructure analysis.In addition,the lightweight functional aggregates was able to reduce carbon fiber content needed to establish the efficient conductive network in mortar and mitigate the reduction of conductivity uniformity and conductive efficiency induced by the acidification damage in the external anode mortar.(3)The acidification damage in the external anode system during the accelerated acidification treatment was investigated.The results indicated that during the accelerated acidification treatment,on one hand,MMO coating was damaged and passive oxide layer formed on the primary anode surface;on the other hand,the acidification damage in the external anode mortar was divided in two areas: the severe acidification and decalcification damage area within a distance of 1000 ?m from the primary anode surface,and less severe acidification damage area beyond a distance of 1000 ?m from the primary anode surface,in which the damage mainly propagated along the interfacial transition zone around the aggregates.(4)The mitigation effect of LFA on the acidification damage in the external anode mortar during the accelerated acidification treatment was investigated.The results indicated that the functional components loaded in LFA maintained the high alkalinity of concrete pore solution,thus on one hand mitigating the damage of MMO coating on the primary anode surface caused by the acidification,maintaining the high catalytic activity and improving the stability and service life of the primary anode,on the other hand efficiently mitigating the acidification damage of different area in the external anode mortar.When the carbon fiber content of 0.75 vol.% and aggregates content of 40 vol.% were used,the acidification damage rate of the external anode mortar based on LFA(0.69~1.39 m V/h)was only about 12~40 % of the acidification damage rate of the external anode mortar based on normal aggregates.(5)Based on Fick's second law on the non-steady electrode reaction process,the influence of different phase compositions in the external anode mortar on the acidification damage process was investigated.The results indicated that the acidification damage rate of the external anode mortar was inversely proportional to the density of cement matrix and the aggregate volume content.The lightweight functional aggregates increased the tortuosity for the ion diffusion and provided partial ion flux for the anodic reactions by the reserved functional components,thus halting the propagation of the acidification damage.During the accelerated acidification treatment with the constant potential of 1.2 V vs.SCE,t the increasing rate of the equivalent diffusion coefficient in the external anode mortar based on LFA was about 70 % of the increasing rate of the equivalent diffusion coefficient in the external anode mortar based on the normal aggregates.(6)The influence of the high performance external anode mortar based on LFA on the operation parameters and protection efficiency of ICCP system w investigated.The results indicated that the LFA-containing high performance external anode mortar maintained the catalytic activity of the primary anode,reduced the polarization potential of the primary anode(after the accelerated acidification for 6 months,the anode potential was reduced about 30 % with the same carbon fiber and aggregates content)and cell voltage of ICCP system(the increase rate of cell voltage was about 35 % reduced),and improved the uniform distribution and volatility of protection current of ICCP system.As a result,the high performance external anode mortar prepared in this study can efficiently increase the stability and efficiency of ICCP system,and further improve the durability of reinforced concrete structures.