| It is well known that corrosion of steel reinforcement is one of the main reasons influencing the durability and service life of reinforced concrete structures.Among numerous concrete reinforcement protection methods,admixed organic corrosion inhibitors are considered as an effective method to halt the corrosion damage of steel reinforcement.However,the normally used organic corrosion inhibitors cannot identify the corroded area of steel reinforcement,thus do not present the intelligent corrosion protection on the steel rebar.In addition,there are some limitations for the practical application of organic corrosion inhibitors,e.g.limited long term corrosion efficiency due to the evaporation of inhibitors through the concrete pore system and negative effect on the setting time,strength and durability of concrete;the inhibition mechanism is also not clear.In order to solve the above problems,in this study,based on the fact that the corrosion initiation of the steel reinforcement is accompanied with a pH drop from high pH value to low pH value at corrosion site,organic micro-nano anticorrosion capsules（OMAC）with pH-sensitivity were designed and prepared.On one hand,before corrosion initiation,corrosion inhibitors are stably encapsulated in micro-nano capsules under the high pH environment,thus their negative effect on the material properties of concrete can be mitigated.On the other hand,after corrosion initiation,the reserved corrosion inhibitors can be efficiently released due to the local pH drop at corrosion initiation sites,resulting in the smart repairing of corrosion damages.In this paper,the relationship between the pH-sensitivity and structure of OMAC,the mechanism on the intelligent corrosion protection of reinforced concrete were also investigated,and the mechanisms related to its effect on the properties of cement-based materials were revealed.The specific research contents in this thesis include:（1）Based on the requirements of OMAC suitable for the intelligent corrosion protection of reinforced concrete,the basic principles related to the selection of the capsule material and encapsulated inhibition component,preparation method,and design optimization methods were proposed to provide the theoretical guidance for the design and preparation of OMAC.Poly（ethylene oxide）-b-polystyrene（PEO-b-PS）copolymers were selected as the capsule material of OMAC,and sodium nitrite（SN）and benzotriazole（BTA）were selected as organic/inorganic inhibiting package components.The OMAC were prepared by dialysis method based on PEO-b-PS,synthesized by atom transfer radical polymerization（ATRP）.Under the consideration of both the encapsulated amount of inhibition components and uniformity of OMAC,the optimal OMAC was selected by controlling the length of PS hydrophobic group and concentration of inhibition components.The molar concentration of the prepared OMAC was 2.824×10-8 mol/L,and one OMAC consisted of about 250molecules of PEO113-b-PS1171.The concentration of the capsule material for the prepared OMAC was 0.5 g/L,and the encapsulated amount of BTA reserved in OMAC was 2.1 mg/mg,about three times of that of SN in OMAC,0.69 mg/mg.The average diameter of the prepared OMAC was in the range of 200-400 nm（2）The pH-sensitivity of OMAC and release process of inhibition components reserved in OMAC were investigated.In aqueous solutions,the release process of BTA encapsulated in BTAC presented pH sensitivity:the release rate of BTA in the solution with lower pH values（pH≤9）was 9%higher than that in the solution with higher pH values（pH≥11）.The pH sensitivity of the release process was related to the higher protonation degree of oxygen groups in PEO moieties in lower pH environment.The release process of SN encapsulated in SNC was not sensitive to the pH alternations in aqueous solution;the reason is that a part of SN was adsorbed around the shell of SNC and the pH sensitivity of the PEO shell did not affect its release process.In simulated concrete pore（SCP）solutions,the pH sensitivity of BTA release process was more pronounced:the existence of calcium in SCP solution with a high pH value of13 is beneficial for both the formation of agglomerates among OMAC and the adsorption of sodium ion around OMAC,hindering the diffusion of BTA reserved in OMAC,which does exist in SCP solution with lower pH value（pH≤11）.The release amount of BTA for 3 days was5 times higher in SCP solutions with pH values lower than 11 than that in SCP solution with a pH value of 13.In cement mortar,C-S-H around BTAC was stable in the high alkaline（pH≥9）environment,and the release amount of BTA was lower than 20%;when the pH value of the environment reduced to 7,most of C-S-H decomposed,thus most of the reserved BTA（90%）was released.（3）The influence of BTAC on the corrosion behavior of steel bars was also investigated.When the rebar was immersed in SCP solution with 3.5 wt.%NaCl solution,the open circuit potential of steel bar in the solution with BTAC was approximately 200 mV higher than that of the control sample,which is to reduce the possibility of corrosion initiation.And the charge transfer resistance of steel bar in the solution with BTAC were also higher than that of the control sample,resulting in lower corrosion current density than that of control sample.After in the solution for 7 d,the inhibition efficiency of BTAC was 97%.When the rebar in cement mortar rebar immersed in 3.5%NaCl solution,the corrosion initiation time of the rebar was delayed by 50%by BTAC,and the corrosion current density was also reduced by BTAC.After in the solution for 135 d,the inhibition efficiency of BTAC was 80%.In both SCP solution and cement mortar,BTAC can remarkably enhance the resistance of the steel bar to the corrosion induced by chlorides.（4）The inhibition mechanism of BTAC on the corrosion damage of the steel rebar was revealed.In reinforced mortar,the inhibition mechanism of BTAC is related to the following three effects:（a）pH-sensitive effect.The local pH drop at the corrosion sites,due to corrosion initiation of steel reinforcement in the presence of chlorides,leads to the release of BTA from BTAC to form a[Fen（BTA）p]m protective film,increasing the charge transfer resistance of steel reinforcement and providing intelligent protection at the local pitting sites;（b）improvement of pore structure.BTAC can improve the pore structure of cement mortar and then reduce the permeability of Cl ions,delaying the corrosion initiation time of steel reinforcement;（c）adsorption effect.BTAC can adsorb on the steel surface to impede the migration and diffusion of water and aggressive ions,thus retarding the corrosion rate of steel reinforcement.（5）Finally,the adsorption characteristics of OMAC on cement particles,and their influence on the micro/macro properties of cement-based materials were also investigated.Almost all OMAC was adsorbed on the surface of cement particles within 30 s,and the distribution of OMAC was overall uniform in cement matrix,however they preferred to adsorb on the surface of the potassium-containing silicate phase and the aluminate phase.The steric effect provided by the adsorbed OMAC reduced the apparent yield stress and increased the fluidity of fresh cement paste.Further,OMAC retarded the hydration of C3A and particle dissolution,and reduced the yield stress and plastic viscosity of the slurry,thus reducing the fluidity loss of the fresh cement paste.In hardened cement paste,the steric effect of OMAC resulted in a more uniform distribution of cement particles,and nucleation effect of OMAC can act as the growing site of C-S-H due to its micro-nano size and surface compatibility.The above effects lead to a more uniform distribution of hydration products,leading to the increased compressive strength and reduced porosity of cement,thus impeded migration of chloride ions. |
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