Study On Preparation Technology And Self-healing Properties Of Composite Structure Microcapsules

As the most important building material in infrastructure construction,concrete materials has been paid much attention to the cracking problem during service.If the micro-cracks in the structure can’t be repaired in time,it will provide a fast channel for the external corrosive substances,accelerate the corrosion of steel bars,and lead to structural damage.The microcapsule cement-based self-healing material can promptly sense and quickly repair the damage in the structure,and prevent the potential hazards caused by cracks in the initial stage,which is of great significance for improving the durability of concrete,structural safety and service life of the structure.In this paper,a new structure of self-healing microcapsule was developed by optimizing the core,wall materials,designing the structure of the capsule and carrying out laboratory granulation film-forming experiments.Based on the prepared capsule,the comprehensive properties of its own morphology,water resistance and hardness were characterized,and its particles were mixed into the cement matrix to explore the effects of the capsule on the mechanical properties and durability of the self-healing material.Combined with three indexes of crack width,water permeability and compressive strength recovery rate,the effects of capsule particle size,content,hardening time of inner capsule wall and thickness of outer capsule wall on the repair effect of matrix structure were evaluated,and the optimal capsule configuration parameters were determined.By means of SEM-EDS,XRD and X-CT detection techniques,the micromorphology and chemical composition of the repair material were ascertained,and the repair mechanism inside the matrix was revealed.The visualization and non-destructive tracking of the self-healing process was realized.The main conclusions of the study include:(1)The core formula of self-healing capsule is DM3,which consists of MgO expansive agent,Portland cement and silica fume;the inner capsule wall material is fast-hardening cement;the outer capsule wall material is GC2-P polyvinyl alcohol,and the mass fraction of coating solution is 5%.(2)The two types of capsule wall are smooth spherical,and the particles have certain hardness and good water resistance.Under the same experimental conditions,the single-layer wall structure capsule has lower water absorption and greater hardness,but the core component is easy to react with the external water in advance,so the overall performance of composite structure microcapsules is better.The results show that the hardness value and water absorption of the composite structure capsules increase with the increase of particle size.The capsules with particle size range of 0.5-4 mm and content of 10%-20%can keep the shape intact,distribute evenly and crack in time after incorporation into the matrix.(3)The addition of capsules is beneficial to improve the impermeability and frost resistance of cement-based materials,but is not conducive to the improvement of the carbonation resistance and chloride ion erosion resistance.The compressive strength and flexural strength of the specimens with capsules are slightly lower than those without capsules,but the decrease range is within the engineering allowable range.In consideration of minimizing the adverse effects,the capsule should be selected with the particle size range of 1.6-4.0mm and the dosage of 10%.(4)With the increase of repair age,the degree of micro-crack healing increases,the relative permeability coefficient decreases rapidly at first and then increases slowly,and the compressive strength recovery rate increases as a whole.The smaller the initial crack width of cement matrix,the more repair agents around the crack and the faster the release of repair materials,the higher the repair degree,the smaller the relative water permeability,and the better the recovery rate of compressive strength of the specimen.(5)Combined with the influence of capsule incorporation on the matrix properties and the influence of four factors on the repair effect of the material,the best parameters of the composite structure capsule for self-healing materials were determined as follows:the inner capsule wall is hardened for 1h,outer capsule wall thickness is about 0.15mm,the capsule diameter is 2.36-3.2mm,and the content is 10%of the mass of sand.(6)Based on the results of theoretical analysis,microscopic morphology-element analysis and phase composition analysis,the determination of the composition of the repair substances was basically consistent and accurate.The products formed by self-healing composite structure capsule are mainly calcium carbonate and C-S-H gel,ettringite and anorthite.(7)The X-ray CT results showed that after curing for 28 days,the capsule content in the matrix decreased from 10.3%to 5.02%,and the crack volume decreased from 1.61%to 1.09%.The repair agent in the capsule is used to fill the micro-cracks,making the crack repair rate up to 32.3%,and the repair effect is remarkable.