Study On Preparation And Properties Of Metakaolin-based Geopolymer Coating Material For Marine Concrete Protection

The durability of marine concrete can not be ignored because it is used in the complex marine environment of dry wet cycle,freeze-thaw,carbonation and multiple ion erosion for a long time.Combined with the damage and failure mechanism of marine concrete,relevant scholars have carried out the researches on the performance modification of marine concrete,the development of protective coating materials,and the establishment of relevant prediction models,in order to effectively extend its service life.As one of the key technologies for prolonging the life of marine concrete,the development of protective coating is very important.Most of the existing coating materials are organic protective materials.The potential aging problems of organic coating materials and the secondary pollution to the environment are increasingly serious.On basis of the excellent durability of geopolymer and its good bond performance as pavement repair material,the introduction of geopolymer into the field of marine coating has become a new research direction.In this study,metakaolin based geopolymer is selected to prepare coating material for marine concrete protection,which is a new technical measure to improve its durability.The effects of supplementary cementitious materials on the reaction kinetics and mechanical properties of geopolymer were investigated via Isothermal calorimetry,setting times and mechanical performances tests.The relationships between the supplementary cementitious materials and the freezing resistance,carbonization resistance,sulfate attack and anti chloride corrosion properties of geopolymers were detected.The optimizing mechanism of the durability of geopolymer was analyzed by MIP,SEM,XRD and BET techniques.The influences of supplementary cementitious materials on the bonding performance of geopolymer were analyzed by means of slant shear,three-point bending,tensile and splitting tests.The bonding mechanism between geopolymer and cement stone was preliminarily explored on basis of the results of ion leaching,micro-morphology and element distribution.The main conclusions of this paper are drawn as follows:（1）The addition of fly ash,rice husk ash and slag as supplementary cementitious materials can effectively improve the mechanical properties of geopolymer.The optimum content of slag used for geopolymer synthesis in this study was 30 wt%.At this level,the compressive strength and flexural strength of metakaolin-slag（M-S）geopolymer were 63.6 MPa and 10.7 MPa respectively,which gained 42.28%and21.59%when compared with the control sample.The optimization effect of fly ash and rice husk ash on geopolymer was comparable.The optimum usage of fly ash in M-F system was 30 wt%,while it of rice husk ash in M-R system was 20 wt%.It was found that the introduction of supplementary cementitious materials promoted further geopolymerization and shortened the setting time,which was related to the high activity of supplementary cementitious materials.Moreover,the improvement of mechanical properties of geopolymers was mainly due to the deepening of geopolymerization.（2）The freezing resistance of geopolymer was closely related to its pore structure.The introduction of supplementary cementitious materials optimized the pore structure of geopolymer,and reduced the volume of harmful pores and multiple harmful pores in the structure.The losses of mass and strength of geopolymer during the freeze-thaw cycles were decreased,and the freezing resistance of geopolymer was improved.The white floccules mainly composed of sodium carbonate and calcium carbonate were formed on the surface of geopolymer samples during the carbonization process.Furthermore,SEM test showed that the original compactness of geopolymer microstructure was destroyed by carbonation,which was the main reason for the strength reduction of geopolymer.According to the pore structure of geopolymer,the porosity of the sample was of great significance to the development of its carbonation resistance.The porosity of geopolymer was dramatically decreased,and the carbonation resistance of geopolymer was significantly improved with the addition of slag.（3）Geopolymers exhibited excellent sulfate resistance,which was mainly attributed to the extremely low calcium content in geopolymers.The supplementary cementitious materials promoted the densification development of geopolymer and improved its micro-properties.It should be noted that the ettringite was easy to generate due to the high calcium content in slag and consequently to damage the microstructure,so the dosage should not be too high.The chloride diffusion coefficient of metakaolin geopolymer was 5.022×10^-12 m²/s,reflecting its good resistance to chloride erosion.The supplementary cementitious materials can effectively reduce the chloride diffusion coefficient of geopolymer and improve the chloride corrosion resistance of geopolymer,which was mainly related to the densification development of geopolymer structure and consequently to the reduction of chloride transport and diffusion channels.（4）The results of mechanical properties of bonding body indicated that a good bond effect was generated between geopolymer and cement stone.The supplementary cementitious material can effectively improve the bond performance of bonding body.Based on the results of ion leaching in alkali solution and the distribution of elements in the transition zone of bonding body interface,it can be found that the recombination of chemical bond was happened at its interface,forming a more stable chemical bond.The improvement of bond performance was mainly due to the enrichments of the composition of raw materials and increasing of reactivity brought by the introduction of supplementary cementitious material,optimizing the interface binding capacity of bonding transition area.