Study On Hydrophobic Modification And Durability Of Cement-Geopolymer Composites

In the northwest of China,there exists a large difference in temperature between day and night,besides,the salinization of land is severe,thus the concrete structure in this area is often damaged by freeze-thaw cycles and chemical media erosion,causing internal cracking and peeling of the concrete skin,which will seriously affect the durability of the concrete structure,therefore,the concrete structure needs to be repaired in time.As a new type of green inorganic cementitious material,geopolymer is increasingly used as a repair material for concrete structures owing to its low energy consumption,high early strength and chemical resistance compared with traditional cement-based repair materials.However,both geopolymers and cementitious materials are hydrophilic,and water is the main medium for frost swelling and corrosive ion erosion of concrete structures,so it is of great engineering significance to study the hydrophobic modification of geopolymer repair materials to improve the waterproofing ability of repair materials.In this thesis,cement-geopolymer composites were prepared with silicate cement and slag as matrix,polydimethylsiloxane(PDMS)as hydrophobic agent,silica fume and polypropylene(PP)fiber as reinforcing and toughening materials,the mechanical properties,water resistance and durability(resistance to freeze-thaw cycles,sulfate erosion and sand erosion)of cement-geopolymer composites were tested,and the experimental results were analyzed in the context of physical phase and morphological characterization.The main research contents and conclusions of the paper are as follows:(1)The effect of PDMS dosing on the performance of cement-geopolymer composites was investigated.The experimental results indicate that with the increase of PDMS content,the overall mechanical properties of the specimens show a decreasing trend,while the waterproofing properties keep improving,when the PDMS content is 1.2 wt%,the static contact angle of the specimen surface reaches 113°,the static contact angle of the fracture surface reaches 134°,and the water absorption rate decreases by 28.2%.(2)The effect of silica fume admixture on the properties of cement-geopolymer composites was investigated.The results of the mechanical properties experiments suggest that the compressive strength of the specimen increases significantly when the silica fume comtent is4 wt%,and the 28 d compressive strength increases by 24.0% compared with that of the blank group.The experimental results of water repellency illustrate that the static contact angles of the specimens are not affected by silica fume,while the water absorption rates decrease continuously with the increase of silica fume content.The durability test results show that the specimen exhibits the best performance in freeze-thaw cycle resistance,sulfate erosion resistance and sand erosion resistance when the content of silica fume is 4 wt%.Considering comprehensively,the optimum content of silica fume is 4 wt%.(3)The effect of PP fiber doping on the properties of cement-geopolymer composites was investigated.The results of the mechanical properties experiments demonstrate that the toughness of the specimen is significantly improved when the PP fiber content is 0.4 wt%,and the 28 d flexural strength is increased by 20.7% compared with that of the blank group.The experimental results of water repellency manifest that the static contact angle of the specimens are not affected by the PP fiber;the water absorption rates of the specimens decrease slightly when the PP fiber doping was lower than 0.6 wt%.The durability experimental results show that the specimen owns the best freeze-thaw resistance when the content of PP fiber is 0.2 wt%;the specimen possesses the best sulfate erosion resistance and sand erosion resistance when the content of PP fiber is 0.4 wt%.Considered together,the best doping content of PP fiber is 0.4 wt%.(4)The effect of blending PP fiber and silica fume on the properties of cement-geopolymer composites was investigated.The results of the mechanical properties experiments indicate that the compressive strength of the specimen increase notablely when the PP fiber content is0.4 wt%,and the 28 d compressive strength increases by 81.7% compared with that of the unmodified specimen.The results of waterproof performance experiment prove that the water absorption rates of the specimens are further reduced by mixing PP fiber and silica fume together.The durability experiments imply that the durability of the specimens are best when the PP fiber content is 0.4 wt%.By comparing the effects of several modifications on durability,it is discovered that the resistance to sand erosion is remarkably improved in the specimen modified by blending PP fiber and silica fume,but the resistance to freeze-thaw and sulfate erosion are not as good as that of the specimen modified by PP fiber alone.