The Chloride Ion Penetration Performance Of Double-Mixed Fly Ash And Slag Concrete

Chloride ion is the major cause for premature failure of concrete constructions in complex marine environment.For this reason,a lot of money has been invested in the reinforcement of damaged concrete structures.Therefore,reducing chloride ion permeability in concrete has become an urgent problem for offshore structures.Based on the crossover design,the effects of the water binder ratio（0.35,0.425,0.5）,curing age（3 d,7 d,280 d）,particle size matching of mineral mixtures（Ⅰ and Ⅱ grade fly ash,S95 and S105 slag）and dosage（40%,60%,80%）on chloride penetration were explored.The concrete samples were put into 3.5%NaCl solution for 28 d,60 d,90 d,120d and 180 d,and then the chloride ion distribution was determined.Grey relational analysis was used to assess the impact of four factors on chloride diffusion.Scanning electron microscope-energy dispersive spectroscopy（SEM-EDS）,X-ray diffraction（XRD）and differential scanning calorimetry-thermogravimetry（DSC-TG）were utilized to analyze microstructure and chemical composition of hydration products.Besides,the reason of low content of binding chloride ion is long curing age or test error.Therefore,soda residue with high content of chloride ion was used as alkali activated material to study the effect of dosage on the binding chloride ion.The results show that the water binder ratio,as the most influential factor on chloride ion permeability,has a negative correlation with the trend of concrete impermeability.The long curing time causes its impact second only to the water-binder ratio,and the longer the curing age is,the better the impermeability is.Too low or too high dosage is detrimental to the impermeability of concrete,and the optimal dosage is 60%.By testing particle size distribution,fluidity of paste,slump and expansion of concrete,the relationship between matching degree and impermeability is established.It is found that the matching degree and impermeability between Ⅰ grade and S95 are the best,while that between Ⅱ grade and S95 is the worst.The contents of Ca（OH）₂and Ca CO₃in each group were quantitatively analyzed by XRD and DSC-TG to reflect the progress and carbonization degree of pozzolanic reaction of slag and fly ash.It was found that the more Ca（OH）₂consumed by pozzolanic reaction,the stronger the impermeability.SEM images reveal that the microstructure is related to the impermeability.The dense microstructure with almost no cracks leads to better impermeability.Slag and fly ash with 60%content give full play to their pozzolanic activity,and have better impermeability and chloride ion binding ability.SEM found that the Friedel’s salt generated by binding chloride ion is wrapped by amorphous C-S-H gel,which is beneficial to the impermeability of concrete.