The Effect Of Supplementary Cementitious Materials Containing Al₂O₃ On Chloride Binding Ability Of Cement Paste And Its Mechanism

The influence of chloride ion on concrete has been a hot spot since the corrosion of steel bars in the reinforced concrete and the utilization of see sand and other chloride containing solid waste in concrete have attracted more and more attention. In these thesis, the effect of Al2O3-containing supplementary cementious materials(SCM), ground slag, fly ash and metakaolin, on the binding capacity of chloride ion of the cement paste and then the effect of alkali-activator on the chloride ion binding capacity of the cement paste were studied systematically, with the results as follows:Firstly, the effect of the replacement of cement with Al2O3-containing SCMs on the Cl- immobilization ability of the paste was studied. It was shown that at the same replacement of cement with SCMs, ground slag was the best in promoting the chloride ion immobilization ability of the paste among ground slag, fly ash and metakaolin. Furthermore, for all the three SCMs, the chloride ion binding ability of the paste after cured for 28 d increased with the increase of the replacement of the cement with the SCMs firstly and then decrease, with the maximum chloride ion binding ability of 3.71mg/g at the replacement of 50% cement with ground slag, 11% greater than the control sample, the maximum chloride ion binding ability at the replacement of 30% cement with fly ash, 2% greater than the control sample, the maximum chloride ion binding ability at the replacement of 40% cement with metakaolin, 4% greater than the control sample, respectively. The chloride ion binding ability was found to increase with the increase of the activated Al2O3 as a result of the increase of Friedel salt in the cement paste.Secondly, the effect of particle size, the co-existing sulfate, water/cement ratio and the concentration of chloride ion in the cement on the binding ability of chloride ion was also studied. It was determined that the binding ability of chloride ion increased with the decrease of particle size of fly ash, that with the fly ash of 800 mesh 2.4% greater than that with the fly ash of 200 mesh. The chloride binding ability was also found to increase with the rise of water/cement ratio, as the amount of C-S-H increased and accordingly the absorption of chloride ion was enhanced. The chloride binding ability the cement was tested to increase with the concentration of chloride ion in the cement, although the binding rate of the chloride in the cement decreased. The co-existed sulfate, however, was determined to decrease the chloride ion binding ability in the paste, with a drop of 21% to the control when the concentration of sulfate is 9.0mg/g cement paste,as the result of the inhabitation of SO42- to the formation of Friedel salt.Finally, the effect of the sodium silicate, NaOH, Ca(OH)2 on the ability to immobilize chloride ions was studied. The result showed that the chloride binding ability increase with the addition of Ca(OH)2 into cement-fly ash paste, as the result of the increase of the content of Friedel salt formed. The chloride ion ability of the paste with the addition of 2% Ca(OH)2 was determined to be 3.73mg/g paste after cured for 28 d, 1.1 times of that of the control sample. The chloride ion binding ability was observed to decrease when the addition of sodium silicate or NaOH increased in the cement fly ash paste, as the result of the decomposition of Friedel salt and the decrease of absorption of chloride by C-S-H in the cement.