Study On Determination Of Chloride Method And Influence Factors Of Chloride Binding Of Cement

Chloride is an important factor to cause corrosion of rebar in concrete. Chloride ion in concrete exist as combination or free state. It is generally recognized that only the free chlorides are responsible for depassivating the reinforcement steel. That is, the more bound chloride ions, the smaller corrosion effect of chloride ion on the steel. The capacity of cement binding chloride is affected by many factors, such as the type of cement, auxiliary cementitious materials, carbonation, chloride, sulphate etc,especially sulphate.The existing forms and types of sulphate in cement are complex, therefore, this paper focuses on the effects of seven kinds of sulphate: sodium sulfate(N), potassium sulphate(K), sodium potassium sulphate(KN), potassium calcium sulphate(KC), dihydrate gypsum(CH) and anhydrite(C) existing in cement additional magnesium sulphate(M), expounding respectively the influence of them on the capacity of cement binding chloride ion.Studying on the capacity of cement binding chloride ion, the most important thing is determining chloride ion concentration accurately, so the test method is the key. Compared with Mohr method, ammonium thiocyanate method is more suitable. Due to the complexity of the cement system, ammonium thiocyanate method is optimized to guarantee the accuracy and precision of the results. The concentration of total chloride ion and free chloride ion is determined in cement admixed different types and dosages of sulphate by optimized ammonium thiocyanate method. The proportion of combining chloride ions in the total chloride ions represents the capacity of cement binding chloride ion, that is, the more the proportion, the greater the capacity. This paper also expound the differences of the cement performance between the cement combining chloride ion and the cement both added sulfate and combining chloride ion. The cement performance is analysed by micro-XRD and electrochemical impedance spectroscopy techniques.The results show that the accuracy of ammonium thiocyanate method is higher than Mohr method, but ammonium thiocyanate method also have some shortages, such as consuming too much time, poor accuracy. So ammonium thiocyanate method is optimized to guarantee the accuracy precision and suitable consuming time by adjusting experiment procedures、concentrate on of reagents、concentration of standardizing reagents. At the same time, it is easier to judge the end point of titration by adjusting preparation procedures of ammonium ferric sulphate indicator. The results obtained by using the optimized ammonium thiocyanate method is more stable and accurate.The study found that, in a certain range of concentration of chloride ion in cement, the greater the concentration of chloride ion, the more capacity of cement binding chloride ion. Adding 1% and 0.4% chloride to cement, we found that the chloride binding of cement increase with the increase of chloride ion concentration; Adding 1% and 0.4% chlorine ion to cement admixed sulphate which are the same dosage(refer to the dosages of sulfur trioxide) but different types, we found that the chloride binding of cement increase with the increase of chloride ion concentration and the age of the cement.Sulphate is also an important factor affecting on the capacity of cement binding chloride ion and different kinds of sulphate have different effects. The general rule is that, alkali metal sulphate(N, K, KN and KC) have greater negative impact on the capacity of cement binding chloride ion. than alkaline earth metal sulphate(CH, C and M), alkali metal sulphate salt(KN) have greater negative impact than alkali metal sulphate(N, K)and magnesium sulphate have greater negative impact than natural anhydrite and gypsum; Different dosages of one sulphate have different impact on the capacity of cement binding chloride ion and with the increase of the amount of sulphate, the capacity of cement binding chloride ion decrease gradually and the degree of the decrease is also different; The capacity of cement with the same type and dosage of sulphate binding chloride ion increases with age of cement.