Effect of chemical admixtures on conductivity of mortars.

摘要

The effect of available commercial chemical admixtures namely ASTM Types A, C, D, E, F and AEAs on the electrical conductivity of mortars was investigated using modified electrical technique. Interpretation of the electrical conductivity (reciprocal of resistivity) of mortars was combined with results from calculating Na+ and K+ ion concentrations in the pore solution of mortars, measuring non-evaporable water content, gravimetric water porosity as well as conductivity of chemical admixtures. The water to cement ratios used were 0.485 and 0.35 with a sand to cement ratio fixed to 2.75. The dosage of chemical admixtures varied from low, medium and high in accordance with the indication by manufacturers.; The results showed that the different types of chemical admixtures exhibited different effects on the electrical conductivity of mortars. In the high water to cement ratio condition, the conductivity values of mortars with added Types A, D and F chemical admixtures were raised due to alteration of the pore solution chemistry, severe set retardation and increase in porosity. The high conductivity of mortars with Type C chemical admixtures was due to the introduction of a large amount of strong electrolyte Ca(NO2)2 to the pore solution which overwhelmed its accelerating hydration rate. On the contrary, the high degree of hydration of mortars with added Type E chemical admixtures offset their high alkali concentration in the pore solution. In the low water to cement ratio condition, the presence of types A, D, E and F chemical admixtures decreased the conductivity of mortars which was attributed to deflocculation, improving the microstructure and reducing porosity. Appropriate addition of AEAs reduced the conductivity of mortars in low and high water to cement ratios owing to the entrained micro-air bubbles in mortars separating the paths of conductivity.