Time-Dependent Gas Permeability of Class C Fly Ash Concrete and Correlation with Its Microstructural Parameters

Based on the field tests of fly ash concrete with a maximum exposure time of 840 days, the effects of fly ash content and exposure time on gas permeability of concrete were studied. A time-dependent gas permeability coefficient model considering the influence of fly ash content was established. The pore structure parameters of fly ash concrete with different exposure times were measured by the nuclear magnetic resonance (NMR) method. In addition, the time-dependent relationship between microstructural parameters and gas permeability coefficient was discussed. Results show that adding 20%-40% fly ash into concrete can effectively reduce the gas permeability and improve its microstructural features after exposure of 360 days; concrete with 30% fly ash has the minimum critical pore size and average pore size, which have the most significant effect on reducing the gas permeability. With prolonged exposure times, fly ash concrete can increase (decrease effectively) the proportion of pore diameter of 10-50 nm (100-500 nm). Total porosity and critical pore diameter can better characterize the time-dependent gas permeability.