Study On Gas Permeability Of Unsaturated Cementitious Materials

Gas permeability is an important indicator for assessing concrete durability,and pore structure is one of the main factors to determine the permeability performance.Studying pore structure and gas permeability of cementitious materials systematically and establishing concrete life prediction model based on gas permeability have both theoretical guiding significance and practical engineering value for improving the durability evaluation system and durability design level of structural concrete.Quantitative characterization of the pore structure of cementitious materials by modern micro-test techniques such as MIP and BET were studied in this work,and a new kind of method of using gas to measure porosity of porous materials was put forward.The gas permeability of cementitious materials under dry condition and at different water saturation were measured by quasi-stationary flow method,and a standard gas permeability test method was established.A bundle model based on MIP original data for predicting the gas permeability of cementitious materials was proposed.Several important research results are as follows:（1）The pore structure and gas permeability of cementitious materials are affected by water-cement ratio,aggregate and mineral admixtures.The intrinsic permeabilities of cementitious materials are between 5.03 × 10-18 to 1.08 × 10-16 m².The porosity and gas permeability are increase with the increase of water-cement ratio.For the same water-cement ratio 0.5,the permeability of concrete is smaller than cement paste while bigger than mortar.The replacement amount of fly ash within 30%and limestone powder within 10%can effectively reduce the gas permeability of mortar,while the replacement amount of limestone powder above 20%can increase the gas permeability of mortar.（2）Considering the pretreatment procedure and main test parameters of cementitious materials,a standardized gas permeability test method was established.Cylindrical samples with different water saturation were prepared by vacuum-saturated and dried at 60℃.The gas permeability test was conducted with confining pressure of 5 MPa and apparent gas permeability under inlet pressure ranging from 5 bar to 25 bar.Each pressure test lasted for 300 s.Finally,the intrinsic permeability was obtained by regression using Klinkenberg formula.（3）The intrinsic permeability of mortars ki has linear relationship withφMIPd2median/τGM,where φMIP,dmedian and τGM are porosity,median pore diameter and tortuosity.The relationship between relative gas permeability krg of cementitious materials and water saturation Sw can be fitted by VGM model.The intrinsic permeability ki and slippage factor b of cementitious materials satisfy the fitting relationship b=α×k_i^-βunder both dry and non-saturated conditions.（4）On the basic of original data from MIP,the bundle model,which takes into account the pore distribution of cementitious materials and different gas flow modes,can predict apparent permeability and intrinsic permeability of cementitious materials.The relative permeability at different water saturation is calculated by using the tortuosity amplification factor,which is in good agreement with the experimental values.