Study On Gas Permeability Of Cementbased Materials

Gas permeability has been widely used as a durability indicator in the field of civil engineering.On one side,gas transport in cement-based materials is the basis of durability issue.On the other side,gas permeability also reflects materials’ capacityof resisting external media intrusion.This paper studied the permeation law and correspondingtheories and models were developed.Moreover,standard gas permeability test methods were also discussed.Firstly,the basic flow modes of gas transport in porous materials,which include viscous flow,boundary slip flow and Knudsen flow,were investigated.Then the paper analyzed the impact of pore size,pressure,temperature and seepage gas species on flow modes,and established the parallel bundle model which considered pore size distribution.Finally,the research built a constitutive equation of gas transport in porous materials and the equation was used to simulate the gas flow.Following frequently-used cement-based materials,hardened cement pastes(HCPs),mortar andconcrete specimens with different w/b ratios(0.3,0.4 and 0.5)were prepared,and the binder of concrete included 100%Portland,70% Portland+ 30% fly ash,50% Portland+ 50% clinker.The pore structure of materials was characterized respectively by specific gravimetry method,mercury intrusion porosimetry(MIP)method,water isothermal adsorption experiment and surface scanning test.The influence of supplementary materials on porosity,characteristic pore diameter,pore size distribution and pore fractal dimension was studied.Furthermore,the gas permeability at different water saturation(W100,W80,W60,W40,W20 and W0)was also measured,then intragroupand intra-group errorwas analysed.With the use of parallel bundle model,the study quantified the contribution of intrinsic flow,boundary slip flow and Knudsen flow to apparent gas permeability.In the quantification process,the intrinsic permeability values kept agreement with the test ones and the pore size distribution was characterized by multi-peak gaussian distribution.The results showed that:(1)Under the non-steady state,the difference in simulation result and actual caseof the gas equilibrium process gradually increased with the improvement of saturation degree;(2)The error of intragroup is larger than that of introgroup.At dry state,intra-group error could be controlled within 30% and intro-group 10%.But the former became greater with the increase of pore saturation,and the latter was still not obvious;(3)parallel bundle model was established.Compared with experimental values,the error of parallel bundle model calculation was within 10% for material at drystate and 20% at wetting state;(4)The research derived moisture film and gaseous pore distribution curves of wetting materials by taking critical pore diameter and film thickness into consideration,and tortuosity amplification factor expressions was also established.