Research On Mixing And Air Entraining Characteristics Of Cement-based Materials In Low Air Pressure Environment

Most of the Sichuan-Tibet Railway was built in Plateau areas,freeze-thaw damage of concrete is considered to be the primary problem of concrete durability in Plateau area,and the use of air-entraining agent is considered as the main measure to improve the frost resistance of concrete.In high altitude with low atmospheric pressure environment,it is difficult for concrete to air entrainment during mixing.However,little research has been done on the fresh paste which mixed at low pressure for air entrainment whether at home or abroad,and it is difficult to form a theory for guiding practical projects.Therefore,this paper simulates the plateau low-pressure environment through a low-pressure environment test box.By changing the mixing speed,mixing time,types and dosage of air entraining agent,the change rules of gas content in cement-based materials in low air pressure environment are researched.Some innovative results are as follows:(1)The changes of cement mortar mixing power with time in different air pressure were investigated.It is found that the mixing power and work at low pressure are higher than those at normal pressure.When air pressure decreased from 101 kPa to 50 kPa,the work increased more than twice(2)The influence of air pressure on the bubble morphology parameters of cement mortar was investigated.It was found that the bubble spacing coefficient and average diameter increased with the decrease of air pressure,and the number and density of bubbles decreased with the decrease of air pressure.The difference of the number of bubbles between ordinary pressure and low air pressure was concentrated in the range of 10-20?m.(3)The influence of air pressure on the performance of air entraining agent was investigated.It was found that the air entraining ability of triterpenoid saponin was weakened the most and that of sodium dodecyl sulfate was weakened the least;the air entraining ability of triterpenoid saponins was the worst at the same dosage,and the bubble diameter was concentrated in 10-20?m,while that of sodium dodecyl sulfate was better,and the bubble diameter was similar to that of sodium secondary alkyl sulfonate,ranging from 10-100?m.(4)The influence of air pressure on the loss of air in cement mortar was investigated.It was found that in different air pressure,the loss of air in cement mortar with air entraining agent was concentrated in 5-10 minutes,and the loss of air in cement mortar with triterpenoid saponins was the smallest.In addition,the lower air pressure was,the smaller the loss ratio of air in cement mortar was.(5)The influence of mixing process on the gas content of cement mortar was investigated.It was found that the gas content increased first and then decreased with mixing speed,while the spacing coefficient and average diameter of air bubbles increased with mixing speed,and the number and density of air bubbles increased slowly first and then decreased sharply with mixing speed.Variation of gas content in cement motar with different kinds of air entraining agents with mixing time is different.