Influence Of New Type Non-alkali Accelerating Admixture On Concrete Performance And Pore Structure

In tunnel and underground engineering,shotcrete is often used for support and reinforcement.In order to meet the high requirements of Shotcrete for quick setting,early strength and quick hardening,the development of special admixture for shotcrete,especially the accelerator,is particularly important in the process of shotcrete technology development.The traditional alkaline accelerator has been gradually eliminated due to its high dust,high rebound,corrosiveness and large strength loss in the later stage,while the non alkaline liquid accelerator has been widely used in the world due to its advantages of low alkalinity and high strength retention rate in the later stage.However,due to the late introduction of wet spraying technology in China,the research on alkali free liquid accelerator is still insufficient.The purpose of this paper is to explore the influence of the new alkali free liquid accelerator on the performance and pore structure of concrete.Firstly,through the study of performance,it is proved that compared with the traditional alkaline accelerator,the new alkali free liquid accelerator has less or almost no effect on the later strength,shrinkage and frost resistance of concrete.Secondly,by testing the pore structure of cement mortar mixed with accelerator,the influence of alkali free accelerator on concrete performance is analyzed from the micro level.The results show that the initial setting time of cement paste is 4min50 s and the final setting time is 6min20 s when the dosage of the new alkali free accelerator is 8%;When the dosage is 10%,the initial setting time is 2min 10 s and the final setting time is 5min30 s.The flexural strength and compressive strength of mortar specimens with different amount of alkali free liquid accelerator were tested at different ages.The results show that the 1D strength of the samples is higher than that of the blank group,which shows the qualified effect of quick setting and early strength.The measurement results of late strength show that the late strength loss of alkali free liquid accelerator is greatly improved,especially when the dosage is 8%.The shrinkage of mortar specimens with different amount of non alkali liquid accelerator was measured within 28 days.The shrinkage of the experimental group with non alkali liquid accelerator was significantly lower than that of the control group,and the later shrinkage tended to be stable,Therefore,it can be inferred that the traditional alkaline accelerator aggravates the shrinkage of concrete,which increases the possibility of cracks in concrete to a certain extent and leads to the loss of later strength.The frost resistance test results of mortar specimens with different amount of alkali free liquid accelerator also prove that the addition of accelerator will not reduce the frost resistance of concrete.Mercury intrusion method and nitrogen adsorption method were used to test the pore structure of cement mortar.The test results show that alkali free accelerator can significantly reduce the porosity of cement paste and reduce the large-scale pores in cement paste.The total porosity of cement paste with alkali free accelerator is only 0.76%higher than that of blank group at 28 d,and even 0.08% lower than that of blank group at90 d,The average pore radius at 90 d is only 11.5nm,which proves that alkali free accelerator has little adverse effect on the pore structure of concrete.