Study On Transport Mechanism And Pore Structure Of Alkali-activated Slag Concrete In Freeze-thaw And Chloride Composite Environment

Alkali-activated material is a kind of green binder material,with significance on environmental protection,energy conservation and waste utilization.And is considered as a substitute for ordinary portland concrete in the future.The problem of durability of concrete based on alkali-activated material has become a hot issue.There are many research results on alkali-activated material,but there are few systematic studies on its frost resistance and post-freezing permeability.In this paper,the slag as the main cementitious materials,the use of seven groups of alkali-activated slag concrete,aimed at the study of alkali-activated slag concrete frost durability and post-freezing permeability.The main contents and conclusions are as follows:(1)The proportion of alkali-activated slag was optimized by orthogonal test,and the ratio of 7 groups was determined in combination with workability,which was the basis of the follow-up test.The influence of alkali equivalent(5%,7%,9%),water glass modulus(M=1.8,M=2.0)and slag dosage on the compressive strength of alkali-activated slag concrete was studied respectively in the optimization process of Mix proportion.Three factors were considered.The results show that when the alkali equivalent is 9%,the water glass modulus is 2,and the slag dosage is 350kg/m~3,the compressive strength of the prepared alkali-activated slag concrete is the highest.(2)Frost resistance of alkali-activated slag concrete in water was studied by rapid freezing method.The quality of water loss after freezing and relative dynamic elastic modulus evolution,using water glass as activator of alkali-activated slag concrete frost resistance is superior to NaOH solution as the alkali-activated slag concrete activator;20%in fly ash against alkali-activated slag concrete of antifreeze.(3)The freeze-thaw damage of alkali-activated slag concrete in 5%NaCl solution was studied.The results show that alkali-activated slag concrete with sodium silicate as activator is better than alkali-activated slag concrete with NaOH solution as activator;20%internal fly ash is disadvantageous to alkali-activated slag concrete frost resistance.Compared with water freezing,due to the presence of salt,the concrete frost heaving is aggravating,thus exacerbating the freeze-thaw damage,but the water glass as the alkali-activated slag concrete activator can still meet the engineering requirements of frost resistance.(4)The capillary water absorption properties of alkali-activated slag concrete after water freezing and salt freezing and the chloride ion transfer performance after water freezing were studied.The results show that with the increase of freeze-thaw cycles,the internal damage degree of concrete specimen increases.The capillary absorption coefficient of alkali-activated slag concrete increases,and the diffusion coefficient of chloride ion also increases.It is observed by electron microscope that with the increase of freeze-thaw cycles,the internal cracks of alkali-activated slag concrete increase.(5)Pore structure of alkali-activated slag concrete was studied by mercury intrusion test and nitrogen adsorption test.The results show that the pore size of alkali-slag specimens is mostly distributed below 100nm,and the pore size increases after freezing is not too obvious,which also shows the excellent frost resistance of alkali-activated slag concrete.