Study On Mix Proportions And Chloride Resistance Of Alkali-Activated Concrete

As a kind of low-CO₂-emission cementitious material,alkali-activated material can effectively solve the problem of high CO₂ emission caused by the ordinary portland cement?OPC?.In recent years,alkali-activated material has been regarded as a new green building cementitious material with broad prospects for development.As a new building cementitious material,its long-term durability needs to be verified in long-term service environment.The damage of concrete is mostly caused by the corrosion of reinforcement steel,and the corrosion of chloride ion is the main cause of the corrosion of reinforcement steel.Therefore,the resistance to chloride ingress is an important index for evaluating the durability of concrete.The existing methods for measuring chloride transport of OPC include natural diffusion method and electro-acceleration method.The results show that for alkali activated system,the electro-accelerated test may cause errors due to the large number of ions in pore solution,but the test period is shorter.Although the natural diffusion test can avoid the errors caused by the applied electric field,the test is more complex and the test cycle is longer.Therefore,the objective evaluation of the results of different test methods becomes the key to accurately evaluate the chloride resistance of alkali-activated concrete.At the same time,at this stage,most of the existing studies on chloride resistance of alkali-activated materials are mostly pure mortar and mortar system.There are few studies on chloride ion corrosion resistance of alkali-activated concrete,and there are some contradictions in the research results.In this paper,the alkali-activated fly ash/slag binary concrete is studied.Firstly,the effects of fly ash/slag ratio,water–binder ratio,alkali concentration and sand–aggregate ratio on the early performance?including slump,7-day and 28-day compressive strength and water absorption?of alkali-activated fly ash/slag concrete were studied.As a result,a series of mix proportions with early performance meeting the relevant standards of concrete are obtained.On this basis,non-steady-state electromigration test and natural diffusion test were used to study the chloride ion corrosion resistance of alkali-activated fly ash/slag concrete.After that,the chloride resistance of alkali-activated concrete with different fly ash/slag ratios,water–binder ratios,alkali concentrations and sand–aggregate ratios were emphatically studied.In addition the pore structure of alkali-activated fly ash/slag concrete was studied and the mechanism of chloride resistance was discussed.Besides,the similarities and differences of the two test methods in reflecting the chloride resistance of alkali-activated materials were analyzed and compared.The results show that the early performance?slump,compressive strength?and water absorption of alkali-activated fly ash/slag concrete are greatly affected by fly ash/slag ratio,water–binder ratio,alkali concentration and sand–aggregate ratio.With the increase of slag content in the composite system,the fluidity of alkali-activated concrete becomes worse,but the strength increases obviously in 7 and 28 days,the microstructure becomes denser and the water absorption decreases.With the increase of water–binder ratio in the binary system,the fluidity of alkali-activated concrete becomes better,its strength decreases in 7 and 28 days,its microstructure becomes more loose and its water absorption increases.With the increase of alkali concentration in the activator,the fluidity of alkali-activated concrete has no obvious change.The strength of alkali-activated concrete decreases slightly in 7 and 28 days,the compactness of microstructure increases,and the water absorption decreases.With the increase of sand aggregate ratio of concrete,the fluidity of alkali-activated concretepresents bell-shaped distribution,which increases at first and then decreases.The change of sand ratio has no obvious effect on the strength of alkali-activated concrete for 7 and 28 days.However,due to the increase of fine aggregate,the specific surface area of aggregate increases,the more water needed to wet aggregate,and the water absorption increases.The chloride resistance of alkali-activated fly ash/slag concrete is greatly affected by fly ash/slag ratio,water–binder ratio and alkali concentration.But,the change of concrete sand–aggregate ratio has little effect on chloride resistance of alkali-activated concrete,and there is no obvious rule.With the increase of slag content in the binary system,the porosity of the system decreases,the pore structure refines,the harmless capillary pore increases,the density of the system increases,and the chloride resistance increases.With the increase of alkali concentration in the activator,the porosity of the system decreases,the harmless capillary pore increases,the compactness of the system increases,and the chloride resistance to chloride increases.With the increase of water–binde mass ratio in the composite system,the porosity of the system increases,the harmful capillary pore increases,the compactness of the system decreases,and the chloride resistance decreases.The non-steady-state migration coefficient of alkali-activated fly ash/slag concrete is in the same order of magnitude as the natural diffusion coefficient,but the natural diffusion coefficient is smaller than the non-steady-state migration coefficient.The influence of fly ash/slag ratio,water–binder ratio and alkali concentration on the non-steady-state migration coefficient is consistent with the natural diffusion coefficient.There is a strong linear relationship between the non-steady-state migration coefficient and natural diffusion coefficient.