Experimental Study On Mechanical Properties And Chloride Ion Penetration Resistance Of Recycled Fine Powder Concrete

Since the beginning of the 21 st century,the construction waste has increased year by year.The contradiction between the low rate of resource treatment and the increasing shortage of construction resources will seriously hinder the development of urbanization.In this study,the recycled fine powder produced in the process of resource utilization of construction waste was taken as the research object,and the physical and chemical activity excitation of recycled fine powder was studied.The effects of replacement rate and curing age of recycled fine powder on the mechanical properties and chloride ion penetration resistance of recycled fine powder concrete（RC）and alkali-activated recycled fine powder concrete（ARC）were investigated.Combined with nuclear magnetic resonance（NMR）test,the microscopic pore structure characteristics were analyzed,and the grey prediction GM（1,4）model of concrete mechanical properties（compressive strength,flexural strength）and pore structure parameters was established to improve the utilization rate of recycled fine powder.Reducing the environmental pollution caused by traditional cement production is in line with the national sustainable development strategy of’carbon peak,carbon neutralization’,which has important academic research significance and practical engineering application value.The main conclusions of the study are as follows:（1）Mechanical ball milling can effectively stimulate the activity of regenerated micropowder,and the excitation effect gradually increases with the increase of ball milling time.Among the activators,when the content of Ca（OH）₂,Na₂SO₄,Na OH and Na₂Si O₃·9H₂O is 2.5%,3.5%and 1.5%respectively,the mechanical properties and activity index of recycled fine powder mortar are the highest.The activation effect of the three activators:2.5%Ca（OH）₂>1.5%Na OH and Na₂Si O₃·9H₂O>3.5%Na₂SO₄.（2）With the increase of recycled powder content,the mechanical properties of RC and ARC increased first and then decreased,and the total electric flux of RC and ARC decreased first and then increased.When appropriate amount of recycled fine powder（10%）is added,the secondary hydration reaction and micro-aggregate filling effect of recycled fine powder are beneficial to reduce the porosity of concrete and improve the mechanical properties and chloride ion penetration resistance of concrete.Excessive incorporation of recycled fine powder（30%）will lead to an increase in porosity of concrete and a decrease in compactness,thereby reducing the mechanical properties and resistance to chloride ion penetration of concrete.（3）The incorporation of an appropriate amount of Ca（OH）₂ is beneficial to stimulate the activity of recycled fine powder,improve the pore size distribution of concrete,reduce the porosity,and increase the compactness,thereby improving the mechanical properties and resistance to chloride ion penetration of ARC.With the increase of curing age,the recycled fine powder is fully hydrated,the pore size in RC and ARC is further refined,the concrete structure is more compact,and the mechanical properties and resistance to chloride ion penetration of concrete are improved.At the same time,the effect of alkali excitation on the mechanical properties and chloride ion penetration resistance of concrete is getting higher and higher.The compressive strength and flexural tensile strength of recycled micro-powder concrete are in the same order of grey entropy correlation degree of NMR pore characteristic parameters,that is,harmless pore>porosity>more harmful pore>less harmful pore>harmful pore.（4）The average relative errors of the GM（1,4）model for predicting the compressive strength and flexural strength of recycled fine powder concrete are 1.519%and 1.423%,respectively,and the accuracy is grade 2.The posterior error ratios are 0.189 and 0.255,respectively,and the accuracy is one level.The predicted GM（1,4）model can accurately predict the mechanical properties of recycled fine powder concrete.The correlation coefficient R² of the power function prediction model between the total electric flux and the compressive strength of recycled fine powder concrete is greater than 0.97,and the average relative error between the predicted value and the experimental value is within 3%,which has high prediction accuracy.