Research On Preparation And Properties Of Recycled Concrete Powder Composite Admixture

With the continuous advancement of urbanization in China,the annual output of construction waste in China is increasing year by year.At present,the main treatment methods of construction waste are still placement and landfill,which wastes a lot of resources and causes secondary pollution to the surrounding environment.It is a difficult problem to recycle construction waste.Among them,the recycling of waste concrete is the primary problem,It is very difficult to apply recycled aggregate and recycled micropowder in practical engineering,and it is difficult to solve the problem of recycling waste concrete resources.In this paper,according to the relevant requirements of JG/T 573-2020 "Recycled Micro Powder for Concrete and Mortar" and JG/T486-2015 "Composite Admixtures for Concrete",the preparation of recycled concrete powder composite admixture is studied to reduce the difficulty of using waste concrete in cement-based materials and improve the utilization rate of waste concrete.In this paper,a new processing method of recycled concrete powder is studied,and the basic properties of recycled concrete powder after processing are characterized.The composite admixture with recycled concrete powder and slag as main components was prepared,and the effects of each component of the composite admixture(recycled concrete powder,slag and alkali activator)on the workability and mechanical properties of mortar were studied.And selecting a target compound admixture according to the actual performance of the mortar;Cement-based composites(cement paste,mortar and concrete)containing recycled concrete powder composite admixture were prepared,and the effects of the target composite admixture on the workability,mechanical properties,drying shrinkage and durability of cement-based composites were studied.And the influence of target compound admixture on cement hydration products,hydration process and pore structure,and explore the influence mechanism of recycled concrete powder compound admixture on various properties of cement-based materials.The particle size distribution,specific surface area and micro-morphology of recycled concrete powder processed by new processing methods were characterized by malvin particle size analysis,Brinell method and scanning electron microscope.The test results show that after the dried waste concrete fragments with particle size less than 4.75 mm are ground for30 min,The fineness of powder just meets the requirements of grade I recycled concrete powder,and the particle size is mainly below 45 μ m.With the extension of grinding time,the particle size of recycled concrete powder decreases and the particles are more regular,but the specific surface area of the powder increases and the water demand ratio is greatly increased.Through the working performance test and mechanical performance test of mortar specimens with different composite admixtures,the influence law of each component on the performance of specimens and the mixture ratio of target composite admixtures are obtained.The test results show that when the proportion of recycled concrete powder in composite admixture is too high or too low,the fluidity ratio of composite admixture is low,and the slag content is higher.The larger the total activity of the admixture is,the highest total activity can reach 87% in 28 days.However,the whole system was not obviously excited by alkali,and after adding alkali activator,the 28-day total activity had no obvious improvement.Considering the fluidity ratio of cement,total activity and the utilization rate of waste concrete,the target compound admixture ratio of concrete powder to slag is 1:1,the grinding time of recycled concrete is 30 min,and the alkalinity coefficient of slag is 1.33.The workability,mechanical properties,drying shrinkage and durability of cement-based composites containing recycled concrete powder were tested.The test results show that with the increase of the content of recycled concrete powder composite admixture,the fluidity and strength of mortar specimens decrease,and the early drying shrinkage decreases.When the admixture content is lower than 20%,The working performance and mechanical properties of the rubber specimen are similar to those of the reference group,and it has good volume stability.When the content of recycled concrete powder composite admixture is 20%,the working performance,mechanical properties and carbonation resistance of concrete specimens are similar to those of the reference group,and the chloride ion penetration resistance and frost resistance are improved to some extent.The hydration products,hydration process and pore structure characteristics of recycled concrete powder composite admixture were determined by XRD,SEM,hydration heat and mercury intrusion method.The test results show that after adding recycled concrete powder compound admixture,compared with the reference group,no new hydration products are generated,the hydration heat of cement is reduced and the exothermic peak is delayed;When the content of recycled concrete powder composite admixture is 20%,the quantity and types of hydration products are almost the same as those of the reference group.Inert powder in the admixture plays a filling role,and the pore structure in cement paste is improved to some extent.