Experimental Study On Waste Concrete Powder As Admixture

Waste concrete piling up at random,resulting in serious pollution of the environment and a huge waste of resources,the use of waste concrete in countries is still mainly limited to the regeneration of coarse aggregate,while the waste mortar as a part of the cost of waste concrete,the use of value has not been fully developed.In this paper,the mortar components of waste concrete are crushed by ball milling,and the basic physical and mechanical properties of the waste concrete are tested.On this basis,the mechanical properties of the mixed concrete are improved by chemical excitation,and the waste concrete with high activity is obtained,the concrete was prepared by changing the ratio of water to cement,and the frost resistance and chloride ion permeability of the concrete were tested.At the same time,the 28 d samples of different waste concrete fine powder,chemical excitation test cement sample for microscopic analysis.Experimental study and theoretical analysis show that:(1)The waste concrete powder has the potential hydration activity,which can stimulate its potential activity by physical way.When the milling time was 210 min,the sieve residue rate of 0.08 mm was reduced to 2.6%,the specific surface area increased to 490 m2/kg,the bulk density was less than that of cement and could be used as cement admixture.The strength of cement mortar decreases with the increase of dosage,but when the content of cement mortar is less than 10%,the cement mortar still own higher strength.(2)Waste concrete powder has an important impact on the physical properties of cement.The addition of waste concrete powder,resulting in increased water requirements of the standard consistency of cement,The initial setting time of the cement increased,the final setting time was shortened,the shrinkage rate of the cement mortar was lower than that of the unmixed mortar powder group,and the shrinkage performance was improved by the addition of the waste concrete powder,which resulted in the increase of the standard consistency water requirement.With the increase of the content of water-cement ratio,the fluidity of cement paste increases with the increase of the content of waste mortar,and the fluidity of cement paste increases with the increase of water-cement ratio and water-cement ratio.The loss of the degree decreased.(3)The effect of different activators on the activity of waste concrete powder was different,in which CaO and Na2SO4 had the best effect,NaOH and CaSO4 were the least,Na2CO3 was the weakest,CaCl2 had no excitation effect on mixed mortar powder.The optimum formula of the combination of chemical activator is:2%CaO+1%Na2SO4+0.25%NaOH+2%CaSO4,the 28 d flexural strength reaches 7.4 MPa and the compressive strength reaches 36.7 MPa.The best mixing ratio of CaO+Na2SO4,NaOH+CaSO4 and CaO+NaOH was 2:1,1:1 and 2:1,respectively,and the compound stimulant CaO+Na2SO4,the cement mortar to stimulate the best.(4)Under freeze-thaw cycle,when the water-cement ratio is less than 0.44,after 25 times freeze-thaw,the compressive strength of concrete is not low but slightly increased.With the increase of waste concrete,the frost resistance of concrete And the content of waste concrete is less than 10%under the same condition of water-cement ratio,which can improve the ability of freezing-thawing failure of concrete.With the increase of water-cement ratio,the mass loss rate and compressive strength loss rate of concrete increase with the same conditions.(5)The influence of water-cement ratio on the resistance of concrete to chloride ion penetration is also obvious.The increase of water-cement ratio makes the concrete electric flux increase and then decrease after the same amount of waste concrete.W/C ratio of 0.44 is the best for chloride ion permeation.When the waste concrete powder the content is 10%,the chloride penetration resistance is still good.(6)After 28 days of curing the waste cement concrete powder,the hydration of the cement phase mainly consists of Ca(OH)2,C-S-H gel and ettringite,The better the excitation effect,the higher the diffraction peak intensity of the hydrated product.When the fineness of waste concrete admixture is the smallest,the hydration product of cement is high and the system structure is dense.When the content of waste concrete is 10%,the peak intensities of C-S-Hand ettringite are the highest,and the microstructure is the closest,When the content is 40%,the peak strength of the main hydration products is the lowest,and the system structure is loose.The cement mortar samples with better excitation effect produced more hydrated calcium silicate and ettringite,and the system had higher density.