Study On The Influence Of The Particle Size Distribution Of Recycled Concrete Powder On The Mechanical Properties And Microstructure Of Recycled Mortar

With the acceleration of urbanization in China,the total amount of construction waste is increasing day by day,and the recovery and utilization of construction waste has good social and environmental benefits.In the process of preparing recycled aggregate from construction waste crushing,a large number of fine powder,that is,recycled concrete powder(RCP),will be produced.The RCP contains a large number of hardened cement paste and incomplete hydration cement,which has some activity.The paper,based on the RCP from the waste concrete,studied its basic properties,the effect on the performance of the mortar and the mode of active excitation.The close degree D(0<D<1,the larger D indicates that the particle size distribution of a powder material is closer to the close packing state)between the particle gradation of a powder material and the particle gradation of close packing of Andreasen is considered as the criterion for evaluating the particle size distribution of the powder material.The effect of particle size distribution on the mechanical properties and micro-structure of recycled mortar was studied when the RCP and mineral admixture were mixed with each other.The results of the study on the RCP and its excitation effect show that the particle size of the RCP is large,the activity is low and the particle size is poor,and its main chemical composition and content are similar to those of ordinary Portland cement.When RCP replaces cement,the worse the particle gradation of RCP-Cement system,the lower the compressive strength of recycled mortar and the looser and porous micro-structure of recycled mortar.When the particle size distribution proximity of RCP-Cement system is more than 55.7%,it has little effect on the properties of recycled mortar and can be used to replace cement.Chemical excitation,thermal activation and physical excitation can improve the activity of RCP,but chemical excitation mode can not improve the utilization rate of RCP.The utilization ratio of RCP can be increased to 20% at high temperature of 800℃ and the excitation mode of ball milling 30 min,and the excitation effect of ball milling 30 min is the best.The essence of physical excitation is to change the particle size of RCP-Cement system.According to the theory of close packing,the RCP without ball milling can be mixed with mineral admixtures for example fly ash and silica fume,which can not only change the gradation of powder particles in mortar,but fly ash and silica fume can also be used as industrial waste,which has more obvious environmental benefits.The results show that the compressive strength at 28 days of recycled mortar increases with the increase of particle size distribution proximity of RCP-Fly Ash-Cement system when RCP are mixed with fly ash.However,the particle gradation proximity of RCP-Fly Ash-Cement cement system is less than 56.9%,which does not improve the compressive strength and micro-structure of recycled mortar for 28 days.When RCP,fly ash and silica fume are mixed together,the compressive strength of recycled mortar increases with the increase of particle gradation proximity of RCP-Fly Ash-Silica Fume-Cement system,and when the particle gradation proximity of powder system reaches 61%,the compressive strength at 28 days of recycled mortar is 2.4% higher than that of blank mortar,and the micro-structure of recycled mortar is better.