Study On Grinding Characteristics Of Waste Concrete With Limestone Coarse Aggregate And Its Powder Properties

With the pace of urbanization accelerated, the demolition and renovation of the houses and the expansion projects is on the increase, which produces mountains of construction waste, and results in the quantity demanded of building materials explosion. The largest component of the construction waste is waste concrete, and limestone coarse aggregate waste concrete is one of the largest amount in the waste concrete. After analyzing the technologies and existing problems of reusing waste concretes, this paper proposed using ground waste concrete powder (GCP) which was prepared by crushing and ball milling all components of waste concrete with limestone coarse aggregate as cement additive. The grinding characteristics of waste concrete have been investigated. At the same time, the method of evaluating the activity of GCP was discussed, and its effects on the performance of cement were studied. On the basis of this, the activation and mechanism of GCP activity were also researched.(1) The 45 μm、80 μm screening residue, specific surface area and particle size distribution of GCP with different grinding time have been analyzed. The results demonstrate that the best milling time of waste concrete is 20 minutes. The specific surface area of GCP is greater than that of cement clinker, limestone powder, slag and fly ash when grinds for 20 minutes. Particle size distribution of GCP is mainly distributed in 0～10 μm and> 65 μm, which is similar to the particle size distribution of limestone powder(2) The chemical and mineral composition of GCP were analyzed. The results indicate that the LOI and SiO2 content of GCP is higher than that of cement, and its major mineral components are CaCO3, SiO2 and Ca(OH)2. Due to grindability difference of each mineral, fine GCP fractions contain a higher amount of CaCO3, while a higher proportion of SiO2 is observed in coarse GCP fractions.(3) Three different methods were adopted to evaluate the activity of GCP, and the test results show that pozzolanicity test and electrical conductivity method, which are used to evaluate the pozzolanic activity of pozzolanic materials, do not apply to evaluate the activity of GCP. Strength activity index method is suitable to use to evaluate the activity of GCP.(4) The effects of GCP on the physical and hydration properties of cement were studied. The water requirement of normal consistency of cement is little affected by GCP, but GCP shortens the setting time, improves the bleeding performance of cement paste and reduces the early shrinkage of cement mortar. The strength of cement mortar decreases with increasing the content of GCP, but the decrease in early strength is small in the range of 0～10% GCP substitution. The incorporation of GCP reduces the hydration heat evolution of cement and decreases the total hydration degree of cement pastes. GCP can be hydrated and the hydration product is C3A·CaCO3·11H2O. At the same time, the reaction of GCP with cement hydration inhibits the transformation of ettringite to AFm.(5) Mineral admixtures commonly used chemical activators, such as Na2SO4,CaCl2 and Ca(OH)2, have little effects on. the activation of GCP. F activator accelerates the reaction of GCP with C3A hydration products by promoting the hydration of aluminate cement minerals. Therefore, F activator effectively improves the activity of GCP.