Research On The Preparation Of Recycled Cement By Using Waste Concrete Powder To Replace Cement Raw Material

The construction industry has been developing rapidly in recent years,and the speed of infrastructure construction and renovation of old cities has accelerated,which consumes a large amount of cement,sand and other construction materials.2021 China’s cement production totaled 2.363 billion tons,consuming a large amount of limestone,clay and other raw materials,and China’s reserves of limestone and other mineral resources are limited,so it is urgent to find new raw materials for cement production to alleviate the shortage of raw materials.At the same time,the demolition and renovation of some old buildings,more and more waste concrete（waste concrete）and other construction waste is generated,and China will produce nearly 3.2 billion tons of construction waste in 2021,which brings huge pressure to the environment.Therefore,the use of waste concrete to produce recycled cement can not only solve the environmental problems caused by construction waste,but also alleviate the shortage of raw materials for cement production,which is a multi-benefit.At present,the resource utilization of construction waste is mostly for the production of recycled aggregates from waste concrete,and there is less research on the production of recycled cement from waste concrete.In this thesis,we study the use of waste concrete powder to replace part of the cement raw material to prepare recycled cement,firstly,we study the decomposition process of the cementitious components in waste concrete at high temperature,and establish the connection between the cementitious components and recycled cement.Then the cement raw material was replaced by the waste concrete cementitious fraction,and the effects of the replacement rate and cooling method on the firing of recycled cement clinker were studied,while the optimal replacement rate was searched for.Finally,full-component waste concrete powders from different regions were used to replace cement raw materials,and the differences between recycled cement clinker and cement plant clinker were investigated to establish a basic utilization scheme for waste concrete.The main conclusions drawn are as follows.1.The decomposition products of waste concrete cementitious components at high temperatures are consistent with those of cement clinker minerals,andβ-C₂S is formed after treatment at 1050°C.The decomposition of C₁₂A₇ and Ca O generated in calcium alumina AFt is about 500°C earlier than the formation of C₁₂A₇ in normal clinker minerals,which accelerates the formation of the clinker intermediate product C₂AS.The strength of the cementitious component has little effect on the nature of the decomposition products,and the unhydrated particles in the cementitious component play the role of crystal seeds in the firing of recycled cement clinker and promote the solid phase reaction.2.The best replacement rate of waste concrete cementitious components for raw cement is 30%,at which the properties of recycled cement clinker are close to those of cement plant clinker,and the waste concrete cementitious components can be used to the maximum extent.Under air-cooled condition,the recycled cement clinker is gray-black and the tricalcium silicate is mainly of M_Ⅰ+M_Ⅲtype,while under water-cooled condition,the clinker is brownish-yellow and the tricalcium silicate is mainly of M_Ⅰtype,and the crystalline type of dicalcium silicate is mainly ofβtype under both cooling conditions.3.After the ordinary waste concrete powder replaces the raw material,its easy burning and easy grinding are enhanced,and the clinker properties are close to those of cement plant clinker.After high-silica waste concrete powder replaces raw material,its easy burnability becomes worse,but it still meets the requirement of free calcium oxide less than 1.5%as stipulated by the national standard.Taken together,waste concrete with coarse aggregate of limestone is most suitable for producing recycled cement clinker,and waste concrete with high crystalline silica content is more difficult to utilize.