Research On The Preparation Of Recycled Cement From Construction Waste

At present,most of the construction waste treatment methods in China are landfilling,and the recovery rate is not high.In this thesis,the construction waste produced by the demolition of existing buildings is used for recycling.A large number of building waste can be recycled as recycled cement clinker,low calcium cement clinker and other raw materials,and waste gypsum from construction waste is used as cement retarder.The solution can improve the recovery rate of construction waste,and thus help to achieve the goal and recovery rate,recovery rate and reuse rate.The principle of the unity of the preparation of recycled cement clinker from construction waste is adopted.Firstly,the first step is to crush the construction waste and the waste concrete,screen by hand（remove impurities,screen the first time by the sieve）,and then conduct secondary crushing and grinding by jaw crusher,and then conduct secondary manual screening to prepare 80 micron recycled aggregate.XRF is used The chemical composition,mineral composition and micro morphology of the main building waste such as construction waste were tested by means of X-ray diffraction analyzer（XRD）and scanning electron microscope（SEM）,which revealed the physical defects and structural properties of the construction waste.Then,the grinding and crushing of the construction waste and the waste concrete and screening were studied as raw materials for preparation In recycled cement,the chemical components were analyzed by X-ray fluorescence spectrophotometer and the chemical composition was allocated.The test conditions:KH value is 0.7-0.9（among which KH value is the coefficient of lime saturation ratio,hereinafter is expressed as KH）,preheating temperature is 800℃for 0 h-3 h,sintering temperature is between 1300℃-1450℃,cooling mode is cooling by air,room temperature cooling and water cooling;for building waste,steel slag,slag,fly ash,the content of calcium oxide is very low.In the second experiment,three kinds of tests were carried out for low calcium cement clinker under different conditions,among which the saturation ratio coefficient of lime was 0.67,the cooling mode after sintering was room temperature,and the sintering temperature was 1300℃-1450℃,in order to ensure that the sample can be prepared,the samples were taken after holding for 2 hours when the sintering temperature was reached.The hydration heat test,free calcium oxide test,XRD test and SEM test were carried out by grinding and screening recycled cement clinker and low calcium cement clinker samples.The mineral composition analysis,relative content calculation and crystallinity calculation were carried out by software.The waste gypsum,which is used as retarder of recycled cement,is added to the clinker of recycled cement with 1%-5%content.The flexural compressive strength of the gypsum at 3 and 7 days and the initial final setting time are tested.According to the chemical composition of waste gypsum,SEM-EDS was used to test the elements and micro morphology of the gypsum,the initial final setting time and the compressive strength of the recycled cement clinker.The results show that:（1）in this experiment,the relative content of C₃S of recycled cement clinker is 37%-80%,C₂S is 15%-37%,C₄AF is 10-18%,and the free calcium oxide content of recycled cement clinker meets the national standard.（2）The optimum calcination temperature of C₃S₂,a low calcium cement clinker mineral,was 1450℃and kept for 2 hours,and cooled at room temperature.（3）The final setting time of cement increases with the increase of the amount of waste gypsum,but the increase of the cement with the increase of the waste gypsum content is smaller than that of the initial setting of recycled cement.Waste gypsum has an effect on the compressive strength of recycled cement,and the best effect is when the gypsum content is 2%,which meets the requirement of gypsum content in cement.