Study On The Use Of Demolished Concrete As Main Raw Material To Produce Cement Clinker And Its Properties

The land occupation and environmental pollution caused by the demolished concretes can be reduced by recycling, which can also supply a new resource of raw material for cement production. As a result, a kind of environment-protecting, resource-saving and sustainable development technology is abtained.Eight samples of abandoned concrete with limestone aggregate were selected from demolishing site. By core-drilling, cutting prior to preparing cylinder specimens,the quality of demolished concrete is evaluated. Each sample was crushed into particles which were then screened out the smaller part by sieve with given size and the residue was kept to use respectively. Most of the residue is the crushed limestone,it was used to replace the natural limestone as the calcareous raw material to produce cement raw meal. Mixed with given amount of sandstone and pyrite cinder, along with a few natural limestones when necessary, taking the usual method to calcine cement clinker samples respectively. And it was compared with the cement clinkers produced by Lafarge plant and Runjiang plant as well as some selected Portland cements on sale. Experimental results were given as follows:①Demolished concrete with aggregate of crushed limestone was crushed into particles,the limestone content in particles is about 75 to 80 per cent. It is suitable to be used as a replacement of natural limestone to produce cement raw meal,and the minority of SiO2 in the demolished concrete is a kind of siliceous raw material.②The cement raw meal produced by separated fractions of demolished concrete in which the majority is limestone, can meet the range requirement of ordinary cement moduli value. The demolished concrete is about 50 to 70 percent by weight in cement raw meal,which can be higher up to 95 percent.③Owing to the removal of the most martors, common production process can be used to finish the calcination of cement clinkers in which the demolished concrete is used as raw material. Furtherfore, the calcination is ease enough as the usual.④The comparative analysis results of X-ray diffractions show that the characteristic peaks of four principal compounds is clear in the cement clinkers produced by demolished concrete. The result is also similar with the common clinkers produced by Lafarge and Runjiang.⑤The analysis results of chemical composition show that the f-CaO of the cement clinker produced by demolished concrete is not higher than the common clinker produced by Lafarge and Runjiang. All of them meet the requirement of China standards.⑥Experimental results on mechanical properties show that the 3 days compressive strenghth of the hardened cement paste using the cement produced by demolished concrete with water to cement ratio 0.5 is up to 18.9MPa, and the 28 days compressive strenghth is up to 51.27 MPa. According to the comparative analysis between the cement produced by demolished concrete and the ones produced by Lafarge and Runjiang, the 28 days compressive strenghth of the cement produced by demolished concrete can reached the strength grade of 52.5 for Portland cement.⑦The drying shrinkage of cement pastes produced by demolished concrete is about 30 percent higher than ordinary clinkers.The above results suggest that the cement produced by demolished concrete is as good as Portland cement. Therefore, the demolished concrete can be used with high additional value and high utilization ratio by using as main raw material to produce cement.