Study On Preparation And Performances Of Recycling Concrete

Concrete is the most widely used civil engineering materials in the world, and it plays animportant role in the development of national economy and progress of human civilization.However, as the amount of concrete increases year by year, it has caused people’s attention tothe sustainable development or the enormous consumption of natural resources and thenegative impact on the environment. So, the development of concrete had to consider theresources utilization and environment protection, only by this way, the sustainabledevelopment of concrete can be achieved in the future.In view of the resource and environmental problems that the cement concrete faced today,a recycled concrete was prepared aimed at achieving completely recycling of concrete. Theconcrete were prepared by the aggregates that with a composition like cement clinker.The high-strength artificial aggregates were prepared, and the influence of raw materialratio and calcination process on the high-strength artificial aggregates performance wasstudied. And the recycling concrete was prepared by using high-strength artificial aggregates,and the mechanical properties and durability of the recycling concrete were studied. Theregenerated cement was made by using the discarded recycling concrete. The cement mineralcomposition and morphology were analyzed by means of SEM, petrographic and XRD. Themain research contents and the obtained results and conclusions are as follows:1. Study on the preparation process of high strength artificial aggregates. Results showedthat the cement raw meal high strength artificial aggregate have best performance when theraw material ratio is steel slag7.7%, coal gangue10.4%and limestone81.8%. The waterabsorption of the artificial aggregates is0.82%, crushed index is6.78%, and apparent densityis2675Kg/m~3. The best calcination temperature of the artificial aggregates is1350°C, theholding time is90minutes. The waste high strength artificial aggregates with about30%steelslag,50%slag,20%gangue, and calcined at1300°C for90minutes had the best performance.The water absorption of the aggregate is about1.55%and both the crushing index (about11.39%) and the apparent density (2672kg/m3) of the aggregate meet the requirements ofnational standards. 2. The study on mechanical properties of recycled concrete was carried out. The resultsshowed that: when the water-cement ratio was0.45, and design strength grade was C40,28days compressive strength of the waste and raw material aggregates recycled concrete were52.8MPa,58.7MPa respectively. Due to the improvement of interfacial transition zone ofrecycling concrete, so its compressive strength are higher than that of ordinary concrete underthe same conditions. With the increase of fly ash, silica fume content, the recycled concretecompressive strength first increases and then decreases, the optimum content of fly ash andsilica fume is30%and15%, respectively.3. Though the study on recycled concrete impermeability, it can be found thatwater-cement ratio was a key factor determined the impermeability of the concrete. Theimpermeability of the concrete became worse with the increase of water-cement ratio. Withthe same water to binder ratios and design strength, because the cement raw material artificialaggregate and cement are the same material, so the concrete structure of the interfacetransition zone can be improved, the impermeability of recycling concrete is better than thatof ordinary concrete.. The impermeability of recycling concrete can be improved by addingfly ash and silica fume, and the optimum dosage of fly ash and silica fume are30%,15%,respectively.4. The research on frost resistance of recycled concrete was studied. The results showthat with the increase of the water-cement ratio, the frost resistance of ordinary and recycledconcrete is reduced. With the same water to binder ratios and design strength, the frostresistance of recycling concrete is better than that of ordinary concrete. If the fly ash and silicafume are added into recycled concrete, the frost resistance can be improved, and the optimalamount is30%and15%of cement by weight respectively. Silica fume as admixture thatimprove the frost resistance of recycled concrete has better performance than fly ash.5. The research on regenerated cement made by using waste recycling concrete. Thecement was prepared by using raw materials artificial aggregate concrete powder, andchemical composition of the concrete powder was tested and calculated. The results showedthat the KH=0.82, SM=2.05, IM=1.26, so the cement clinker can be obtained by calciningconcrete powder directly without adding other materials. The regenerated cement clinker wasgrinded with5%gypsum and the specific surface area of cement clinker was tested, which is 396.5m2/kg. Initial and final setting time was140min and345min, respectively. And thesoundness remains qualified. The samples was molded and cured according to the cementstandard test method. At the age of3,7and28days, the compressive strength were29.8MPa,45.1MPa and51.3MPa respectively, the bending strength were5.8MPa,8.4MPa and8.6MPa.And its flexural, compressive strength can meet the requirements of42.5ordinary Portlandcement.The chemical composition of waste artificial aggregates concrete powder wasdetermined. And the results showed that KH=0.16, IM=1.87, SM=1.63, respectively. Withadding calcareous raw materials limestone, the regenerated cement clinker was prepared. Theregenerated cement clinker was grinded with5%gypsum and the specific surface area ofcement clinker was tested, which is387.2m~2/kg. Initial and final setting time was130min and320min, respectively. And the soundness remains qualified. The compressive strength ofcement paste was tested. At the age of3,7and28days, the average value of compressivestrength were47.1Mpa,59.8Mpa and75.6MPa. The samples was molded and curedaccording to the cement standard test method. At the age of3,7and28days, the compressivestrength were23.6MPa,38.7MPa and48.9MPa respectively, the bending strength were4.8MPa,6.9MPa and8.0MPa. And its flexural, compressive strength can meet therequirements of42.5ordinary Portland cement.