Characterization of high quality recycled sand from construction and demolition waste

Abstract

Construction and demolition (C&D) waste was used in large scale in the reconstruction of cities after de Second World War. However, improvement in the recycling processes, mixing and dosage procedures were intensified at the end of the 1990's. The current demand for recycling is owed to the increasing consumption of aggregates, exhaustion of deposits close to large cities (average distances >50km) as well as the prohibition of dumping inert materials in landfills or by rising of disposal cost. This scenario contributes to the vertiginous raise of natural aggregate costs, which in São Paulo was more than 100% in the last four years, with a direct impact on infrastructure and housing costs. The existing construction waste recycling technologies and standards are focused in the production and use of the coarse fractions of recycled aggregates. However, fine fraction represents 40 to 60% (in mass) of the total processed construction waste. In many cases this fraction is rejected as residue or used as a road sub-base which low or even negative value. Therefore, the improvement of the quality of such product is essential to both financial and environmental efficiencies of recycling process. However, the progress of recycling of the fine fraction has been slow.The present research explore the potential of adapting mineral processing technologies already applied by the mining industry to produce only high quality recycled sand instead in focusing on producing mixture between coarse and sand aggregates. The method includes the improvement of crushing and grinding, as well as the use of classification processes to remove of deleterious fractions. Porosity, rheology and chemical parameters will be the quality indicator of the produced sand. The proponent group, with the support of CNPq, has studied different alternatives of mineral processing in order to increase C&D recycling rates (Brazil <5%), focused on the production of high quality recycled sand (low porosity and better shape - roundness). The development of the processing technology comprehends the evaluation of distinct unit operations, such as: optimized comminution, classification (wet and dry), attrition for the removal of the attached cement paste, high pressure washing system (combination of attrition and classification) and mineral separation to remove less porous particles. The present proposal comprises the definition of an accurate analytical technique to characterize the products and tailings from different mineral processing operations, specially particles porosity and density. The products will be characterized according to: grain size distribution, chemical and mineralogical composition, content of attached cement paste, water absorption, porosity, density and envelop density, distribution in density classes of heavy liquid separation, particles shape (roundness). Rheological and application studies of the recycled sand will be helpful on the definition of the best recycling process and further optimization of each unit operation. The results will enable the comparison between the rheological properties of the mixtures with the recycled sand properties (shape, grading, porosity, density), allowing a detailed assessment of the effectiveness of the mineral processing. The benefits of recycling go beyond the increasing of service life of landfills and prevent destruction of natural environments due to sand mining, which is sometimes illegal. Recycling aggregate will also generate more jobs than natural aggregate extraction. Besides, the possibility of implementing recycling plants close to the market contributes markedly to the decreasing of transportation distances, which is decisive on the final aggregate costs and CO2 emissions. (AU)