(143u) Development of a New Recycling Process of Fine Aggregate from Waste Concrete Particles Using High-Pressure Carbon Dioxide Solution

The recycling rate of waste concrete in Japan has reached as high as almost 100 % after the enforcement of the ?Construction Material Recycling Law? in 2000. The waste concrete has been reused as a roadbed material, of which the demand is expected to be on a declining trend in the future. On the other hand, the emission of the waste concrete by the demolition of buildings is expected to be in a rapid increasing trend for the next decades. Because the aggregate, a major component of waste concrete, is a depleting resource, development of an effective recycling process of the aggregate component from the waste concrete is an urgent issue from both aspects of waste management and resource conservation. Two types of aggregates are used in the concrete depending on the particle size, namely, coarse aggregate (5~40 mm) and fine aggregate (0~5 mm). Recovery of coarse aggregate from the waste concrete is an established process; coarse aggregate can be separated by crushing and classifying the raw waste concrete. However, recovery processes of fine aggregate and cement contents from the finer particles after recovery of coarse aggregate is still under development because of the difficulty in the separation of these components. In this study, a new type of recycling process of fine aggregates from the waste concrete particles has been developed. Waste concrete particles obtained as a by-product in the recovering process of coarse aggregate were used as the raw material for the recycling. These particles are called ?5-mm under? because of the particle diameter being below 5 mm. The waste concrete particles were mainly composed of hydrated cement contents and fine aggregate. The recycling process is composed of three types of treatments, namely, treating with high-pressure CO2 aqueous solution, grinding with a ball mill, and classification. With the treating with high-pressure CO2 solution (~ 3 MPa), the cement contents, mainly calcium compounds, will be leached out into the CO2 solution as a carbonate. The remained cement content in the waste concrete particles will be more fragile after leaching, and it will be easily crushed into fine powders with the ball mill grinding. After grinding, fine aggregate content will be recovered by classification. A preliminary experimental study demonstrated that the water absorption ratio of the feed waste concrete particles was significantly reduced by the present process, and in some cases, fine aggregate of which the water absorption ratio is below 3.5 % was obtained, which satisfies the Japan Industrial Standard (JIS) for recycled fine aggregates. Effect of the operation conditions of each treatment on the properties (water absorption ratio, calcium content, morphology) of the recycled aggregate was experimentally investigated. Based on the experimental studies, power consumption and cost for the recycling process were estimated to optimize the recycling process.