Abstract
The carbonation of recycled aggregate was accelerated by sparging with supercritical carbon dioxide (scCO2) to reduce the amount of time needed for carbonation, which is necessary for the pH neutralization of recycled aggregate. To accelerate the carbonation process, pressurized scCO2 was sparged into two different types of recycled aggregates immersed in water for 1 h, followed by standstill for 2 h (in total, a 3 h treatment process). The reduction of the pH of the treated aggregates due to carbonation was investigated using batch extraction experiments. A continuous column extraction experiment for the scCO2-sparged recycled aggregate was also performed to identify the effect of pH reduction under the condition of non-equilibrium reaction. From XRD, SEM/EDS, and TG/DTA analyses, much of the portlandite in the recycled aggregates was consumed. In its place, calcite was created as a secondary mineral during only 3 h of treatment (1 h scCO2 sparging and 2 h stationing), indicating satisfactory carbonation of the aggregate. The results of the batch extraction experiments for both of the two recycled aggregate types also showed that the average pH of scCO2-sparged aggregate decreased from 12.0 to <9.8 (the tolerance limit for recycling). The pH of the eluent from the column packed with the scCO2-sparged aggregate also remained as <9.8, suggesting that a 1 h scCO2 sparging process is sufficient to carbonate waste concrete aggregate and to create an alternative construction material resource.
Highlights
As the by-product of industrialization and urbanization, a large amount of construction waste has been produced worldwide
Even the change of XRD peak intensity for a specific mineral did not quantitatively match the change of its amount in the compounds. These results indicate that the transportation of sparged supercritical CO2 (scCO2) and water into the inner parts of the recycled aggregates occurred actively and that they were trapped even in micro pore spaces, where they accelerated the carbonation of the aggregates
Several previous studies have demonstrated the process of carbonation of concrete using scCO2 treatment, but more than 10 d of treatment time was required for successful lowering of the pH of the concrete or the recycled aggregate
Summary
As the by-product of industrialization and urbanization, a large amount of construction waste has been produced worldwide. To overcome the limitation of the conventional carbonation time, the carbonation process of the concrete aggregates was optimized by sparging pressurized scCO2 into recycled aggregate immersed in water under supercritical condition (0.8 MPa and 50 ◦ C). H+ and carbonic acids (HCO3 − and CO3 2− ) that penetrate much better into the micro pores of the aggregate to neutralize the hydroxides dissolved inside them This results in decrease of the pH of the water and the precipitation of CaCO3 on the aggregate surface in a relatively short time. The use of the scCO2 sparging process as the accelerated carbonation process has never before been attempted, even in the laboratory, and this study presents a new technology for effectively shortening the time for carbonation of concrete waste to overcome the time limitation affecting its reuse at real construction sites
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