Carbonation for recycled fine aggregate by using the sparging process of supercritical carbon dioxide

Abstract

Recycled aggregate derived from waste concretes occupies about 60% of domestic construction waste in Korea and is mainly utilized as the construction material. However, due to the cement pastes attached on the surface of recycled aggregate, the pH of recycled aggregate becomes high so that it is necessary to neutralize the pH below 9.8 (Korean tolerance limit) for the recycling in the field. Previous neutralization processes using supercritical carbon dioxide (scCO2) to maintain the pH of recycled aggregate below 9.8 require long reaction time and their application in real field has been limited. The objective of this study is to reduce the reaction time for the neutralization of recycled aggregate up to three hours by using the scCO2 sparging process. Experiments conducted in this study can be divided into two parts. The neutralization experiment for the mortar cube (1 x 1 x 1 cm in size) was performed by using the sparging of scCO2 (hereafter‘scCO2 sparging process’). The scCO2 was stored in a 75 L of stainless steel tank at 50 ℃ and 110 bar. Mortar cubes (30 g) and distilled water (60 mL) were mixed in a high pressure stainless steel cell (100 mL of capacity) and the scCO2 was sparged at a pressure of 90 bar (Δp: 10 bar difference) from the tank to the bottom of the cell through the inlet valve of the cell for 1 hour. The cell was maintained at 50 ℃ and 80 bar by consistently discharging gaseous CO2 at 10 bar through the outlet on the top of the cell during the scCO2 sparging. Microbubbles of the scCO2 formed in water by the sparging process can penetrate deeply into the mortar inside and can accelerate the dissolution of alkaline Ca(OH)2 from the mortar. After 1 hour of the scCO2 sparging, the cell was left at 50 ℃ and 80 bar for 2 hours to induce the precipitation of CaCO3 from Ca2+ oversaturated solution. The dissolution reaction among the scCO2-water-mortar without the scCO2 sparging (hereafter‘dissolution reaction with the scCO2’) was also conducted with mortar cubes for 3 hours and its results for the pH lowering of the mortar were compared with those of the scCO2 sparged mortar cube. From experiments, the scCO2 sparging process was able to dissolve Ca(OH)2 more than 1.7 times and produce CaCO3 more than 1.4 times, compared with the dissolution reaction with the scCO2. As the second part of experiment, batch and column extraction tests for recycled fine aggregate sparged with scCO2 were performed to prove the pH lowering of the recycled fine aggregate treated by the scCO2 sparging process. For column experiments, the column (2.4 x 2.4 x 14 cm in size) was filled with 300 g of the scCO2 sparged recycled fine aggregates and the distilled water was injected from the top of the column at a rate of 1.5 mL/min. The average pH of scCO2 sparged recycled fine aggregate was 8.83 in batch extraction tests and the average pH of the effluent from the column was 9.68 in column extraction test. From results of batch and column experiments, the pH of recycled fine aggregate sparged with scCO2 for 1 hour was effectively maintained below 9.80, suggesting only one hour scCO2 sparging process has a great capability to efficiently neutralize the recycled fine aggregate.