Simultaneous productions of high-purity calcium carbonate and amorphous nanosized silica-rich gel from waste concrete powder by alkaline treatment and carbonation.

Simultaneous productions of high-purity CaCO3 (calcium carbonate) and amorphous nanosized SiO2-rich (silica-rich) gel from waste concrete powder remain quite challenging, and thus the high-value added utilization of waste concrete powder is significantly limited. A novel technology involving a combination of alkaline treatment and carbonation, has been proposed for the greatly enhanced utilization of waste concrete powder. Through alkaline treatment, CaCO3 and SiO2-rich gel formed during carbonation of concrete powder were transformed into Ca(OH)2 (calcium hydroxide) and Na2SiO3 (sodium metasilicate), respectively. Subsequently, Ca(OH)2 and Na2SiO3 were separated. As a result, CaCO3 with a high purity of 98.54% and amorphous nanosized SiO2-rich gel were obtained via carbonation, and the recovery ratio of CaCO3 was 81.46%. Furthermore, the concrete powder exhibited a high CO2 (carbon dioxide) uptake efficiency of 0.24g CO2/g. 30.31 Mt CaCO3 and 2.77 Mt SiO2-rich gel productions as well as 38.54 Mt CO2 emission reduction could be realized via recycling a quarter of annually generated waste concrete powder worldwide.