Ultrasound is considered to be an effective agitation technique to enhance the rates of reaction and mass transfer for various chemical processes. The literature reports several successful ultrasonic application cases, including extractive metallurgy. The present study investigates the effect of sonication (referring to the application of ultrasound) on the dissolution of calcium from steel slag to maximize the amount of CO2 sequestrated. The results are compared with results from similar tests with mechanical agitation. When calcium bearing phases liberate their calcium, the remaining silica and other common impurity species in the slag form a porous layer on top of the calcium-liberating surface. This porous layer and in some cases, mechanically weak surface can be removed effectively by sonication. Hence, 96% of the calcium in the smallest particles (50-74 mu m) of the slag was extracted by ultrasonic, and 65% by mechanical agitation. While from the largest particles (500-1000 mu m), 38% of the calcium were extracted by ultrasonic and 18% by mechanical agitation. Thus, sonication significantly enhances the extraction of calcium from both small and large particles. (C) 2014 Elsevier B.V. All rights reserved.