As can be seen, commercial CaO (Fig. 4a) was mainly built of bulky aggregates with irregular sizes above 400 nm. In contrast, the samples derived from carbide slag exhibited distinct morphologies. The Raw CS sample (Fig. 4b) was found to be stacked and consisted of irregular aggregates. After calcining the Raw CS at 650 °C, the compact and irregular aggregated particles were observed for CS-650 sample (Fig. 4c). Compared with Raw CS, random pores were also visible for CS-650 sample because of the dehydration of Ca(OH)2. Upon increasing the calcination temperature to 800 °C, the agglomerates turned to platelet shapes with sizes below 100 nm. As could be seen, the platelet turned into branched aggregates after the calcination at 950 °C. Eventually, the cone-shaped carbide slag with larger size was observed after calcination at 1100 °C. The increase of crystalline size with calcination temperature was in agreement with the XRD results, and the corresponding compact morphology implied the decreased surface area, as also evidenced by N2 sorption result.