In Fig. 3 and Table 2, the CE loading of 20 wt.% did not provide
enough active site (basic site) for transesterification due to its lower
total basicity and BET surface area. However, the CE loading of 50 wt.%
catalyst still had poorer catalyst activity because of the coverage of the
basic sites by excessive CaO [27], which was further supported by the
disappearance of Ca2SiO4 crystalline phase, as shown in Fig. 3.