**2.2 Synthesis of cockle and mussel shell in powder form**

Wasted mussel and cockle shells were washed and brushed several times with tap water to remove any foreign matter and impurities present. The cleaned shells were dried until no moisture was left then proceeded to crush into smaller sizes. Dry-milled both shells for 45 minutes at 500 rpm in batch mode. Then, the crushed shells were sieved to a particle size of ≤75 μm using Cooper's TG-0411/0412 Sieve Shaker for an average of 10 minutes. The powdered cockle shells then were mixed with 50 mL of ethanol in a 250 mL stainless steel jar and were wet-milled at 500 rpm for 4 hours equivalent to 240 minutes in a clockwise direction. The same condition was set for powdered mussel shells except for the volume of ethanol used is 30 mL. Dry and wet milling processes were operated in a planetary ball mill (Pritsch Pulverisette 6) to gain nanosize powder [14]. Both shells were purified *via* the decantation method four times every 20 minutes before filtration. Then, the fine particles were dried in a drying oven at 40°C for 3 days corresponding to 72 hours. After cockle and mussel shells dried, the samples were calculated in a muffle furnace (Protherm-Furnaces) of 950°C with a heating rate of 10°C/min for a whole 4 hours. The obtained CaO powder was cooled before being stored in a sealed zipper bag.
