On this page of the solution Dragon Ball Z Kakarot, you will discover the complete progress of thesub story Renovations of the Hall of Spirit and Time.
Episode: Cell Saga - Episode 10
As Son Gohan, go see Dende in the Divine Palace (picture1). You need to find ghost sand, green crystal and red crystal.
You will find green crystal in the ore deposits of the western forest area (pictures2and3).
The red crystal is found in the ore deposits of the central plains area (pictures4and5).
Finally, go to the southeast mountains area and collect the ghost sand not far from Yamcha's lair (pictures6and7). Then eliminate the robots (picture8), then bring the items back to Dende to close this sub story.