It can be seen from the figure that stalagmites are damaged because of the dissolution of CO 2 in surface water, which can dissolve stalagmites and generate Ca(HCO 3) 2, while Ca(HCO 3) 2 will decompose to generate CaCO 3 when heated, forming stalactites and stalagmites. Using alkali which can react with Ca (HCO _ 3) _ 2 to repair stalagmites damaged at the bottom of the cave. Therefore, the first step of the experiment is to select a suitable alkali, and then prepare an alkali solution with a certain concentration for the repair operation ① ④ ⑤.