What's the fourth application of Calcium Carbonate according to Senku ishigame? In the second episode of the anime, Senku Ishigame said the Calcium Carbonate they were gathering had four applications. Fertilizer for growing crops, raw material for building houses and soap. 

What's the fourth application of Calcium Carbonate according to Senku ishigame?
The fourth application of Calcium Carbonate is explained later in the manga/anime and it's a bit of a spoiler.
But it's used in :


   The making of Gunpowder. It's explained in Chapter 7 The gunpowder adventure and the third episode of the anime Weapon of science.