I can't teach you everything you need to know to understand about the drink in one or two posts. That is why you must study all the important posts on this forum with ratings of BSA R RR RRR. We will guide you but you must do your part or you will not be successful in all of this.
It will take you much study to understand why we need calcium. I can't explain in one post. Sorry, I wish I could though. Don't forget to do searches within the forum on calcium or limewater.
This is a long study....not something you will understand in one day so please be patient. :)