chemistry Through riddles

Riddle#6

I am a white amorphous solid. Water

Starts boiling when brought in contact

With me and finally leaves a product ,

Whose aqueous solution is used

extensively on the walls of concrete

buildings. if you open a coca-cola bottle

Keeping the same aqueous solution

near its mouth, the clear solution turns

Milky, but prolonged exposure under

the same condition makes it clear again.

Questions

a)Who am I ?

b) Why water boils when brought in

    contact with me ?

c) Name the product formed due to

    action of water on me ?

d) Tell the reason for milky appearance

    of the solution and subsequent

    clearing of it.