Putty lime gets hardened under water

Hello. I just made one bucket of lime putty out of hydrated lime powder the other day. By following the regular procedure, putty was in a creamy consistency right before i filled the remaining few centimeters with water and closed the lid off.

Earlier today, i lifted the lid up and poked the putty with a long screwdriver or i should say i struggled to do it as it was "suspiciously" hard underneath the layer of water.

I could only made as much 10cm deep in the bucket and as i was poking it on various spots, some putty with the regular creamy consistency has come to the surface.

I was also able to smush the lumps in my hand pretty easily. So that gave me some hope but all in all, is this all normal? Or it went wasted?

Hi alt say and welcome to Permies. What you've discovered is totally normal and expected. The calcium oxide in the lime putty is reacting with dissolved CO2 in the water to form a skin of calcium carbonate. Underneath this layer the putty is unmodified, so when you want to use it just peel off the skin (you can use it as aggregate if you're making mortar or plaster).

Stonemasons in Roman times would mix up lime putty and leave it in a pile in the cellar for their sons or grandsons to use. The protective skin helped keep the material underneath damp. I've read accounts of heritage building restorations in the UK where they found batches of mortar stashed away in wall cavities that may have been several centuries old. Underneath the shell, the lime was the best quality they'd ever seen...time works wonders.