What's the risk of water/rain falling on a recently placed tilework that's not grouted yet

Question

I'm currently working on renewing a concrete staircase going to the garden. The shape of the stairs make it a very time-consuming job, as every step needs several custom cuts. Currently I've 3 steps done with the tiles (glass mosaic) on flex thinset and no grouting yet.

I've always heard that it's critical to prevent water going into the thinset while it's drying and they are forecasting rain for tomorrow. Should I be concerned and take measures to cover the worked steps? Or I just need to let it dry well before grouting?

Answer 1

Once the initial set/superficial hardening has taken place, water should not be a problem. Excess water is a big problem when it can still alter the mix of the cement or wash it away - after that, it won't have much effect.

On the other hand, it will be difficult to "dry the steps well" with water trapped under the tiles, so covering would probably make sense - 3 steps can't be that large of an area to cover.