Very cute idea. A nice mixture of 3 parts sand, one part portland cement, and enough water to make it a brownie dough consistency should make a smooth finish, assuming that your mold has a smooth finish. Water amounts can vary depending on the humidity, how much water is in the sand, etc. So start with much less than 1 part water, you can add more as you mix until it is the consistency you like.

Once you pour or spread your concrete mix, then use your hand or something to pat the concrete. You should be able to see it sort of liquify. This makes for a smoother finish. Slight vibration or bouncing makes for fewer air bubbles in the finish. Post another picture when you try again. I like the one you did though.

Glenda

Love your idea. You also might consider adhering some solid round disks (cut from a wooden dowl rod or some other material) to the shelf area of your mold that are the diameter of your tealight candles so that they create little round holes/inserts to hold the candles instead of placing them on a solid shelf (if that makes sense).

Then only the candle's flame/glow would be visible...if you like that look.