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

DETAILS, we need details!! LOL!

Cement is an ingredient in hypertufa, concrete, mortar, etc.
Concrete is cement, sand and gravel. Unless you've got a mag trowel you'll probably see the gravel.
What recipe did you use, cement and water???
Are the cracks/voids in the side facing out in the picture the "terrible finish and consistency" that you're referring to?

Thanks both for the help.

I used straight cement and water by mistake. I thought I had bought a pre-mixed concrete.

The cracks, voids and general crumbliness are the horrible consistency...I don't mind if it looks rough, but this looks more decayed than rough. I might be able to salvage it by resurfacing when I make an actual batch of concrete. But ideally I want a smooth finish and I may attempt to polish it if I can get past just making the thing correctly .

Concreteblue, I don't have advice but I like the looks of the fireplace. Wish winter was nicer so we cold do more projects.

This may not fit your situation but thought I would offer it anyway. I have made a couple sphere's by applying the cement mix to the outside of a ball. After removing the ball, I finish the outside using a mix of plain portland, water, some color and admixture. You need to experiment to get the consistency so you leave a smooth finish using your gloved hand. It is more liquid than the mix you are accustomed to using. Also must be sure of your proportion of cement to color because you need to make several small batches. Hope this helps. Marie

I would suggest using melamine forms. I have made a countertop on melamine, and it came out as smooth as granite, with only a few minor imperfections.
NickTexas

Nick, I am curious about the melamine. How did you use to make a counter top? Thanks. Marie

Hi Marie,

When using melamine, you make the countertop upside down and then turnover. I built the form where I put a sheet of melamine on the bottom of the form, which ends up being the top of countertop. Even when doing it this way, there will be some holes due to airbubbles. I patch those immediately after turning the concrete over and allow a day to dry. Then using a sander or a polisher, smooth the top. The top will be as smooth as granite. After sanding, put on sealer, let dry, and then wax. Nick

Just another thought, try using Quickrete Resurfacer, should be available at Lowes, Home Depot. The smoother the texture of your mold, the better. If You could cover mold sections with plastic wrap, you would have an even better chance but will have some lines in your surface. Even try aluminum foil

I've used concrete in many situations for a decorative finish. To get it smooth you either need to form it in a smooth form and flip it over or hard hand trowel the surface with slight misting of water. I've also polished concrete before but it required some expensive polishing tools. I did a kitchen countertop before and got a super smooth finish with a high build epoxy coating - It was the enCountertop Concrete Countertop Mix with their enPoxy. It turned out great!

Here is a link that might be useful: Concrete Countertop Mix