We were recently commissioned to make a Reception Desk for an Estate Agents and I put forward a design with a polished concrete worktop. I love the natural look of cement and for this design it was perfect, I was really pleased when the customer approved the design.
Polished Cement Surface
Firstly I made a few small moulds to test run different concrete mixtures to find the correct colour and finish. It was like having a little laboratory, I tried different colour sands and proportions of mix etc. I was amazed at all the different finishes I managed to create.
After I chose the final finish for the desk (considering the strength in the cement that I needed too), I tried different methods of sanding, sealants and gloss finishes on my chosen sample.
We made a large mould out of Melamine which is relatively cheap, it has a smooth gloss finish which is perfect for this job. I have read that you can use standard Ply and coat it yourself but I figured why muck about when Melamine does the trick.
When making your mould remember that the dimensions of your surface will be the measurements of inside the mould and also everything inside will be printed and cast onto the surface of your concrete.
I sealed all the inside joins with general use, clear silicone caulk, this not only makes it watertight which is essential for keeping the water in the cement (the last thing you want is dry areas, this will weaken your worktop), it's also essential for the finish to your edges. Remember whatever is inside your mould will define the cast for your cement, so it has to be neat! This is a really important step, don't skip it and remember to use CLEAR SILICONE!
Give it a good clean, I used water and arm grease as any chemicals can effect the cement.
To strengthen the cement I added a metal framework with wire mesh, it's recommended to use a wire mesh to add strength but as our worktop wouldn't be completely supported as it's a desk I figured I'd add something with a bit more structure. The metal was attached to the wooden mould by wire and screws that are attached externally to the top of the mould which you can see in the picture further down. The bottom of your mould will be the surface of your counter so you don't want the mesh sinking through your cement mix, securing it will make sure it's hidden within the cement worktop.
Make sure that your mould is on a flat surface as you don't want a wonky cement top when it's dry. I used a level to make sure that it was completely even before pouring in the mix.
For my small samples I mixed the concrete by hand but for the main worktop I used a cement mixer, this is essential when working to large scale, as again you don't want dry or weak areas. I went for a standard cement mix and white builders sand, mixture of 1 cement to 3 sand, this is for a really strong bond. I always overbuy and then return what I don't use, there's nothing like being caught short. When adding the water I just kept going until I got the consistency I required from experimenting on my smaller samples (if you have the time it's well worth doing this), I used my hand to gauge this, the best way to describe this high tech process is you want it to ooze through your fingers not run haha hope that helps.
It took a lot more cement and sand then I though it would, you really need to be precise otherwise you'll have lines down the sides if your next batch is slightly out. I found this out with my small samples and then did it on purpose to make a pretty cool subtle stripey slab.
Level the cement mix with a large baton, getting rid of any excess mix (I'm sorry my photos are pretty poor quality, I think at this point my hands where covered in cement and I was using a tea-towel over my camera to take the pictures.... ever the professional).
You need to make sure that you get all the bubbles out, again they can cause the worktop to have weak areas and also if they don't rise you'll have potholes on your surface. Ideally you would use a vibrating table but I don't have that luxury so I made my own. My mould was on a strong table, I then used my bum and hips to body bop the mould haha, it worked amazingly you can literally see all the little air bubbles rise and pop. After the body bopping session, I used a hand sander on the outside of the mould although I have to say the bopping seemed much more effective and it was highly entertaining although tiring haha.
It's then good to leave, I left this for a week it was inside my house so it was pretty warm.
We then dismantled the mould by undoing the screws and finally cutting our wire that was supporting the internal metal framework as close to the cement as possible. I then treated and lightly sanded the bottomside before flipping it over to reveal the top of my worktop. I nearly passed out through holding my breath at this point, it's scary not knowing what's on the other side. It's also extremely heavy, you'll require some manpower to help lift it.
I then left it for a following few days before I gave it a good sand, sealed, polished and buffed it to my desired finish. I used a standard cement sealant and a wax rather then a gloss as I think it looks more natural. I then buffed it with an electric hand tool until it was super shiny.
It was a really messy (that might be a personal trait) job and it really tested my nerves with all the waiting but it was well worth the effort. I've decided that I'm going to (eventually when I get the time) make my kitchen worktops like this, they really are beautiful and this surface looked just how I imagined when it was placed on the rest of the desk.