The project begins with tracking down all of the needed materials:
- Forms - I used 3/4 inch plywood to build my forms.
- Cement - This time around I wanted to try white cement and go for a very light colored look. After making alot of phone calls I found white portland cement at the local White Cap.
- Sand - I opted for silica (fake) sand for it's white color.
- Aggregate - Since this is a table I wanted to keep the weight down with a light weight aggregate. I originally planned to use styrofoam beads, but read that it's hard to get them to mix in well since they float. It sounded like perlite (basically pumice) would be a better option, but after calling all over town I couldn't find any. I ended up picking up 1 gallon sacks of pumice at the nursery.
- Reinforcing - with no openings that would require rebar, I just used a sheet of half inch sqaure mesh.
- Sealer - I used NeoMix Sealer which is a latex based combination surface/penetraiting sealer specifically for countertops.
I built the mold so that a sheet of 3/4 inch plywood is embeded in the bottom of the table. The concrete wraps over the edges making it appear thicker than it really is. The concrete is 1.25 inches think without the edge, but I think I could have gotten away with making it quite a bit thinner to save more weight (maybe .75 inches thick). Once the form was built I sprayed it with a fast drying laquer to seal up the wood and prevent it from drying out the concrete. I also sealed the seams with silicone. The laquer worked OK although it did stay on the concrete in a couple of places when I pulled off the forms and the silicone made kind of a mess.
I based this on a seven sack mix which is very rich, and then rounded up:
- 3 parts aggregate
- 1.5 parts sand
- 1 part cement
The Messy Part
Since I had miscalculated the volume I had to keep mixing a little bit more concrete until I got enough. I also got the concrete a bit wet(dryer is stronger) but even so it's pretty difficult to get it completely flat. Once things are screed off and smoothed out it's time to let the concrete sit for an hour or two before troweling. However, with the combination of white cement and the soft pumice I wasn't able to do much with the steel trowel. When I put any real pressure on it I would get black streaks, so I had to give up making it really smooth.
I don't know if it's possible to get the edges smooth without patching but I've never gotten close. I also ended up with grain patterns on the edges from the plywood. Since I hadn't been able to trowel the surface I mixed up some cement and water to the consistency of drywall mud and spread that all over with a wide drywall knife, filling in the voids. This is also the method for patching up the edges. Next I sanded with a 60 grit sanding block (it's like a rock hard sponge). I went thru two whole blocks pretty quickly. Wet sanding is definately better - I started out dry sanding and the dust is aweful. After about 10 days I put on the sealer. I was able to put on four coats before it seemed to stop absorbing.
I will definately use something like coated mdf board for the forms next time to prevent the wood grain patterns. That might prevent the need for the laquer as well. The pumice worked OK. I think the top weighs around 100 pounds - 1 cubic foot of regular concrete should weigh 150 pounds - so it definitely helped with the weight. I would recommend the pumice for a darker mix, like the slate grey I did last time. I am still interested in trying polystyrene and will probably experiment with that sometime. I found a product called B-Lite which looks neat, although I've emailed them for info and never heard back. I don't think the silica sand probably makes enough difference to bother with the toxic dust.
The sealer I used seems to work pretty well. We've had minor food spills (which we've wiped up quickly), but don't have any stains yet. It is recommended to use wax over the sealer but I haven't done that yet. The one problem we had with our countertops was water spots on the wax, so I haven't decided yet if it's worth it for the extra protection. Last time I used V-Seal and it seems like the NeoMix does provide more protection. Overall the table looks a bit like a very light marble and turned out really well.