Yesterday I made really good progress towards my goal of creating my own DIY faux stone facade for the porch base area. I still haven’t poured any of the concrete blocks (more on that later in the post), but I have some of the silicone molds finished and ready to pour the concrete. (I’m actually going to make aerated concrete.)
To recap: Yesterday I showed you how I used the real stone on the front of my house…
…to create a structured shape from silicone stone…
…this is what it looked like when it was dry and peeled off the stone.
This part worked out really well, but as you can see, the textured silicone mats I created don’t give me a way to actually pour concrete. If I tried to pour concrete onto this mat it would just run over the sides. So making these mats was just the first step towards creating real molds that I can use to pour concrete. Now I’ll show you how I turned this mat into a real shape.
For the purposes of this post, I’ll show you the shape I created using the textured mat I made from this particular stone at the front of our house. You can see how textured this stone is.
I measured the textured portion of the mat and determined that the finished interior dimensions of the mold should be 5″ x 16″. So I used my table saw to cut a piece of plywood to exactly that size. Then I cut sides out of PVC panels. I used PVC because I already had those on hand and didn’t need to purchase additional supplies. You can also use melamine-faced plywood for these areas. I used 1″ x 4″ boards and cut the end pieces to 5″ wide, and then I cut two pieces for the long sides to 17.5″ wide.
I screwed all of these pieces together to create a very simple box. I didn’t use any glue to build the box because once it’s dry I have to take it apart very easily and remove the concrete block. Then I have to be able to easily put the box back together to cast the next one.
Once my box was assembled, I cut my textured silicone mat to fit inside the box. This silicone is very easy to cut with scissors, but I also used a utility knife with a new blade for some cuts. Both worked very well and the silicone was very easy to cut.
I cut a piece of each to make sure it fits somehow well. You can always cut off more, but if you cut off too much there’s really nothing you can do about it.
After cutting it to fit, I had to make a few adjustments. You can see how wavy the silicone mat is at the edges.
How to build a mold with a homemade textured silicone mat for casting artificial concrete (air-concrete) stones.
And of course this guy made my project a little slower and more demanding. He doesn’t really understand that I’m out working on projects. He always thinks I’m out petting him and letting him sleep on my lap.
Anyway, back to the project…
The reason it is so wavy is because the backs of my silicone mats are not flat. I actually tried to make them flat. The first one I made, I tried placing the silicone/cornstarch ball on a piece of plywood and then pressing it onto the stone, but that didn’t work at all. Not only would this have required about twice the amount of silicone per brick (and silicone caulk isn’t cheap!), but I also wasn’t able to apply enough consistent pressure to force the silicone into all the structures and crevices of the brick. This process might work for stones that aren’t as heavily textured and don’t have as much high relief, but with these stones it was a no-go. So I ended up forgoing the plywood and flat back plan and just pressed the silicone onto the stone with my hands. This meant that this was very irregular and definitely the case not flat back on each silicone mat.
So I had to add a step to compensate for the irregular backing of the silicone mats. First I used a fine Sharpie marker and marked the highest point. And then I removed the silicone mat and marked a solid line around the inside of the box at this height.
And then I used paving sand to raise the low areas. I simply used a butter knife to slide between the box and the mat so that I could lift the mat in the lower areas, drop some sand under the mat in that area, and then push the mat back down onto the sand. It was a messy process, but I kept going until the edges of the mat lined up with the black Sharpie line around the shape.
Here you can see it from a different perspective, aligned with the line around the box.
After aligning the mat all the way around, I realized that when I took the mat out of the box, I should have taken the opportunity to draw a second line 3/4 inch above the first line to indicate how far up I would be pouring the concrete. I forgot about that and there was no way to remove the mat once it was just right, so I did my best. But this would have been much easier before I added the mat and sand.
After removing the excess sand with my vacuum cleaner, the final step in preparing my mold for pouring the concrete was to place a bead of sealant in the corners of the box as well as where the mat meets the box. I used silicone caulk for this, but I think that was a mistake. This stuff dries so slowly!! So I think from now on I will use a quick-drying, siliconized latex compound for this step. I can’t wait several hours for the caulk to dry between pours.
I got this far yesterday. I have a few more molds to build today, and then I’ll start pouring the actual concrete “bricks” (air-concrete). I hope I can have a drink before the end of the day today.
I was going to pour some of it yesterday using the molds I prepared, but I think I’ll change the aerated concrete mix I was going to use. I’ll share more about the process when I’ve made some, but the formula I use for the aerated concrete calls for using Portland cement, so that’s what I purchased. The problem with Portland cement is that it takes 24 hours to dry. I can’t pour six stone a day. At this rate I will never finish this project. I need quick-drying concrete. Therefore, I will test the aerated concrete formula with Rapid Set Cement All, which hardens in 15 minutes and can be demoulded in an hour.
I don’t know if it will work because I have no idea if the additives that make this product a quick setting cement will work with the aerated concrete formula. My brain is not designed for chemistry, so these additives could affect the aerated concrete process. I have no idea, but I’ll test it and see if it works. I really hope so because I need these stones to dry very quickly!
