DIY Concrete Faux Stone Facade, Part 3 — Pouring The Faux Concrete “Stones”

If you’ve been following along, you know that I had gotten as far as creating my own silicone textured mats using the actual stones on the front of our house and then turning those silicone mats into molds that I could use to pour my own concrete stones for the skirting.

I tried three different times before I had success, so let me tell you about the failures and what went wrong before I show you the success.

Test #1 — Aircrete faux stones with a quick-cure concrete — FAIL

I was so focused on this process that I forgot to take pictures of this first attempt. But my original idea was to make my own aircrete (which is about 50% lighter than concrete) to make my stones. I came across this very helpful video showing a very easy recipe for DIY aircrete using items you probably have in your kitchen (especially if you’ve ever done a ketogenic diet).

My one issue with the process he shares is that he uses plain Portland cement to make the aircrete, and Portland cement takes a long time to cure. I was hoping to find a way to make aircrete with a quick-drying concrete formula instead of the very slow-curing Portland cement so that I could get through several pours with each of my six molds in one day.

Before I tested this, I left a comment on that video to ask if he thought it would work. He seems to know a great deal about chemistry, while I’m very ignorant about the topic. I wasn’t sure if the extra ingredients in a quick-dry formula would negatively interact or counteract the aircrete formula. He actually responded within minutes and said that he thought it might work as long as the quick-dry mix doesn’t have an aggregate in it.

Unfortunately, as far as I can tell, there’s no such thing as a quick-dry cement mix with no aggregate. The closest thing I could find at my local Home Depot is Rapid Set Cement All, which uses a super fine sand as an aggregate. So I decided to give it a try.

Rapid Set Cement All, when mixed with water according to the instructions, sets within 15 minutes and is supposed to cure in about an hour. But again, that’s when it’s mixed with plain water. But I tried it mixed with the aircrete liquid formula from the video.

It didn’t work. Or I guess it would be more accurate to say that it probably would have worked had I given it more time to cure, but giving it two hours to dry, it had only reached the consistency of a firm moist beach sand, kind of like the consistency that you would want to build a sandcastle on the beach. So the aircrete liquid formula definitely cancelled out the quick-cure feature of the Rapid Set Cement All, and that’s the exact feature that I needed.

So at that point, I had two options left. I could use the exact aircrete formula using plain Portland cement, which would allow me to pour six stones per day, or I could just use the Rapid Set Cement All and mix according to the instructions (i.e., using plain water) and be able to pour several stones per mold per day. I decided to go with the second option.

But since I knew I was going to be adhering these faux stones to the skirting area of my front porch, I thought it might help the adhesion to add some texture to the back of the stone. I did this using my utility knife, and I just barely ran the blade over the surface of the concrete.

That ended up being a mistake. And then I made a second rookie mistake, again, because I’ve never worked with concrete before (correction: I have, but it’s been over a decade). I gave it the hour that the instructions say, and it felt very hard to the touch. But I noticed it also still felt very hot, meaning the chemical reaction that cures concrete was still happening. But I was anxious, so I ignored that and tried to demold the stone.

As I was removing the stone from the mold, it snapped in two right down the line shown by the arrow below.

Since it was ruined, I decided to test out the strength of the Cement All at that point, so I picked up the two big pieces and dropped them onto the concrete, and it continued to break. From the front, it looked like random breaks.

But these were not random breaks. When I turned the pieces over, I could see that the breaks on the backs were exactly along those texture lines that I had made on the back.

You can see here that I didn’t even make those texture lines very deep at all, but they definitely affected the integrity of the concrete.

So this was a very helpful test, and a very important learning experience for a concrete newbie like me. The first important lesson is that you never remove concrete from a mold if it’s still hot to the touch, even if you’ve waited the amount of time the instructions say to wait. As long as it’s still hot, the concrete is still curing. You have to wait until that heat has completely dissipated before removing it from the mold. The second important lesson is that you shouldn’t mess with the back, probably especially on something as thin as 3/4″ faux stones.

With my new knowledge gained from this second failed attempt, I was ready to try a third time.

Test #3 — Rapid Set Cement All again — SUCCESS!

Again, I mixed up a very small batch of the Cement All and poured it into my mold. And by the way, before pouring the concrete into the mold, I sprayed the mold with WD-40 to act as a mold release. This time, I left the back alone.

And this time, I also waited not only until it not only felt really hard, but it also felt cool to the touch. I’m not sure how long that took because I got busy on something else. I would guess it took about an hour-and-a-half or two hours. Then I removed the small end piece of my mold and removed the stone.

This was a success! For this test, I used two of my molds and poured two stones, and they both came out great. I think the WD-40 caused some discoloration, but since I’ll be priming and painted these stones, I’m not concerned about the color at all.

I was so excited when I pulled these two stones out of their molds and saw the finished result.

Other than the color, they looked just like they could fit right in with the stone on the front of our house.

Because of the discoloration, it’s a little difficult to get a good, accurate picture of the stones. You might just have to take my word for it. 😀 They look so good!

UPDATE: Here’s a comparison of the smaller one with the original stone that I used to create the silicone textured mat.

Doesn’t that look good?! I really couldn’t be happier with how this turned out. The funny thing is that the natural color of the Cement All with the discoloration caused by the WD-40 actually has a very natural Austin stone look. But I’ll be priming and painting mine to match the rest of the house. And there’s no need to worry about the WD-40 affecting the paint. I’ll be using my favorite shellac-based primer (or maybe my second favorite oil-based primer) first, and it will seal that WD-40 so that it won’t cause any problems with the paint.

Now that I have my process down, I need to set up an assembly line in my carport with all six of my molds so that I can pour six at a time every two hours.