Q&A for How to Make Fake Rocks with Concrete

You could use a hand held grinder with a masonry blade and cut the rock or 'score' it and then use a heavy hammer and break it.

Plaster may work well as an alternative to mortar. However, plaster should not get wet after drying.

The strength of your imitation rock will depend more on the underlying structure, and the rigidity of the material that you use to make the shape/structure. Use heavier-gauge wire mesh or combine this with wire fence material, or go big and build a frame out of wire/metal rods with welded contact points. Trying to add thickness to the cement won't achieve the result that you want if you have a weak underlying structure.

You can, but you would need to use a heavier wire mesh to keep the rock stiff. This will prevent cracking.

A fake rock is easier to move around. You can also make a small opening to store things inside of your rock.

Because carving a hole inside a real rock is hard.

You can buy molding rubber (which is rather expensive, very easy to use, and works very well), or you can try using wet sand as a molding material. If it's a interlocking stone though, it might be complicated. (Videos on YouTube can help with this though.) The molding rubber is expensive, but if you need to make more than one stone, it might be worth it. If it is one only, try one of the recipes on the internet where you use alternative material to make a cheaper rubber from silicone and soap to make the mold.

Yes, my artificial rocks are part of my garden pond and they get wet all the time. My rocks are not opened at the underside; they are hollow and have concrete all around so water can't get inside.

You're going to have an idea in your mind of how you want it to look, so draw it out and build from there. Make sure your materials are pliable, yet durable, and able to withstand water.

Fill the holes with glue, let the glue harden, and then paint over it.

Yes, I have done it and it works very well. Make sure that you put a pond liner under the area of the waterfall, as concrete may leak. I made several small concrete batches with different tones and colors of concrete powder dye to the last application of concrete to the waterfall rocks. You can apply it using a trowel, hand (with plastic gloves) and sponges for different textures. Do not apply concrete sealer, as it may kill your plants and fish; just let it cure outside for about two weeks, hosing it thoroughly with no soap and scrubbing several times with a brush.

Trowels, different texture and size brushes, plastic gloves, sponges. All for applying concrete and for texture. If you are making a big batch of concrete you need a shovel for mixing concrete or a drill and mixer. If you make a small batch for small rocks, you can mix the concrete in a pail and use a trowel.

There will be caustic leachate, toxic to fish/plants. Depending on how much time you have, consider 'aging' the project. If the rocks are made separately, soak them (in plastic garbage bins, perhaps even your 'wheelie bin') for up to a month, changing the water out whenever practical (initially, not onto plants/lawn). This also allows them to fully cure to maximum strength, if not allowed to dry out in meantime after making. Then expose rocks to the elements to further leach/neutralize, hosing when possible. Some people even go to the extent of adding a weak acid to the soaking water.

What sort of "sheet"? A 'sheet' as in bed linen? which is coated in mortar mix to 'soak/coat' it in cement? If so, is that so you don't have to have a 'core' of paper/waste plastic? Then "yes". Consider though that natural fibres (e.g., cotton/linen) will decompose, especially if the 'rocks' are exposed to moisture (not only ponds, but also general garden use). As for synthetics (manmade 'plastic' based fibres) these may, more than likely, become brittle due to the caustic nature of the cement mix, so ensure your rock 'crust' is substantially stronger than you initially think.

Artificial rocks are always much lighter than real ones because they are hollow and are made of lightweight materials. So yes, they are suitable for props unless the shapes don't work for your play.

You could but the texturing and coloring will appear quite a bit different from real bricks. Look at a real brick while you apply the mortar, to try to get the shape and texture accurate.

Papier maché works the best to make faux rock, especially one that needs to be portable. Follow the methods in Create Papier Mâché and related articles.

Use an upside-down container (the size needed), cover it with some bits/chunks of Styrofoam, cover that with wire mesh and then put the concrete mixture on the mesh. You'll get a concrete faux-rock cover with ease this way.

Rather than fiberglass, use Rapid Set Cement available at Home Depot or Lowe’s. This material sets very quickly so mix small amount to begin with. Put material in a metal or plastic container (brush the interior surface with Vaseline so when it sets it will just break free from the container) then add water in small amounts and mix. Add more water slowly and stir until the mixture has a thick pliable texture similar to play dough or clay. Now apply the cement mix to fill the holes. Have a two-inch brush handy to feather the rapid set into the rock surface. When you are finished, use acrylic paint (available at Michael’s) to match the rocks color or create a color of your choice.

Yes, you can leave the cardboard base inside of the concrete. The cardboard will likely break down over time, but it won't cause any issues with your fake rock.