How to Make Fake Rocks Using Concrete

The best concrete mixing ratio to make realistic rocks & boulders

Co-authored by Ryaan Tuttle

Last Updated: December 14, 2024

Making fake rocks is a rewarding hobby as you can build yourself any pond, waterfall or statue that you can imagine. The key to creating these features, or even more complex projects like climbing walls is all dependent on the quality of your concrete mix. This article outlines the best concrete mix to use for making fake rocks that are strong, durable and ideal for sculpting and texturing.

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The best concrete mix for making fake rocks from concrete is actually a mortar mix and not concrete at all. Gravel is the main difference between a concrete mix and a mortar mix. For making fake rocks you will want a mortar that is free of larger aggregates like gravel. This allows you to trowel the concrete at any thickness without gravel disturbing the process.
The best mix ratio for faux rocks is the strongest ratio which is approximately 3 parts sand to 1 part cement. This will produce a finished product concrete that is as strong, or stronger, than regular concrete containing gravel.

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Use sharp sand. The type of sand that you choose will have a large impact on the workability of the concrete as well as the finished strength. Sharp sand, like a brick sand is ideal as the sharp edges of the sand allow for a very strong finished product. Using tumbled sand where the sharp edges have been removed such as with play sand will result in a substantially weaker mortar mix. Using tumbled sand would be similar to attempting to glue rubber balls together. The sharp edges of brick sand allow for a much better bonding process.
- Another major advantage of sharp sand or brick sand over another variety known as masonry sand is that the brick sand is very well graded to remove any potential larger stones which could negatively impact your ability to trowel smooth, finish and stamp texture into the concrete rock.
One of the properties that you are looking for in concrete for making artificial rocks is you want the mortar to be as dense as possible. Using concrete additives such as silica fume (densified) as well as fly ash, water reducer and acrylic polymer can substantially increase the density, strength and ability to sculpt, texture and trowel the concrete.
From a strength perspective the very best thing you can add to your concrete mix is the addition of glass fibers. Glass fibers, or even fiberglass is fantastic for increasing the overall strength of the mix. The glass fibers help to minimize hairline cracking as well as increase the tensile strength of the finished product.
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Mix in the following ratio:
- 1 part type 1 (type N or type 10 depending on where you are)
- 3 parts sharp sand
- 1 part silica fume
- 1/3 part fly ash
  - This is the advanced mortar mix designed for strength. The amount of glass fibers, liquid polymer and water reducer will depend on your application and how much strength and slump you want the mortar mix to have.

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How much water do I need to mix with the concrete?

Community Answer

There should be instructions on the package of the concrete, so you can refer to that for the specific amount of water needed.

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Can I use blasting sand?

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Blasting sand would work great, but it is much more expensive. Unless you just have a bunch laying around, it's not worth the price difference.

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What are "fly ash" and "silica fume?"

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Fly ash is produced in small dark flecks and is carried into the air and silica fume is a byproduct of silica metal, often found in concrete.

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The addition of calcium at .5% will vastly increase the set time of your concrete and can be used in cold weather applications to assist with initial set times and additional heat in the concrete which aids in the curing process.

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Warnings

The concrete will need a full 28 days to cure to full strength however you will have 75% strength after 4-5 days time.

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