Get the right concrete. 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.
Use the best ratio. 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.
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.
Be sure that your mortar is dense. 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.
Add glass fibers (or fiberglass). 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.
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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