By popular demand, here's a simple "formula" to begin experimenting with your stepping stone mortar ingredients:
1/2 cup White Portland Cement
1 cup Fine, Light-Colored Sand (minimum)
1/4 to 1/2 cup Washed Pea Gravel (optional)
1 tbl Dry Calcium Chloride (optional)
3 tbl Liquid Acrylic Admixture
1/2 tsp Superplasticizer (optional)
1 tbl Compacted Silica Fume (optional)
1 tsp Dry Cement Pigment (optional)
1/2 tsp (packed) 3/4" or 1/2" Nylon Fibers
3 tbl Clean Water (this will almost certainly vary)
Remember, appropriate quantities may be very different depending upon the actual, specific components you use. Read the manufacturer's instructions! I also encourage you to read more about these ingredients on my Stained Glass Stepping Stone Mosaic Mortar Material Matters web page.
Though some folks refer to this as "Cole's formula," it is actually just an introduction to blending/mixing information readily available throughout the concrete industry. I do not suggest it here as an optimal mixture, which depends upon your application, but rather as an example to help you to start learning.
For far better and more complete instructions on how to make stepping stones read Angel's Silicon Folly Studio's Stone Manual
It doesn't matter much how you mix this stuff together, as long as you do it very thoroughly. This is merely the way I've come to prefer.
Thoroughly mix together the cement, sand and calcium chloride and set it aside. Combined the superplasticizer with the acrylic and mix the silica fume into the result. Add the pigment and fiber to this and mix thoroughly. If you're using compacted silica fume, let it soak awhile, then mix some more. Combined the acrylic/superplasticizer/silica fume/pigment/fiber with the cement/sand/calcium chloride and blend well. Mix gently, but thoroughly for several minutes with gloved hands or stiff spatula, if necessary adding teaspoons of water until the consistency is like smooth, thick pancake batter. The bubbling should have slowed down a lot by the time you stop mixing. Pour the stuff into a disposable plastic container lightly lubricated with something like Vaseline or a really good release agent. Repeatedly tap and/or vibrate for several minutes, or until you stop seeing bubbles rise. Wait until well after the test block is hard before removing it.
If necessary, alter the proportions and try again. Once you have a block or two that is the way you want it, multiply your recipe to whatever size you need for your real mold. Remember: 3 teaspoons (tsp) equals 1 tablespoon (tbl) and 16 tablespoons equals 1 cup. You can easily determine the amount of mortar you'll need by counting how many cups of water it takes to fill your mold to the level desired. Compare that to the volume of your solid test block, not the volume of the un-mixed ingredients. Make sure your first actual artistic work using any new formula or ingredients is simple (quick) enough that you won't mind too much if it doesn't turn out.
This mortar formula suggests a relatively rich ratio of cement to sand (1 part cement to 2 parts sand, which is 1/3 cement) where redi-mix commonly uses less (about 1 to 3, or 1/4 cement). The difference being the definition of "best" depending upon whether you're the company maximizing profit selling the redi-mix, or the artist maximizing quality and protecting your reputation. It's also true that there can be too much of a good thing. The sand plays an important role, so you don't want to use more than 1/3 cement when mixing your own mortar. Also, it's important that at a good portion of the sand not be too fine in such a rich mix. The point being that you can successfully cheapen your mix a bit and it will be good and strong. Alternatively, you can add pea gravel to your mortar after pouring in the "face" to decrease the cost in a better manner than using different layers of mortar and concrete as some folks do. Adding up to the same amount of gravel as the cement you used (if it was 1/3 of the original mix) would be OK. If the gravel has been washed (you can buy it that way) then there shouldn't be a significant change in the color of your mortar which otherwise would be noticable on the edge of your stone.
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