For those folks who want to mix their own high quality stained glass mosaic mortars and grouts rather than use expensive products like DiamondCrete...
If all you want is a simple formula, then here is one. However, I recommend reading a bit more about...
Some important components:
White Portland Cement is available from your local lumber yard or concrete company, if you have one. This is the stuff that binds everything together and yields a strong, solid, and durable product. Other more specialized types of cement, such as CTS' Rapid Set, can be used for additional protection against cracks due to freezing or shrinkage.
Fine, light-colored sand. White silica (quartz) is usually preferred, though you might find colored quartz useful for special projects. This should be graded to fairly uniform sizes. Those I use most are #30 and #70. Its inclusion is especially important since it is the first line of defense against shrinkage. Including some extra fine sand will improve the surface smoothness, but using only such fine sand is likely to cause hairline cracking. Sand is also significantly less expensive than cement. Avoid the temptation to use too much however, as doing so will weaken your finished product.
Superplasticizers, or high range water reducing admixtures, are highly efficient dispersants or deflocculants which decrease the water requirement in the mortar mix thereby decreasing shrinkage. That is, they help eliminate the main cause of cracked glass mosaic elements. They keep individual cement particles from clumping together which also helps the mortar to flow between mosaic elements. Pigment dispersion is also improved. Melamine-based dispersants are recommended for high performance grout, mortar and plaster, as opposed to napthalene-based ones for general purpose concrete. They are also available as a dry white powder for convenient batch pre-mixing and easier mixing of pastel colors. Find one that does not retard the set time as those from batch plants often do. (Visit my Mortar Component Links page for sources)
A set accelerator such as calcium chloride or more modern non-chlorine (non-corrosive) curing agent is especially useful to counter the retarding effects of polymer admixtures. This is what makes it so that you can remove your mosaic (e.g. stepping stone) from its mold quickly instead of the next day. It is also available in dry form for convenient batch pre-mixing. Some accelerators should not be used in colored mortar. Set acceleration weakens the finished product, which for maximum strength should dry out very slowly, so it amounts to an economic trade-off. (Visit my Mortar Component Links page for sources)
An Acrylic or Epoxy polymer admixture for increased strength and to decrease moisture absorption. These polymers also increase the grout's adhesion to the mosaic elements. Most readily available polymer admixtures are liquid, but some are available in powdered form. There are many different types for sometimes subtly different purposes. The liquid acrylic mortar & grout admixture readily available from the tile section of your local building supply store will most likely work great. (Visit my Mortar Component Links page for sources)
A bit of Pozzolan material for increased strength and a smoother surface. Light-colored Silica Fume (microsilica) is more practical for mosaics than the more readily available, but darker fly ash. Most Silica Fume is dark too, but you can also get it in an off-white which is great for pastel colored mortars. The pigments mask the grey pretty well as long as you want dark mortar. This stuff fills in the tiny spaces between the other components, as well as chemically interacts, yielding a smoother and stronger product. The compacted form works fine, is more efficient to ship and less hazardous to use. Silica Fume and fly ash have different properties and effects. Here's an excellent explanation of Silica Fume Cement. Be careful to read and follow the manufacturer's safety precautions for this material. (Visit my Mortar Component Links page for sources)
Other reinforcement material such as nylon, polypropylene, or glass fiber lets you get away with a thinner mortar base. Adding fibers dramatically reduces the likelihood of the product cracking. Fibers are also far superior to wire mesh for products like stepping stones and most other mosaic work. Don't waste your time, materials, and reputation using wire. I like nylon far better than polypropylene since it is much more flexible and lays into the mortar better without leaving a hairy surface. If you're concerned about durability over centuries, glass or carbon fiber is probably the way to go. (Visit my Mortar Component Links page for sources)
Pigments come in a wide variety of colors and forms. Some which would be too expensive for most other cement-based applications are within reason for small mosaic projects. Beware: some pigments are extremely hazardous, especially if inhaled. Learn about what you're using before you use it and take appropriate precautions. (Visit my Pigment Links page for sources)
Release Agents can have a dramatic effect on the quality of the final surface of your mortar. You can use readily available lubricants like Pam, soy lecithin or Vaseline. However, more appropriate release agents especially formulated for cement will yield a smoother finish with noticeably fewer of those annoying little holes. (Visit my Mortar Component Links page for sources)
Washed Aggregate such as pea gravel can be added to the mortar to decrease its cost without sacrificing quality. If a grouting layer is poured before mixing in the aggregate, make it as thin as possible to avoid warping due to the different degrees of shrinkage between the top and bottom.
Lightweight Aggregate, such as perlite or vermiculite, can be added for special applications requiring overall weight reduction, additional thermal insulation or sound absorption. However, special care must be taken as such aggregates dramatically reduce concrete's strength. (Visit my Mortar Component Links page for sources)
As for how to use this stuff,
I'm a strong believer in starting from the manufacturer's directions and experimenting. It's really very easy if you don't let yourself get intimidated. It's also lots of fun, especially if you have kids of an appropriate age. Your specific formula will depend on exactly what products you're able to find locally or obtain from a distance. There are hundreds to choose from, but don't let that slow you down. Whatever you choose is likely to yield a superior product for substantially less cost than a heavily marked up alternative marketed for your "convenience," which simply takes advantage of your fear of the unknown. Even buying the smallest quantities at full retail, your total cost is likely to be less than half what you'd pay your favorite stained glass supplier and you can make up whatever colors you want whenever you want them. If you measure and record what you mix you can easily duplicate your favorite colors later.
The most important thing about mixing mortar is not to use too much water. If you do, your stone will shrink as the cement cures and drys and your glass will crack. The same may well happen if you use too much water with your DiamondCrete. Sometimes the amount called for in the instructions is too much. Basically, don't make the mix any runnier than you have to in order to get it to flow between the mosaic elements. Using some superplasticizer really is worthwhile since it cuts down on the water requirement. Your local cement guys may get a chuckle about asking for the stuff (same for the silica fume). Or, if you're lucky they may be really helpful. The acrylic and fiber reinforcement shouldn't be considered optional, while the silica fume is. While it's good to support your local vendors, you may well be able to save a bundle ordering from someone else.
One way to come up with your own "formula" is to carefully measure very small batches (e.g. with a half-cup or so of cement) and time how fast each sets up as a block in the bottom of an old milk carton (actually I like Cool Whip containers). If it's not set in about an hour, skip it and try again, adding more set accelerator. You can have a few test mixes going at the same time such that you should be able to come up with a good recipe in a few hours. Once you have some that sets up at the rate you want, multiply the recipe proportionally and make a very simple full sized stone as a final test. If you change your ingredients (e.g. to another brand of acrylic admixture), you may have to change your proportions a bit.
Most any proportion you use of most of the components will be fine as long as you're fairly consistent and it's something close to the manufacturer's recommendation. If the stuff doesn't come with any instructions, or they're in units that aren't useful, try something like this formula to get started. The main trick is to get the amount of set accelerator right while keeping the rest of the proportions the same, especially the other admixtures since they may dramatically change (usually lengthen) the set time.
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