Ok, since I'm retired from a sometimes miserable day job in the chemicals for concrete industry, I guess I should chime in on a couple things. So here comes another "book length missive from Leo". I highlighted topic in the "chapters" in case you want to just skip over sections. Pour yourself a coffee or drink, sit back and have a read.
definition -
Concrete is the end product of the mixture of cement, sand, gravel, water & optional chemical additives. Cement is the fine powder that has the pozzolanic properties when exposed to water. Its not a cement sidewalk, its a concrete sidewalk, (except West Virginia, the WV Dept. of Transportation uses the word cement where 49 other states use the word concrete in their construction specifications) To my surprise, to this point in the thread, everyone has used the term concrete and cement in the correct context - you guys almost sound like you work in the field.
Grout can be a number of things, some grouts include cement, as used in concrete, plus sand. No gravel is used in grouts. Technically, most concrete pots are made with a cement based grout, because they tend to not include the gravel component. But that is a minor detail. When you say concrete pot, most everybody knows exactly what you mean.
Concrete normally does not have Calcium chloride in it, unless it is added to speed up the set time. It is forbidden to use calcium chloride in concrete that will come in contact with steel, so cements & aggregates are all manufactured with near zero chloride. Calcium chloride is only added after the fact to low end use concrete mixes to speed up set times. In pre-packaged, & pre-mixed concrete - where you just add water - it might very well have calcium chloride, or it might not. If you mix your own cement, sand, and gravel, there will be zero calcium chloride unless you put it there.
However, the concern about what leaches out of fresh concrete is a valid concern. The Safety Warning!
Cement is an anhydrous, finely powdered blend of various calcium and magnesium oxides, silicates of aluminum, sometimes some iron, and if dealing with ciment fondue, sulfates of iron and aluminum. Fly ash from coal fired power plants also has a pozzolanic reaction and is often used to supplement the cement portion of concrete mixes. When water is added, these materials hydrate, forming a matrix that becomes the binder for the concrete. The hydration reaction releases hydroxide ions, which are very basic. Water coming out of or off freshly placed concrete will be pH 12.0 to 14.0 - this is caustic enough to cause chemical burns to the eyes and skin. Always wear gloved and eye protection when working with cement powder and fresh concrete. The initial water to leach out of concrete is not only exceptionally basic, it also can contain heavy metals that are trace contaminants of the cement (and fly ash if present). The initial leaching from concrete is quite toxic to plants. So this is why concrete pots must be cured before use.
If you play patti-cake with bare hands in fresh concrete for any length of time, not only will the skin of your hands slough off that evening, (painful) you will experience nausea, sweating, dizziness, and headache from what has leached through your skin into your blood stream. It can throw your blood pH off with disastrous effects. You must wear gloves. (been there, done that)
The initial hydration reaction is pretty quick, normal set times concrete will be at about 40 to 60% final compressive strength at 48 hours. At 30 days it will be in the 75% to 90% range, and then the hydration reactions slow down, it take many years to reach 100% hydration, the whole time the compressive strength of the concrete is increasing. It is important to keep the surfaces of the concrete damp for the first 48 hours, if the top layer dries before sufficient hydration happens, the surface will flake off in time. Leave your concrete pot in its mold or forms for a minimum of 48 hours, if you are too quick to remove the form, you can break the pot before it is fully set up.
The hydration process releases heat, so don't be surprised if your concrete pot feels hot the first day or two. Concrete sets best between 60 F and 80 F. Outside that range, for structurally critical work, chemicals are added to compensate for issues caused by temperature. (when its cold, accelerators like calcium chloride, and when hot, retarders like corn syrup) Amounts added are small, very small at the pot size batch level, I wouldn't bother if I were to do it at home. Avoid making concrete pots on 100+ F days or in freezing weather and you will be ok.
Curing concrete
Generally the common way to cure concrete pots is to place them in a bucket or other container of water, and let them soak for a month or two, changing the water once a week. After a month, you can check the pH of the last change of water, and if it is less than 9, it has leached enough to be used. If you don't have a pH meter, you can taste the water - if it tastes ok, its good. Or you can throw a goldfish or minnow in, and if it lives 24 hours - its good. Once concrete is cured, no further significant leaching will occur. Cured concrete pipes are routinely used for drinking water supplies in some areas with no ill effect. Many old fish ponds were made of concrete, and of course swimming pools.
A sealer can be applied, let the pot dry out a few weeks, then apply the sealer. There are many acceptable acrylic, latex and other concrete coatings that can be applied after curing the pot if you want to guarantee no further leaching. Home Depot, Menards, Lows all have a bewildering array of various coatings or sealers. A common old school sealer that can be used is linseed oil from the paint department of your local box store. It works well. Coat the outside of the pot with the sealer too, to aid in freeze-thaw resistance. Sealing a pot will significantly aide freeze-thaw resistance.
Concrete blended with peat moss, paper or other organic products is not freeze thaw resistant, and there is no point in applying a sealer to this type of mix. It will crack, flake and slough off layers over time. Usually these are made with thick walls, and the deterioration becomes part of the organic look, along with the moss that grows on the pot. These are cool, especially in areas where they have mild winters. Temporary display pots in the north. Usually you can get several years from one of these, even in Chicago.
The suggestion to use pre-packaged
concrete repair products instead of ''regular'' concrete is an excellent suggestion. Traditional concrete is not durable when less than 4 inches thick, something has to be done if used in thin applications. Often repair products include a latex, or vinyl or acrylic polymer to provide the elasticity needed for smaller, thinner applications, like a pot or patch. These polymers make the concrete slightly flexible. In addition most concrete repair products have fibers added that help hold the concrete together. Some use bits of recycled plastic, if these leave sharp points and edges on the surface of the pot, just hit it with sand paper, (a wheel or pad sander is great) and they will smooth right out. Construct internal wire frames for larger pieces, place them so they are encased in the concrete. These will help support the concrete. Concrete has great compressive strength, but not much flexibility, and tensile strength is poor too. Internal frames are a must for anything over a foot or so long if you want them to last. You must make armatures and frames for large slabs. Fiberglass mesh like that used in plaster and in fiberglass boat building is good also for internal support. For small pieces this is not so important.
Tinting and coloring concrete. Use dry powders, either made for tinting concrete (hard to find retail) or dry pigments from an art supply store. Earth pigments are very natural looking, burnt umber, raw sienna, red ocher, etc. Acceptable are earth pigments suspended in alcohol solutions, aniline dyes in alcohol solutions can be used, though some colors are fugitive in direct sunlight. Don't use oil based pigments, spaulding and pop outs will occur. Tinting with potter's clays may weaken the concrete unless they are the 'pure' earth pigments, I don't know them well enough to be certain on usefulness. Tints for colored grouts should work very well.
I haven't made any concrete pots yet, but this is what I can offer from my experience "in the business". One series of concrete repair products available will mention being acceptable
for making vertical patches, I would try one of these first. They will have the highest concentration of polymer in the cement matrix. Since I haven't done it myself, I can't recommend brands, but knowing generally how they are made, this is what I would try first.
