FreeShip- Tan Art Concrete #251, Complete Concrete Ready-Mix - (Prompt rebate on orders with 3 or more FreeShip items!)

DESCRIPTION-> Click "Learn more about this item" for article & instructions!
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This is one of 4 custom concrete ready-mixes made by us. There is a 5th called "Cement-All" that was added after the descriptions for the 4 custom mixes were written. Note that this #251 mix is formulated for quick setting of small castings 2 inches thick or less but with an adequate working life for placement of the concrete in the mold. There is a chance that thermal cracking can result in much larger masses than 2" due to curing exotherm. This is not made with white portland cement and cannot be colored as brightly as the color tiles shown in the #80-102 White Art Concrete listing. It awaits testing to see how well it can be colored with pigments. Probably it will do adequately with the iron oxide colors like red, black, yellow and a few others, especially the earth tones.

PICTURE #1- Sample bag quantities of this listing #251 concrete.

--This #251 concrete was used with a range of water ratios by weight:
20.3 water/100 concrete powder (conversion factor = 0.169) (upper value)
18.9 water/100 concrete powder (conversion factor = 0.159) (median value)
17.4 water/100 concrete powder (conversion factor = 0.148) (lower value)
(The conversion factor = water/water+powder (20.3/120.3 = 0.169)
(See below "NOTES ON WATER RATIO, WORKING TIME, AND MIXING"
for detailed explanation). Mixing time should be about 2 minutes.

PICTURE #2- Tiles of our four Art Concrete mixes with this listing on bottom.

PICTURE #3- Tiles of Tan Art Concrete Mix #251 integral and surface variations:

258 "Tan Art Concrete Mix", 20.3/100 water ratio, no additional aggregates,
Burnished with brass stylus.

283 "Tan Art Concrete Mix Under Silver Mica", non-integral mix, layered.
https://www.etsy.com/listing/500052386/mica-pure-medium-coarse-flake

138 "Tan Art Concrete Mix", 20.3/100 water ratio, no additional aggregates,
linseed oil test longevity compared to shellac sealant/coating (deeper)

271 "Feldspar Chunks, Small" 120%, integral mix, exposed agg by brushing.
https://www.etsy.com/listing/1315552674/freeship-feldspar-chunks

275 "Tan Art Concrete Over Large Mica", crushed, non-integral mix, layered.
https://www.etsy.com/listing/1231706843/freeship-muscovite-mica-clear

268 "Large Mica, Crushed" 0.13%, integral mix, exposed agg by brushing.
https://www.etsy.com/listing/1231706843/freeship-muscovite-mica-clear


PICTURE #4- Tiles #251 or #252, surface variations in texture, shape, & reflectivity.
Opaque reflective copper and silver metal leaf micro-flake,
Left to Right, Top to Bottom:

A) "#251-#252 Concrete Under Copper Micro-Flake", non-integral mix, layered.
Naturally formed bas-relief fugitive copper metal leaf micro-flake.
Sidewalk in Copper.

B) "#251-#252 Concrete Under Copper Micro-Flake Coated Coal Slag",
non-integral mix, layered. Natural texture 30/60 mesh coal slag with
copper metal leaf micro-flake. Darkly copper ore sand.

C) "#251-#252 Concrete Under Copper Micro-Flake", non-integral mix, layered.
Controlled form, textured bas-relief fugitive copper metal leaf micro-flake.
Reptile fossilized skin transmuted from sedimentation to Copper.

D) "#251-#252 Concrete Under Copper Micro-Flake", non-integral mix, layered.
Controlled form, mid-relief wrapped copper metal leaf micro-flake.
Middle ages Austrian baroque, abstract in copper.

E) "#251-#252 Concrete Under Silver Micro-Flake", non-integral mix, layered.
Naturally formed bas-relief fugitive silver metal leaf micro-flake.
Moon from orbit.

F) "#251-#252 Concrete Under Silver-Copper Micro-Flake", non-integral mix, layered.
Controlled form, bas-relief silver-copper metal leaf micro-flake.
Solder flows distressed.

G) "#251-#252 Concrete Under Silver Micro-Flake", non-integral mix, layered.
Naturally formed bas-relief fugitive silver metal leaf micro-flake.
Erosion reveals rivers, ancient lahars.

H) "#251-#252 Concrete Under Silver Micro-Flake", non-integral mix, layered.
Controlled form, bas-relief silver metal leaf micro-flake.
A bounty of cells under a microscope, frozen.


PICTURE #5- Tiles #251 or #252, surface variations in texture, shape, & reflectivity.
Opaque reflective gold metal leaf micro-flake. Graphite sheen micro-flake.
Left to Right, Top to Bottom:

A) "#251-#252 Concrete Under Gold & Graphite Micro-Flake", non-integral mix, layered.
Controlled form bas-relief gold metal leaf micro-flake with graphite inclusion.

B) "#251-#252 Concrete Under Gold Micro-Flake Coated Coal Slag", non-integral mix,
layered. Natural texture 30/60 mesh coal slag with graphite micro-flake.

C) "#251-#252 Concrete Under Gold Micro-Flake", non-integral mix, layered.
Naturally formed, textured bas-relief fugitive gold metal leaf micro-flake.

D) "#251-#252 Concrete Under Gold Micro-Flake", non-integral mix, layered.
Controlled form, bas-relief shapes in gold metal leaf micro-flake.

E) "#251-#252 Concrete Under Graphite Micro-Flake", non-integral mix, layered.
Naturally formed bas-relief fugitive graphite micro-flake.

F) "#251-#252 Concrete Under Graphite Micro-Flake", non-integral mix, layered.
Naturally formed mid-relief fugitive graphite micro-flake.

G) "#251-#252 Concrete Under Silver/Graphite Micro-Flake", non-integral mix, layered.
Controlled form high-relief graphite micro-flake w/post applied copper micro-flake.

H) "#251-#252 Concrete Under Graphite Micro-Flake", non-integral mix, layered.
Controlled form, high-relief with post applied graphite micro-flake.

PICTURE #6- Tile of #251 or #252 Art Concrete Mix:

258 "Closeup Concrete Art Mix", 19.6/100 water ratio, no additional aggregates,
Burnished with a brass stylus.

PICTURE #7- Tile of #251 or #252 Art Concrete Mix:

"Closeup Concrete Art Mix, Concrete Foam", integral mix, (a work in progress).
4 crystals protruding from the concrete mass of post applied graphite micro-flake.

PICTURE #8- Tile of #251 or #252 Art Concrete Mix:

"Closeup Concrete Art Mix Under Copper Micro-Flake Coated Spherical Aragonite",
Non-integral mix, layered. Post exposed-aggregate, brushed white aragonite.

PICTURE #9- Tile of #251 or #252 Art Concrete Mix:

"Closeup Concrete Art Mix Under Graphite Micro-Flake", non-integral mix, layered.
Naturally formed mid-relief fugitive graphite micro-flake.

PICTURE #10- Tile of #251 or #252 Art Concrete Mix:

"Closeup Concrete Art Mix Under Copper Micro-Flake", non-integral mix, layered.
Controlled form, mid-relief wrapped copper metal leaf micro-flake.


Note that a binder was applied between layers of coated micro-flakes when they were post applied for making the above sample tiles. The binder I used was natural shellac because of its ease in handling, lack of toxicity, and alcohol solubility. For production, a strong binder like a transparent epoxy or urethane varnish should be used for its toughness and longevity. For the ultimate in binder/finishes go to an auto paint supplier. They have the highest quality finishes made (and most expensive). Finishes for automotive use are unsurpassed. Many of the samples had no post applied flake materials. If none are applied for production, a high quality finish should still be applied. It should be used only after evaluation determines it will uphold the beauty and protection of the product and will be compatible with your materials and methods.

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To jump down to the main article about this material, go to the next "****************", below.

NOTES ON OPAQUE SURFACE TREATMENTS FOR CONCRETES #251 AND #252

Since both #251 and #252 can have darker ingredients, they cannot be colored with pigments (with the possible exceptions of red, black, and other iron oxide derived pigments for the lighter of the two concretes, #251 Tan). They can have a greater choice of pozzolanic and strengthening additives and so are potentially stronger than #63-101 or #80-102 (but whether they are depends on whether they have added colorants or aggs, time mixed, water content, and other factors).
Since integral bright and painterly pigment addition would be wasted on them for the most part, I've viewed them more in terms of their 3-D sculptural possibilities instead of 2-D pigment coloration. Sculptural properties include surface texture and shape, which can vary from simple low bas-relief to extreme high relief. Color is still an important factor in any sculptural treatment but it's more often used in tandem with texture which plays with light by reflecting, scattering, breaking, absorbing, or otherwise modifying light. And as for shape, these darker concretes seem to beg freer, organic shapes, at least for me. I read about (and repeat) "playing with art" nowadays, but it still boils down to specific and easily described methods that have been in use by many people for many years. Yes, there are a few materials being combined in new ways using novel methods; a good example is the creation of "Cells" using epoxy resin, silicone oil, and alcohol. But those materials have all been around for a long time and the new methods are a result of patching together old methods that have been used for decades. The old saying that there is "nothing new under the sun" seems to be more true with the passing of time, due perhaps to the steady increase in human population combined with the existence of the internet and the increasing ease with which it disseminates information to the "masses". The "nothing new..." proverb means (according to Wiktionary) "There is nothing truly novel in existence; every new idea has some sort of precedent or echo from the past."
I only bring that up because I've been making concrete samples using ways to cast the darker #251 and #252 and have "discovered" what (to me) are new methods of mold making and casting these concretes. In truth I've only "re-discovered" new methods. "Nothing new..." also means people are independently re-inventing the same things all the time.
See the above picture notations for basic descriptions of the ingredients used to make the trending, more 3-D type samples with basic methods (deconstructed).

NOTES ON WATER RATIO, WORKING TIME, AND MIXING:

--This #251 concrete was used with a range of water ratios by weight:
20.3 water/100 concrete powder (conversion factor = 0.169) (upper value)
18.9 water/100 concrete powder (conversion factor = 0.159) (median value)
17.4 water/100 concrete powder (conversion factor = 0.148) (lower value)
Mixing time should be about 2 minutes.
Soft Set times at a given water ratio:
Recording of Soft Set times was not diligent: ~20 minutes @ 19.6/100 (midway
between 20.3/100 and 18.9/100) at a somewhat elevated ambient temperature of 76F to 78F.

Note that the given amounts above are what I used in my testing and will be the optimal ratios for this #251 concrete mix. But, you are free to use whatever proportions give you the best results for your application. Don't want to be bothered by carefully measuring units of weight? Then use measuring spoons for proportioning by volume. You will need to experiment using your spoons to see what will give workable mixes for your process. Just keep in mind that an optimal mixture will be the thickest that your process allows. Making soupy pourable mixes will result in weakened concrete and if you are adding pigments, it will lighten the colors. Also be aware that volume measuring is not accurate from batch to batch if you allow the amount of packing of powder in the spoons to change each time you measure a batch. What follows is for those who have accurate scales that will measure small amounts of weight (using grams is preferrable):
20.3/100 is the highest water content. More water in the concrete mix makes it more pourable, makes it weaker when cured, and can lead to more settling out of the dense ingredients. When adding pigments it will lighten the color slightly. But when adding extra aggregate it can allow more aggregate to be added and still have a workable mixture.
17.4/100 is the lowest water content. Less water will give a thicker mix, will make it stronger when cured, and will discourage settling out of the heaviest ingredients (the difference in water content here is not a large amount).
Using the ratios to determine the proportions for a desired total amount of concrete mix can be done in two ways. The least specific is to just divide the numbers of the water ratio by the same amount (example: dividing each number of 20.3/100 by 10 gives a correct mix amount of 2.03/10 water/powder (by weight; I used grams). The easiest way to find the proportions for a desired total amount of mix is done by using the conversion factor: for example, if a total amount of 16 grams of concrete mix is wanted, multiply 16 by the conversion factor, then subtract that number from 16. So, for 20.3/100, conversion factor of 0.169 times 16 = 2.70, which is the water amount. Subtract 2.70 from 16 = 13.30, which is the concrete powder amount. That gives you a water ratio of 2.70/13.30 water/powder to use for a total mixture of 16 (grams or other unit weight).
Above are relative times of what I call "Soft Set", which is somewhat equivalent to "working time" (plus a degree more). It is the time when a very small spatula poked in slightly to the wet concrete surface lifts up a small amount of mix and the mix does not settle back into the concrete mass on its own. You will probably find that your Soft Set times do not coincide with my figures. Take the given times as a relative indication of which concrete mixes we carry set up faster or slower than the others. Differences in temperature cause the greatest variance, but other factors like how effective your mixing technique is (and how long you mix), whether colorants or other additives are being used (they can act as accelerators or retarders), whether you are keeping freshly poured concrete covered with a plastic sheet or wet paper towel suspended above it (a good idea during the initial hardening phase of the concrete) and even the relative humidity since very dry conditions can rob some moisture from the concrete mix during stirring, placement, and settling (prior to covering).

NOTES ON WHAT THE PICTURES SHOW:

The 1st picture shows this listings concrete ready-mix powder in some of the sizes it's available in (the smallest sizes).

The 2nd picture shows all 4 of our concrete "ready-mixes" in sample tiles so you can compare how they look fully cured:
The tile that goes with this listing is at the bottom of the picture. Most of the tiles have a darker vertical section on the right side of the tile where we have brushed on a sealer/shellac to show how a "varnish" painted onto the cured concrete darkens the color.
Here's a list of the 4 concretes with the part # of the concrete :
-- Mix #80-102 = A White Art Concrete with some of the aggregate left out. We call it an "AddAgg" mix. It's the best mix for people who want to add some of their own agg (a special size, shape, or color), or want to add a pigment to make their own custom concrete blend.
-- Mix #63-101 = A White Art Concrete with all of the agg included. Best to use as a stand alone white cement concrete.
-- Mix #251 = A Tan Art Concrete with all of the agg included. It has light colored additives that make it especially strong as a "Pozzolan" type of concrete. The additives are colored enough to make it tan but not dark gray like common Portland cement concretes.
-- Mix #252 = A Dark Art Concrete with most of the agg included, but "cement-rich" enough for more agg if desired. It has additives that make it especially strong as a "Pozzolan" type of concrete. It is unique because it will remain a dark color when fully cured. Standard dark gray Portland cement concrete is a dark gray powder but cures to a light gray. This concrete contains some dark additives, and also has black pigments to force it to stay dark when cured. It works well when a dark background color is desired to match dark surface aggs or special powders like graphite, or to provide a contrast for light colored aggs.

The 3rd through 10th pictures show tiles made with sized particulates and flakes (mica) that can be used as aggregates, metal leaf bronze micro-flakes, and graphite flake, all of which we carry that are added to this listings concrete #251:

They show #251 with a "decorative" agg added in one of two ways: 1) "Exposed Aggregate"- variations are: mix the decorative agg with the wet concrete mix (integral addition), then when the concrete is semi-hard, brush away the surface of the concrete until the embedded agg is "exposed", or wait longer and chemically remove the concrete with a weak acid like vinegar, or wait even longer and remove the surface of the concrete by grinding with diamond abrasion until the agg is exposed (large surfaces use other techniques like applying a spray of retarder to the top surface, then when the body of the concrete is set enough, hosing off the top layer of the concrete with a spray of water), 2) "Non-integral Addition"- lay the decorative agg on a surface and pour the wet concrete mix on top of it, then after it's (partially or fully) cured, flip it over and the agg will be on the top (large surfaces use method 2 performed by spreading the decorative agg on top of an already placed layer of wet concrete that's thickening).

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MAIN DESCRIPTION:

This is a strong and hard tan colored concrete mix which is specially formulated for small objects like jewelry, small sculptures, small vessels, and the like. It has small sized, scaled down aggregates so the mixture can fill any fine details which will be narrow cavities in your mold that would not fill if aggregates ("agg") are too large.

Our concrete mixes combine several ingredients to make a concrete "proportionately strong" for the small sized objects it is meant to cast. If you cast a 12mm (1/2") detailed jewelry pendant with the general purpose "High Strength" concrete mix found in 60 or 80 lb bags from masonry suppliers, you will probably fail, not just because the aggs are too big but because the concrete is not strong enough, mainly it's flexural and compressive strength is not great enough. Like it says on the bag of those mixes, for sizes 2" or thicker. It's strength is only high enough when its mass is large enough, in multiple inches.

Is our Tan Art Concrete mix considered a "pozzolan" mixture? Yes. Pozzolans are defined as:
"....a broad class of siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide (Ca(OH)2) at ordinary temperature to form compounds possessing cementitious properties. The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity." That is from Wikipedia.

Details about our Tan Art concrete:

- Has multiple types of cement and additives which produce a denser and stronger concrete. An example of such an additive is a "water reducer" which allows a given amount of dry concrete mix to become fluid and workable using less mixing water. Excess water in concrete contributes to porosity. And porosity means lower density and lower strength. An object much smaller than sidewalks and walls, such as elements in jewelry or small sculptures need concrete which is dense for reproducing tiny details. And it needs high flexural strength to resist breaking when in thin sections.

- Is made with relatively light colored cements, and light colored additives to produce a tan colored concrete.

- Needs only water to produce either a pasty or fluid mix. Concrete made with small amounts of water is very strong, but our concrete does allow for higher than normal amounts of water when needed to be quite fluid and still have surprising strength and hardness. Tan art concrete can accommodate the addition of different aggregates. The amount it can take depends on the specific aggs in question. It is best found by testing. If the amount added dries out the mixture to an extreme degree, don't go overboard with the addition of significantly more water. Try first the addition of a wetting agent (the dishwashing additive product called "Finish" is an easy to find example) with a small amount of additional water. If additional water is still needed, add it in small amounts in steps. Superplasticizers (aka "water reducers") are already included in #251, so they will not help. Whether a given desired mixture will be strong enough can only be found out by testing (see the next paragraph below this).

- Has high hardness. We formulate and test our concrete with small (1.75" x 1.75") thin tiles. After they're cured, the tiles allow us to do inexpensive testing by holding them at various heights and dropping them onto a slab of polished granite. We see how resistant to breaking they are and also how hard and dense they are by the sound they make when hitting the granite. The hardest samples have the highest pitched vitreous ringing sound when they hit the granite. We have another test of flexural strength which is simply attempting to break the tile with forceful bending by both hands.

- Can be heated to 120 F under a light or in an oven to speed it's cure and drying time. It's best to allow it to harden enough to demold at room temperatures. Depending on the concrete mix, the water used, the configuration of the mold, and the room temperature, objects can be demolded in 45 minutes to 4 hours. Surface treatments like exposed aggregate can be undertaken shortly after demolding. Coating/finishing/wetting with natural oils and finishes like linseed or shellac can be done after 2 to 6 hours depending on ambient temperature and humidity. Burnishing works best (hard concrete can be burnished with various metal tools) after full cure which may take 8 hours to 3 days (contrast that with construction concrete which must be covered to retain moisture and will not be fully cured for 1 to several weeks, depending on the type of concrete used).

- Has lower shrinkage than pure Portland cement concrete. It is also less subject to efflorescence (crusty whitish mineral salts deposited on the surface by the movement of water through the concrete) than construction grade concrete.

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Rubber mold material considerations for concrete:

Rubber mold materials for casting concrete:
-- DIY custom silicone, platinum based, thick walled molds
-- Pre-made thin walled silicone molds
-- DIY custom urethane molds
-- DIY custom latex molds

Portland cement concrete mixes (either with or without other cements and high strength additives) can and should be cast into rubber molds for ease of demolding, especially with decorative or sculptural objects that are detailed. Large objects usually are cast into custom urethane molds. If shrinkage from the original object can be tolerated custom latex is an option if the proper release agents are used. Small objects like those that our concrete mixes are intended for more often use silicone rubber molds these days. Using the pre-made silicone molds with very thin walls that are available all over the internet are not a good choice for production molds but are great for a limited number of castings. More on that just below.
Urethane is less expensive, thus is more often used for larger molds. Latex is even less expensive than urethane. But besides cost, urethane has several things going for it that make it superior to silicone.
Urethane is much tougher (with greater tear strength, assuming you use a quality version) than silicone, but most urethane rubber must be used with adequate release agents for each casting made. The same can be said for latex molds. Silicone molds are most often not used with release agents and will more accurately reproduce fine detail as long as they hold up. But concrete is rough on rubber molds of any type. Concrete tends to stick to more substrates than plaster or gypsum cement and concrete is highly abrasive, unlike plaster and gypsum cement. It will dull shiny silicone molds if you are production casting scores of castings. For around twenty or fewer castings you should not have any trouble casting concrete into bare silicone molds and getting easy release with no significant mold deterioration, although mold shine will probably deteriorate before that.

The low-down on silicone molds:

The big advantage of silicone is that theoretically, you don't need any release agents. In fact, it's very difficult to get water based release agents to spread evenly on new silicone because of the extreme hydrophobic nature of silicone. And oil based releases are problematic because of silicone's tendency to absorb oils and swell, distorting the mold and also start a vicious circle of weakening the mold surface leading to pitting, roughness, and sticking of the casting. Urethane, most types of which need a release agent for each casting is very easy to release-coat.
The all-important outer surface of a silicone mold can start dulling (if shiny) and take on unwanted textures from wear in less than production casting 10 pulls, depending on the quality of the silicone and the presence of fillers. At that point, silicone molds will require release agents to get any further life from them.
Be wary of the pre-made thin-walled silicone molds that flood the craft market. Good quality pure platinum cured silicone rubber is expensive. Third party mold makers are free to add fillers to lower their costs. Some fillers such as fumed silica and carbon black can enhance certain forms of strength, but all fillers (with the possible exception of small amounts of pigment) will increase the permeability of silicone which is the root cause of silicone absorbing solvents from release agents and the casting material itself. Fillers will speed up the onset of the mold wear described above. A translucent, platinum cured, thick-walled silicone mold made yourself will greatly outlast a thin-walled mold (especially if it contains fillers). And, if you can find translucent premade silicone molds you should choose them instead of colored molds. They indicate lack of fillers.
Another peculiarity of silicone (and some other polymers) used to cast high quality concrete is a "whitish" appearance that castings can take on. Not many sources cite this phenomenon but it exists. Some people who notice this relate it to efflorescence (which is a well known problem), but it's presentation is not at all like traditional efflorescence, which most often appears on masonry walls. Urethane molds do not seem to share this odd property. You will find that castings in urethane tend to have more "color". I personally can't explain why this occurs. The only commonality with efflorescence is the whitish color. And, it's not a problem if you're wanting your castings to be white! If you're adding colorants to your concrete then it becomes a consideration and something to be tested for depending on your particular castings and processes. And, it's a problem you can do something about if you're making your own molds and have a choice of whether to use silicone or urethane.

Surface treatment of concrete castings:

Concrete has a wider variety of possible surface appearances than its cousins, plaster and gypsum cements, unless you chose the few gypsum cements that are made with the ability to accept aggs. Even if you're casting with the lower strength construction concretes that come in 60 or 80 lb bags, you can use exposed aggregate surface treatment to get a different appearance. But you can't effectively and inexpensively color gray Portland cement based concretes with anything but the red or black of the iron oxides that are the most saturated of the iron oxide pigments.
"Exposed aggregate" is a surface treatment that's been around since the early 1900's. "Aggregate" (or "Agg") refers to the sand and gravel of varying sizes that is necessary to make concrete. The strength of concrete is increased most effectively when the aggregate grain sizes are a mixture varying from fine to coarse. The other ingredient of concrete is the "cement" which binds the agg grains together. Portland cement is the type most often used as the binder, but there are others with different properties.
Exposed aggregate involves removing the outermost layer of the concrete binder and its smaller aggs to a depth that reveals the integral larger aggs, effectively "exposing" those aggs, making them appear to have emerged from the concrete body. The "larger" exposed pieces of agg can be medium or coarse. Most often they are coarse enough to visibly distinguish individual agg particles. The resulting surface will have a "bumpiness" dependant on the size of the largest agg grain. Agg particles will be partly submerged into the mass of concrete and partly protruding upward away from the mass. The protrusions will result in a slightly bumpy surface which can be smoothly rough if the agg is made of rounded particles or sharply rough if the agg is angular.
The exposed agg can be over the entire surface of the concrete or only over areas of a pattern that is walled off by strips of dividing borders, curved or straight, which define the pattern. Parts of the pattern can consist of agg of different colors, sizes, or particle shapes.
The tricky part of exposed aggregate is in removing whatever depth the binder and smaller aggs need to be removed accurately. There are several methods employed to remove the concrete surface: by chemical means, by brushing before full cure of the concrete, or by abrasion.
A chemical that reacts with the alkalinity of the binder ( a mild acid) is applied evenly over the surface which dissolves the binder to the desired depth immediately followed by a pure water rinse which carries away the dissolved binder and smallest agg grains, leaving the now lower outer surface of the concrete intact. Timing is crucial to avoid weakening the concrete too deeply.
For small objects vinegar is an acid that will work for removal of depths 1 mm or less. Either brush the vinegar or immerse the object into a container of vinegar for 1 minute or less, and then use a soft brass brush to get rid of the softened concrete. Follow this with a thorough brushing with a non-metallic brush while the object is soaking in a container of pure rinse water or by brushing under a stream of running warm water.
An alternate method that avoids the use of a chemical reaction is to demold the object as soon as possible, while the concrete is hard but not cured. Us a thin bristle wire brush to abrade the concrete surface and small agg grains away to a shallow depth. This method depends on demolding the object soon enough and it won't work on objects with fine details, it will ruin the detail. Also on detailed objects you won't want to demold early because you need the concrete to be hard enough to withstand the demolding process without breaking off the detail.
Removal of the surface layer of concrete by abrasion can be done in several ways. One is by blasting with hard grinding material such as aluminum oxide, silicon carbide or other loose abrasive media in a sand blasting cabinet. Using this method the blasting nozzle "gun" which is held by hand is easy to control and areas to be removed by varying depths can be accurately accomplished. Areas to be left shiny (if the concrete is polished before going into the blasting cabinet) can be protected by adhering rubber sheeting onto the object where the concrete is to be left polished. Patterns can be created consisting of brightly polished areas against a varying depth matte surface finish
Another way to remove surface concrete by abrasion that is not strictly an exposed aggregate method involves grinding away the surface concrete with diamond pads or burrs (for curved surfaces) after the concrete is fully cured. Often this method goes further, to actually polish the concrete. It differs from exposed aggregate in that the resulting surface is smooth and the agg grains themselves will have been ground, revealing the interior color of the agg pieces. This method works to full effect when the agg consists of larger pieces.
Another less often used method of concrete surface treatment is burnishing. A cured concrete surface, either "as is" from the mold, or with an outer surface that's been ground smooth, is hard enough to be burnished with metal tools rubbing and compressing the surface to a bright, very polished-appearing state, leaving a mark of metal ground on the concrete surface that the burnisher stylus is made of. If softer metal styli are used more metal will be deposited and line drawings can be made on the concrete surface. A brass pencil-like stylus, for example, would leave brass lines that are drawn upon the surface. Wider tipped hard metal tools can be made to compress without transferring any of the metal. A professional mechanical burnisher usually works using a tip that is a ball or roller that rotates against the surface,

Coloring Concrete:

Please see our concrete mixes #80-102 (recommended), or (secondly) #63-101.

Exposure to Sunlight:

All materials containing colorants will be affected by sunlight. It is only a question of when, not if. Our art concretes when mixed with water and cured have not yet been tested by the ravages of time in the sun, they have not been around for a year so far. We recommend interior use only. In any case it is inevitable that a concrete containing cements other than regular Portland Gray will not hold up as well as construction type concrete when used outdoors. Our White Art concrete will be more affected than our Tan Art and Dark Art concrete.
When colored by pigments, White Art Concrete should be kept from direct sunlight as much as possible. Pigments are the first to be affected in any material to be colored, including paint mediums, plastics (such as epoxy "resin"), glass, and cementitious materials like plaster or concrete. The lightfastness of colorants varies depending on the chemistry of the pigment, its opacity or transparency (pigment versus dye), the concentration in a medium, the chemistry of the medium (linseed oil versus epoxy versus white concrete), the direction of the sunlight (a sun high in the sky will transmit more UV radiation than a sun low on the horizon), and other factors.
But it can be said that pigments used with organic mediums such as natural or synthetic polymers (linseed oil, epoxy, etc) will fade faster than pigments used with inorganic materials such as concrete. And of course, organic pigments in themselves will fade faster than inorganic pigments. Let's finish on a positive note: although pigments will fade when exposed to sunlight, a given pigment that is added to linseed oil and put on a painting's canvas will be more subject to fading than the same pigment added integrally to a compatible white concrete.