I was not sure where to post this question, figured I'd get the radiant heating, passive solar and other subsection folks all in one here.

Situation:  Passive solar house, concrete ground slab (insulated of course plus radiant hydronic heating via PEX tubes - rebar not WW).

Looking for:  A way to color concrete black (polishing and sealers are on the table), though color preferred over surface stain.  A matte finish dark gray floor with specks in it would also be an favored option over solid black. 

Desired outcome:  absorb and retain the sun's heat more effectively than normal concrete mixes.

Background: I've mixed small wheel barrel sized batches of concrete before, fence posts, chimney caps and so forth.  Aside, with fiberglass/foam composite projects I've added West Systems Graphite Powder to their epoxy mix and made a super slick coating, also added micro-balloons and made a sort of sandable light weight carbon foam.

Research:  So far I found a carbon graphite nano-tube additive with some bold claims on performance improvement but not specifically on the qualities I'm looking for (see link).

Expansion:  Would the expansion rates be greater with black concrete?  Would I then need larger pre-mold joints at the perimeter, and more saw-cut control joints in the middle of the floor area to control cracking?

Possible Final Outcome:  Through color concrete I assume is best, as I don't want to have white cracks one day, could be polished, hoping the glare doesn't blind anyone (if this additive smooths out the top layer somehow so you don't have to polish it - huge win), may or may not add strength, will absorb a greater percentage of light energy hitting it.  A matte finish or gloss finish is secondary or third place consideration, it will be what it wants to be.

History: Back in the early 1980's a solar energy magazine published an article featuring an architect/builder offering up his home brew concrete stain cocktail, one of the main ingredients was his used motor oil. I'm not kidding, he thought putting known carcinogens into the home and heating them up to be released into the air was a good idea.  My point, I don't want to use something that will one day be considered bad for people/pets, and have to be encapsulated or removed.

Maintenance: a gloss black floor is bound to expose flaws in workmanship, and show dust.  I'd favor a matte finish charcoal gray floor with specks in it over something demanding perfection at either end.

What is out there?

Have you used it or know someone that has used it?

On a side note; one of my college professors, a landscape architect advocated the use of asphalt driveways in Michigan because his asphalt drive cleared it's self of snow before his neighbors light gray concrete drive did.  This saved him a lot of shoveling and laying down of salt, and he gloated about it with the neighbor that paid more for his concrete drive.  I'm thinking black or dark colored driveways could be another use for colored concrete.

There are many concrete color additives, most of them developed for southern climates with the result that they don't do so well when subjected to road salt used in northern climates.  That salt has a way of making it's way indoors on the bottom of boots and shoes.  The result could be bad staining of any product with a different Ph or porous in nature.  Concrete is porous.

Feel free to address anyone aspect of the topic, I don't expect any one person to know it all. Some of my concerns or question may admittedly be silly or over concern, just information collecting at this time.

Side benefits: if the mystery additive affects the flat-work floating finish so that no grinding or polishing is required........ that would be double bonus points and a powerball winner. 

We frequently use black concrete for our passive solar HR heated slabs and for slabs in other building floor areas too. We use a 4000 psi 0% air pump mix and we have not had any issues with expansion joints, etc. These days it seems our clients tend to prefer stamped floors more than polished floors. You can color/stamp the concrete to make it look like whatever final floor finish is desired (e.g., stone, tile, wood plank, etc). We like the Davis Colors for the base concrete color and ButterField Color for the antiquing release color. The color of the concrete surface does affect the thermal mass solar absorptivity and emissivity, and these parameters are required for properly accomplishing the passive solar thermal mass performance analysis and the associated passive solar heat gain analysis.

Thank you sailawayrb for the on the spot reply.

I have only known colored concrete and the surface applied coloring agents to be used on exterior applications, walks and patios.

I designed a house addition for a friend last year, and this fall he was down to doing the concrete walks. He was planning to use what he described as a powder on the surface which was to be power washed a few days later. Last I heard he missed his window and let the what I'm assuming is an antiquing agent remain all winter long. I'm going to have to see how it turned out.

I'm assuming that these antiquing release agents add color depth or marbleizing to the surface and aid in getting the texturing mat/mold off (and no power washing inside the house).

Here are the two links I'm poking around in right now.

Davis Colors
https://www.daviscolors.com/concrete-colors/

Butterfield Color
https://www.butterfieldcolor.com/product-category/antiquing-and-release-agents/

NOTE: The textures I've used in the past on my projects were all rather course and crude exterior patterns and unfit for interiors in my opinion.  It will be interesting to see what is being used and offered these days. 

Yes, the antiquing color provides the contrasting texture to the base concrete color. You can just use the antique color powder directly before stamping or you can mix the antique color powder into a liquid release agent and then spray it onto the concrete before stamping. We typically use the powder method for exterior flat work as this eliminates the additional expense of the liquid release agent. We typically use the liquid release agent method for interior slabs because it is way less messy and doesn’t have to be power washed off like the powder method.

We use standard outdoor stamps for the interior finish. However, we can apply painters plastic over the recently stamped surface and then lightly sweep the plastic to soften the texture if the client desires a smoother finish (photo of this technique is attached). The concrete can be sealed with a satin, semi-gloss, or gloss finish as desired. I personally prefer the coarser texture and the satin sealer on the stone looking stamped concrete and the softened texture and the gloss sealer on the wood plank stamped concrete. I like the rock looking stamped concrete to feel and look like real rock and the wood plank looking stamped concrete to feel and look like real wood. However, this is very subjective and we just do what the client wants after discussing all the finish options.

It's all subjective as you said, personal taste and all.

As much as I'm in awe of some of the things being done with concrete and tile to mimic other materials it feels a bit disingenuous to me most of the time.

The closest thing I came to actually liking on the Butterfield site does not even show an interior use.

Heavy Stone Texture Mats & Touch-up Skins
https://www.butterfieldcolor.com/tool/heavy-stone-texture-mats-touch-skin/

I'm going to keep looking around.

EDIT: I'm favoring the black concrete acid washed, polished and sealed versus black concrete with water based eco staining, polishing and sealer.

Yes, there was a time when I also felt that way too. However, I have learned to appreciate how wonderful it is to have a wood plank looking kitchen and dining room where you don’t ever have to worry about damaging the floor. We prefer to put the real wood in places that are more prominent and that don’t see feet, fallen objects or are prone to water damage. It is also nice to have stone looking floors in other rooms as it conveys a sense of castle permanence. Concrete is a great product that can last forever and can look amazing when done well. When the walls are concrete it also makes sense to make the floors concrete too, especially with passive solar and HR floor heating. These days nearly all our builds are slab-on-engineered/elevated grade. If you find something more durable than concrete floors that is also low cost and has excellent thermal mass/HR heating qualities and fire/flood resistance, please let me know.

Black acid washed concrete doesn’t stay black for very long when exposed to sunlight. Neither does black colored concrete unless you pour 0% air concrete. Anyhow, make sure you consult with your concrete color manufacture before proceeding with your project to ensure that it will be acceptable for your intended application.

Ive done a dark floor using Dura Stain acid stain in "leather" color, also done "redwood" and "western brown" of theirs. Leather was pretty dark. They do offer a black. No polishing, just several coats of TK Bright Kure and Seal after its neutralized and dried. Both products are nasty to work with but can turn out pretty sweet, you kind of get what you get with acid.

Wood plank stamped looks alright, but Ive seen hairline cracks across the "boards" and it sticks out and looks terrible, control joints are bad enough. Ive done colored interior stamped once inside per request, random stone pattern, and it looked sweet but livibility wasnt great.

Also had a powder type stain done by a sub, that is kind of an art, it can look awesome or look like Dexter started mopping the blood up and quit halfway through.

Greentree, I am curious why you felt livability wasn’t great? I am of the opposite opinion. I find HR heated stamped concrete floors to be the most comfortable, livable and best value floors I have ever experienced. You can easily keep them clean with the central vacuum system, clean up any spills with a moist rag, reseal them about every 10-15 years and they never have to be replaced.

Yes, you definitely want to avoid getting any uncontrolled cracking in a wood plank stamped concrete floor. This is why we only use 4000 psi 0% air concrete and #5 rebar 12 OC. You can’t do too large an area and all the control joints with the wood plank areas are made coincident with the plank lines. To do larger rooms, we create 14” boarders using a solid stone stamp. Anyhow, it can be done well, but you have to understand how to work with the concrete and not exceed the limitations.

Livibility with smooth concrete is fine, stamped issues dependent on pattern, depth, ect can make cleaning difficult (wiping up spills), chairs rock, tough on bare feet, other complaints like that. Although I havent done a lot of indoor stamped, I mainly do basements which is where acid stain is a low cost upgrade to garage or basement floors and an easy sell, but what I have done and some other guys I know, consensus seems to point to feelings of "it looks great, its ok, but for the money I would spend it elswhere if I built again" and I would tend to agree after its all done. Walk across your garage floor barefoot than go walk on some texured stuff, I wouldnt want that everyday unless its a hallway with a rug over the whole thing.

Maybe if I did alot more the opinion would sway the other way, hard to say.

I know a guy who tries to stamp colored plank under porches or commercial overhangs all the time so the plank runs the long length since the traditional perpendicular orientation wont really work unless you want to cut up a set, forcing control cuts across the planks. Its not that bad when its an outdoor walk. It would look odd in a living room all gridded out though.

We did mostly polished concrete floors years ago. These days we do mostly stamped concrete floors and this is about the lowest cost floor option in our area, especially if you are already starting with a concrete reinforced slab rough floor. For $6.50/sf total, one can have a 5” thick stamped concrete reinforced floor.

Some of our elderly clients weren’t happy with how slippery and fall prone a polished concrete floor can be. They actually prefer having the extra texture that the stamped concrete can provide. Again, the amount of texture can be varied with how you finish the concrete.

We have 100% stamped concrete floors in our home and we are very happy with them in terms of how they feel underfoot and cleanability. We haven’t had any complaints about unevenness either. We actually prefer a rougher texture in the barefoot bathroom areas just like we prefer a sandstone pool liner over a smooth concrete or tile liner. Somehow the texture seems more natural and less sterile than the typical conventional floor finishes (e.g., carpet, engineered wood, polished concrete, tile, etc). All of our stamped concrete floors are warm in the Winter and cool in the Summer and that likely also helps with our high comfort level perception too.

As I wrote earlier, I guess it is a subjective thing and perhaps also in how well the stamped concrete is actually done.

I walked a regional indoor mall yesterday that used a lot of the water-based (eco-glaze), it looked nice and was forgettable. Meaning it did not call undue attention to it's self, a good thing I suppose.

Sailawayrb, sounds like you are saying the surface layer finishes and release stains (glazes) protect better against UV fading than plain colored concrete. Something I will look into.

I did see some modern looking passive solar interiors on-line, you know the ones with white walls and natural wood ceilings - looked great with simple polished black floors in my opinion.

Plain black can be overbearing, I can see how an eggplant with leather-like finish would be a nice adaptation or compromise between highly textured and colored schemes versus smooth monotone black.

I did a search a few days ago on leather textures for concrete and found nothing. Closest I came was that "heavy stone textured mat" by Butterfield.

I watched a fascinating Youtube video yesterday using a similar flatrock/leather texture but it was small batch fiberglass reinforced, I was looking into perhaps making some hang-able rainscreen panels as an experiment one day. The textures I could add such as leaves, sticks and things (fishbones?) has always been of interest to me ever since I did my first slump test in 1982.

https://www.youtube.com/watch?v=KUJT6GLMzG4

No, I was just saying that you should verify with the manufacturer of your color finish selection that it will hold up well when exposed to intense sunlight. Some colors and products do much better than others.

When I wrote “Solid Stone” previously, I should have wrote “Heavy Stone” as this is one of the stamps made by Butterfield that we commonly use and shown in some of the photos that I previously posted.

If you like polished concrete, you might also like Terrazzo and the appearance/design possibilities that it can provide:

Terrazzo

I will also note that Terrazzo can be applied on the top of any concrete slab. So if you manage to do something that you end up not liking, Terrazzo might be a good recovery option too. Of course, it will add another significant expense layer to your concrete slab floor project.

Thank you Sailawayrb for the clarification on the UV aspect, words of caution good for any project and not just a passive solar one.

Terrazzo and it's excellent wear reputation for a product specific to the task of being flooring is most certainly a good one. It's always been an upper end institutional product but I see it's branched out and has been liberated.

In that direction, in-lays of glass and other material similar to some artsy concrete counter tops could be on the table. I've seen some crazy epoxy/acrylic resin things done mixing different materials together as well.

Embedded floor lighting and electrical outlets a consideration.

Crossing that line between a construction project and an art project can be a risky thing though, the whole risk/reward thing comes into play.

Someone once said the difference between art and engineering is that with art the outcome is always uncertain.

From what I can tell most of these concrete floors have an element of art or them or uncertain outcome, and people that want more certainty can always install tile or other manufactured flooring versus a created by craftsman's hands flooring.

Many combinations and options out there.

I will add something that I am sure one will dispute, but a passive solar home is not compatible with In floor radiant heat. During the night as the building cools, the heat comes on and warms the floor. When to sun comes up you have no heat storage as the floors are already warm.

Any Builder with knowledge of high efficiency homes will tell you this. Here in Oregon having a radiant heat system automatically disqualifies you from receiving a tax credit for a passive solar home. Is there a reason you want to keep that much heat in the slab? during a sunny day they might get quite warm. Once the energy passes the windows it is inside the home to be utilized. You will not attract any additional heat because of the color of the floor. If not absorbed by the slab it will be utilized in the space. We have smooth concrete floors, saw cut on 4 ft centers, and sealed. If the sun hits them, they are plenty warm. Look good, and perform. Total electric home 2300 sq ft, $480 electric bill per year.

Yes, I will dispute this again and I will give you the same explanation I gave you previously on the Thermal Mass thread:

GBT Thermal Mass Discussion

I am very familiar with the flawed OR passive solar tax credit application. It incorrectly derates HR heated floors for passive solar application. We had ASHRAE write them a letter several years ago to clarify and correct this which they indicated they would eventually do. However, government agencies don’t always move very fast and this tax credit will end this year before they made the clarification/correction. Even so, we still received deviations of this HR heated floor derate for many clients.

Passive solar and HR heated floors are not incompatible if designed correctly, but most companies don't know how to accomplish it or don't want to get involved with it as some engineering is required. The most simple way of accomplishing this is to zone the passive solar heated areas separately from the other areas and not call for heat in these areas based on slab and/or air temperature. The better but more complicated way of accomplishing this is to extract the excess heat absorbed by the passive solar heated slab and move it back to the boiler return line for use in other heating zones that are calling for heat. The temperature of a concrete slab doesn’t affect how much heat that it can subsequently absorb. The slab just keeps getting warmer and it can overheat the room if something isn’t done about it. Anyhow, we have been successfully doing this for over 10 years now and it works very well. In fact, passive solar and HR floor heating design integration is what we specialize in...that, and water project integration (i.e., water rights, hydro power, hydraulic ram pumps), which is why our company logo looks the way it does...

One needs to accomplish an integrated design that fully considers passive solar design, thermal mass design and HVAC design for the given location to achieve great results.

I will add something that I am sure one will dispute, but a passive solar home is not compatible with In floor radiant heat.

Yeah, I have to dispute it because I have a passive solar with radiant in the PNW and it works spectacularly.
As soon as the sun comes up and hits the fenestration, the zones in the sunlight get the idea and shut off because the wall thermostats report it's just fine in here. If you think about it, the heat capacity of a concrete slab is not maxed out at the temp it is radiating at.