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What would be a good way to make cement with lots of hollow areas? I want it to resemble a sponge when cured. I’ve see a similar material used as siding in buildings.

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    Do you mean a material like this: en.wikipedia.org/wiki/Autoclaved_aerated_concrete ? – samcarter_is_at_topanswers.xyz Jun 23 '19 at 17:50
  • @user36296 yes but with much larger holes. Maybe 1.5-2cm – Isaac Wasserman Jun 23 '19 at 20:39
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    Not hollow, but there are thin wall glass beads used to lower the density of concrete ; And I have added Styrofoam beads to concrete - it takes a lot of beads to make much density difference. – blacksmith37 Jun 23 '19 at 23:41
  • This is going to be really hard. Anything you put in the concrete to leave voids will float when the concrete is liquid. All the stones will fall to the bottom, leaving (for example) ping-pong balls loosely stuck together with weak mortar. – Robin Bennett Jun 25 '19 at 11:46
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This is an old question, probably already overtaken by events, but I'll toss in a couple of other ideas in case anyone else has a similar need.

  • Tiny holes
    Aircrete, or foamed concrete, is concrete that's been greatly expanded with bubbles of gas. The holes are tiny, but there's enough of them to expand the concrete to 5 to 7 times its volume (it's so light, it floats on water, but it's actually very strong and is used in construction to replace regular concrete). There's tons of information online if you search "aircrete". It's typically made either by blending soap foam into the concrete, or adding a little aluminum power to the concrete (that reacts with the concrete to produce gas). To give you an idea of what's involved, here's a video of a guy who made the soap foamer out of cheap plumbing supplies, mixed the aircrete, and then cast it into things like bricks: https://www.youtube.com/watch?v=aSLJCPR1Prw (there are a lot of such videos on YouTube).
  • Visible holes
    If the objective is something that resembles a coarse sponge, you can make it by coating a sponge. There are very coarse sponges that resemble sea sponges, often sold as bath sponges or for washing cars. They can be really cheap sponges, they just provide the shape.

    enter image description here

    Mix up fairly thin cement (usually just Portland cement and water). Immerse the sponge and squeeze and release it until it has absorbed the cement throughout. Then shake out the excess so it looks like a cement colored version of the sponge, without cement filling the holes. Put it in a container that will hold in the humidity and let it cure for a week.

  • Big holes
    If you need big holes, you need to be careful about how you use the resulting concrete because you are likely to get some really thin walls between the holes that have no strength. But a good approach to making it is to mix into the concrete some balls the appropriate size made of a material that will disappear once the concrete has set. Elmy's suggestion of water beads is an example. If you don't want any remnants, like dried beads, here's an untested idea to try (I don't know how this material will behave in concrete).

    You will need some kind of mold for balls of the desired size (hobby mold or make your own Oogoo mold from something the right size). The material for the balls is what is used for making a homemade version of "Canned Heat" (gelled alcohol).

    There are a number of different recipes for gelled fuel. The version here is made from calcium carbonate (egg shells, chalk, powder, or antacids with an extra step), vinegar, and alcohol. There are a lot of Google links (try "diy gelled alcohol" or "diy sterno"); here's one: https://www.survivopedia.com/diy-gel-fuel/.

    The basic steps are getting the calcium carbonate into a powder, dissolving it in vinegar, filtering, boiling to reduce the volume (calcium acetate solution), then adding alcohol. It turns into a gel and then, with the right ratio of ingredients, into a solid. A tiny amount of calcium acetate forms a matrix that holds the alcohol and water.

    If it sets up in a mold, it holds the shape. Left exposed to air, the alcohol and water evaporate (or get absorbed by the concrete), leaving a miniscule residue of calcium acetate wisps. And that's water soluble, so any that's visible will dissolve and disappear if exposed to water, leaving nothing.

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You could try Orbeez / water beads.

Thoroughly water them to make them inflate as much as possible. Then mix them into the moist cement and let the cement set for several days. The moisture retained in the orbeez should slowly evaporate through the pores of the hardened cement, leaving behind a sponge-like structure (and the shriveled, dried out Orbeez of course).

Here's a video of someone mixing Orbeez into transparent epoxy resin (start watching at 11:00 minutes) and a closer look at the result in a second video (trigger warning for Trypophobia).

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I do not have personal experience, but I read about something similar some time ago (I do not remember the details of that story, could have been about food).

The base idea is to mix the cement, while pumping / injecting into it compressed air. While being mixed, the air forms bubbles. Adjusting the mixing speed and the pressure and volume of the compressed air, you should be able to control the size of the bubbles.

However, I think you cannot get "arbitrarily large" bubbles, because of the physical properties of the cement.

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