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I bought a bag of quikrete mason mix for creating some planters from molds. The wood panel molds look something like this, but the spacing is a bit narrower and they're smaller:

enter image description here

Since I'll be pouring the mixture into a mold, I would prefer the mixture to be as "liquid" as possible without it affecting the integrity of the mold.

Question: how much water should I mix the concrete with so it pours without issue? The more "watery" the mixture, the easier it would be for me.

Also, the pots are square at about 14" per side, so it's not like they need to be super strong.

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    Vibration will fluidize a relatively thick concrete mixture and settle it into recesses. Just tapping on your wood frames should do it. – blacksmith37 Jul 23 at 15:30
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Concrete is strongest when the mix is as dry as possible, and weaker when the mix is wetter. A highly liquid mix is therefore almost always a bad idea.

This is well understood/documented/tested in the industry, tends to be poorly understood by homeowners and some of the less educated pros.

Likewise, the importance of plenty of water for the curing process after the concrete has set is well understood by industry and poorly by most of the same people. Once the concrete has set, all the water you can supply is ideal, with the best option being to submerge it.

On a larger scale it's practical to use air-entraining (bubble/foam) admixtures that help the concrete flow as if it was wetter, but which do not impact the strength so much (and actually help with things like freeze/thaw cycles.)

On a small scale that's not very practical (AFAIK nobody sells a tiny packet of foam precursor) but vibration is highly applicable - shake, tap, hammer on the outside of the mold, pick it up and drop it a short distance, and rod (literally run a rod up and down inside the forms to distribute) the concrete inside it. On larger jobs a vibrating thing to insert in the concrete can be rented, but not likely to be useful here (too big.)

Note that mortar (what you show a bag of) is not concrete, precisely.

The linked wikipedia article sugges a ratio of weight of water to "cementitious material" (which is only a fraction of your bagged pre-mix, with the rest being aggregate, primarily sand in mortar mix) of 0.45 to 0.60

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    Am I wrong in thinking that mortar mix is the wrong product for this project? – JPhi1618 Jul 23 at 15:16
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    No, I quite agree with you that it is not what's best suited to the project. – Ecnerwal Jul 23 at 15:48
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Here are some thoughts, not exactly addressing what you ask, but which may help you succeed.

  • Ecnerwal's point that you want concrete, not mortar, is a good one.
  • Read the concrete bag. I think it will recommend a minimum 3" thickness, which it does not look like you have. Maybe you can make it work anyway, but understand it's not a recommended practice to make it thinner.
  • Getting rid of voids in a small space like that will be a challenge. If you are going into production, get a mini-vibrator. If not, work in maybe 2" layers, using a rod or stick to thorougly jiggle the air pockets out of what you put in. Tap, poke, rock, way more than you think it needs. Wash, rinse, repeat.
  • The inside of your form is going to be hard to remove. Can you design it so one piece is free to be removed, unlocking subsequent pieces? I don't think you will be able to slide it up and out intact.
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To strictly answer your question : Add the most amount of water per the manufacturer instructions and no more.

The better answer is that you don't want to rely on water to control your workability. I hope you are planning to do this a few times, because you will have some trial and error. Your mortar will want to shrink, so it might crack. Excessive water increases cracking. Follow the process on the site where you got the picture of the planter form. It's pretty good. Use silicone caulk on the corners of your form to prevent leaks. If you can, put a draft (angle) on the plug in the middle so you don't have to pry it out piece by piece. Dish soap functions as a plasticizer (helps it flow better) and can increase the strength of the concrete if you entrain the right amount of air.

As others have said, your mortar mix is relatively weak. Buy the highest strength premix you can find. If you need more strength in a thin wall, you can add straight Portland cement to your ready-mix.

Good luck!

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