Hi. I'm a refractory designer. I have over 30 years of experience in creating linings for industrial furnaces with bricks and monolithics, including what we would call 'Precast shapes' of which this would be included.

Those are definitely air pockets stemming from the pour that were not vibrated out. It could be your aggregate size is making it difficult. I don't know the product you are using, but often there are 'fine' versions that feature a smaller grain size just for better consistency when pouring. I would never recommend gunning something like this.

As far as vibration goes, they sell industrial vibrators with long rubber mixing tips. I might have video from a few of my precast shapes showing a large precast shape getting vibrated. Everything about it just screams dildo and you feel silly, but it is what it is, and it does work.

You'll know if you add too much water. It will be brittle and fail if you can even get the mold released. More water is never a solution. Better PH is superior to more water.

Vibration can be achieved by building an outer box for your foam insert. Wood can work for something this size probably, but metal would be best. You can then use any vibratory method that works. Pour direction matters too, but it opens ugly doors like lifting and drying.

Consider lubricating the inside of the mould so it'll release better and some reinforcing mesh. eg, cooking oil spray, raw fiber glass.

Haven't vibrated it enough. 100%. But it's interesting because your void distribution is pretty consistent... im guessing you vibrated the mould rather than the mix.

Your problem is not mix consistency, it's entrained air. Mould vibration (esp with a small mould) is not good at removing entrained air.

Try a needle vibrator on your next one. Maybe an old tooth brush?

Edit: don't forget to treat your forms. You want to apply specialised compound to reduce adhesion between concrete and form, so when you strip forms, pockets of uncured mix don't 'pull away' from your piece.

Out of curiosity, what are you making?

That many voids implies not enough vibration to me

The vibration required is pretty serious. If you look into how they make concrete bricks you need a lot of force and control for the frequency to make it effective.

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1024&context=mesp

I used this as a basis designing a table to vibrate epoxy-concrete molds for machine tools. You'd want the dildo attached to a hammer drill style vibrator for small scale application. https://i.imgur.com/RcIcqk1.jpeg I used a steel plate here, 2hp Craigslist motor, bicycle tubes, rubber and wood. My plan was for larger molds..

That, and the aggregate ratio also being the two critical parameters. Edit: the whole requirement is also dependent on your needs. You can change a lot if you don't need serious strength to better target surface finish.

how do you vibrate? do you try to vibrate your whole mold? like using a "concrete vibrating table"?

maybe you can build you a equivalent of a "handheld concrete vibrator" in even smaller, and just put it into the concrete?

I'm no expert but maybe also need some more larger particles in the mix. Is this just cement and sharp sand?