Update on the above -- I was impatient to try it, so I thoroughly dried out some sand -- spread out on a board in the sun, weighed it and added 9% by weight Portland cement and 4% by weight water.
I then tried to extrude a 5/8" x 5" cylindrical core using a piece of PVC pipe split on one side as a core box and the slit closed up with two hose clamps, per Doubleboost's video.
I used a 1/2' dowel to ram the mix, and to act as a plunger when trying to push out the core after the clamps were released.
I had little luck -- if I rammed it hard it wouldn't extrude at all. If I rammed lightly and it did extrude, it had no wet strength and crumbled to bits.
I played around with more water and cement, and got a little better wet strength, and I widened the slit in the pipe with a hand saw which made extruding a little more likely -- but still was unable to get a single solid core. I suppose I could widen the slit further -- the only problem with that is, the clamps make the finished core diameter smaller in inverse proportion to the size of the slit -- the bigger the slit, the smaller the core. I wanted one about 5/8" -- which is just about exactly what you get from a 1/2" nominal water pipe here.
So, questions:
I'm wondering if the mix recipe assumed damp, instead of thoroughly dry, sand (which would have increased the proportion of both water and Portland cement).
Or was based on volume instead of weight?
Also whether this type of core actually needs to come out of a mold before curing?
Maybe extruding is the wrong idea. Since it is Portland cement based, it can cure in an enclosed space -- so maybe that's what ironman does.... ?