Pianissimo Del Vento So you might be able to make a frame out of reinforced concrete, but that will be more complicated to make than cast iron
At home it would be very easy, compared to cast iron.
I made reinforced concrete myself in various occasions. It's easy to make it sloppy. It's extremely hard to make it right, and especially so if you want a clean, good-looking, high-resolution and not crumbling surface. For example, you have to carefully control temperature and humidity, and actively so because the curing chemical reaction of concrete changes the temperature and humidity of the area where it is. That is why the most critical reinforced concrete structures are pre-cast in a (huge) lab and brought onsite ready, rather than cast onsite (assuming that the size allows, obviously, otherwise the civil engineer just resort to "overkill" by designing something that is sturdier than required, assuming the conditions for curing would be non-optimal).
And after all of that, I believe your "reinforced concrete frame", assuming you can make it, make it right etc won't be much lighter than the cast iron one. Sure, in volume concrete is about as half as cast iron, but it does not have the right mechanical properties, so you have to make it thicker. Plus, even for the parts where you could mechanically make it thinner, you can't: you can make cast iron much thinner than you can realistically make concrete. And piano frames are made that thin.
Pianissimo I was talking about alternatives and DIY,
For this purpose, I think you have to really think hard: do you really need a piano as loud as the "regular" acoustic ones? I am convinced the answer is no, and that opens a gazillion of realistic possibilities, not trying to make weird things such as a concrete frame. Have you looked at the keybird link I gave you earlier?
It's ironic that most of the advances in the piano have been to make it louder which is something that the regular home pianist does not need! And at least some of that loudness comes with a corresponding heavier requirement. If you don't need that loudness you can then explore many different things. Regarding your "can't do cast iron at home" thing, for example, the first easy thing you can do in a piano is reduce the number of strings. Instead of having two or three strings per note, use just one, all the way from bass to treble. Instead of using a high tension scale, use a low tension scale. All the sudden, cast iron becomes unnecessary! If you have it in sections, two or three strong wood frames (e.g. one for bass, another for middle, and a third for treble) will be sufficient. Alternatively, you can use a (single) steel frame, made out of steel extrusions. A bit more work than working with aluminum extrusions, but doable at home, and certainly sufficient for a 88 low-tension strings, which is also much safer because you will break at least one string and will have it flying around while working on it…. With less strings and less tension, you risk less to injure yourself while working on this project