Crumbles when soft

I'd stick to KISS, if we keep to the idea that the moon base come first
anyway you'll have plenty of lunar regolith already pre-processed on hand
there, so melt that with solar furnaces and make ship structural parts out
of basalt. No mess, no water, no shipping from earth and good solid stuff to
work with. SSI has concluded that basalt is a good material for use of lunar resources. For the most part, when people talk about lunar-derived concrete, I think most of them are talking about that rather than water-based cement. Just throw parts straight up to an assembly facility in orbit
with a magnetic accelerator, or even better, assemble the ship on the moon
and take off from there. Not sure why you would say "or even better". Assembly of large, massivestructures in 0-G should prove easier than in gravity, even the 1/6th gravity of the moon. In fact, the best bet is to just throw up raw ore, and do your smelting (or in the case of basalt, casting) in HEO, where you have 24/7 availability of solar power for your furnaces. Also, if you're only accelerating up raw ore rather than finished parts, you can get by with greater accelerations, which means a shorter mass-driver, which means lower start-up costs. the only real
issue is where to source uranium for the fuel-bombs, ship it from earth
won't be much of an issue in terms of mass, but I can see some problem in
regular launches of rockets packet full with military-grade fissile
material... ;) Yep, I guarantee you that's a non-starter, politically speaking, even if advocates are right in saying fuel elements can be safely packed for launch. I don't think it's wise to assume any kind of such nuclear drives unless such ships are fueled from ET sources, and under some strict prohibition against Earth-intercepting trajectories. On the plus side, assuming there are extra planetary sources of suitable
fissiles it would take only a relatively small mining operation to source
them compared to any other fuel, in terms of mass anyway... does someone
knows if there is uranium or something else suitable somewhere in the solar
system we know of for sure, other than on Earth, that is? I've only heard a possibility held out for Thorium on the moon. Other sources are probably out there, somewhere, but are presently unknown.