Crumbles when soft

> >But solar power is cheap in space. Why not sinter to make ceramics
> and forget the concrete?
> ...
> I just dont see the molds standing the heat of a sintering process
> for such a large structure.
>
> >Or use metals, which can be vapor deposited using focused sunlight,
> which can reach 6000 K and so could vapor deposit any (and all)
> element(s).
>
> How would the structure look like which has to take this heat to carry
> the metallic deposits? Wouldnt it smelt also?

Simple to solve. There are three ways that heat is dissipated:
conduction, convection, radiation. Create a very thin mylar balloon in
the shape that you want to build. The easiest shapes would be spheres.
So think of a large spherical balloon attached to a contraption that
melts the raw material and sprays in inside of the balloon. Probably
some sort of solar power sterling cycle electrical generator with the
smelter / sprayer inside of the balloon.

At first the balloon is filled with a gas. Doesn't matter what it is as
long as it doesn't get chemically involved with the raw material. The
spray directed toward the walls is going to lose heat rapidly due to
convection with the gas. The gas is going to carry the heat mostly
evenly to the entire sphere. The eddie currents due to the spray and
other gas currents due to the fans we have installed keep the gas
moving. By the way the sphere is rotating slowly or there is a static
charge between the sprayer and the balloon. The raw material forms dust
and settles on the walls of the balloon and slowly accumulates.

When the dust accumulates to ? (say a 1 cm) most of the gas is removed.
The sprayer continues but the spay now reaching the wall is melted
liquid because it is only losing heat due to radiation on the trip to
the wall, a much slower mechanism at the temperatures of interest than
convection. At the wall it loses heat due to conduction and
solidifies. The heat per unit area is kept below the melting point of
the initial balloon material until the smelted material can hold its own
shape. When there is enough material accumulated to hold shape while
the inner surface is being smelted, then the temperature and volume of
the spray can be increased. The spayed material can be alternated with
other material or special patterns can be built up.

There is a lot of control mechanisms required for this. But there are a
lot of variables to work with: spray droplet size, speed, and direction;
gas pressure; how often any one section is sprayed, material mixture.
This method doesn't use water which is going to be in short supply for
space construction for quite some time. It does use sunlight, vacuum,
and zero gravity environment all of which are in plentiful supply.

Mitchell James
www.InnerTransit.net (Home base for collaborative engineering on the web)