In 2004 the Lauter "cement" piano was a subject in the MMDigest.
At that time it was very hard to find more information about this.
It's easy now, thanks to Google, which now offers the possibility
to search a giant database with (USA) patents (use: Google patents)

I searched for "Lauter piano", and found a patent with number 1799675,
describing a method of producing a body for a grand piano, using
a mould.  In fact, the patent says "Having this described my invention,
what I claim is: 1) A piano including a case composed of molded plastic
material..."  Which, earlier in the text, was given as: "With the
sections of the molding apparatus thus assembled the operator applies
a plastic material..."

The patent is from 1921, and "a plastic material", could be almost
anything that could be used to fill a mould, and hardening after that.
In order to protect an invention, it's clever to describe the possible
"plastic material" as well.

So the patent is giving the following suggestions:  "The plastic
material of which the case is formed may be widely varied..." and
"I have found that a well-known composition, used for manufacturing
floors, may be used, for example, a composition of equal parts by
volume of sawdust of wood floor, silex or very fine sand, and
a suitable binder, such as glue, Portland cement, or a gum, for example
shellac...".

So it looks that the "cement" Lauter grand in fact had a moulded case,
and a "plastic material" in which Portland cement was indeed used as
binder for a composite of quite cheap materials, like sawdust and sand,
would do the job, and could easily be seen as "normal" cement or
concrete.

Since a case also has a lot of influence on tonal qualities, and
I doubt if such a moulded case, apart from the weight, would bring those
qualities for the more expensive kind of piano as the grand.

By the way: Google patents does give quite a lot of Lauter patents.
Just use Lauter for a search.  Or piano.  Or...

Have fun!
Jan Kijlstra