Someone recently said they had a certain cement good for fixing exterior
marble. Could we get into this a little?

Dan

That was me Dan. Magnesium oxychloride cement. I did a patch for
terrazzo and a four inch hole in white (w gray swirls) carrerra marble.
Mixed in titanium dioxide white pigment and fine white sand (dolomite) then
troweled in the gray while still wet. No smell, UV resistant and really hard
(can take a diamond buff). Do you think there is a market repackaging for
the stone care industry? Would you like an evaluation sample (since you are
actually in this industry). Its real sticky too - without any latex, and
shrinks very little. Great for terrazzo because it fully "cures' in a day,
unlike portland cement, 28 days.

Another method is, oddly enough, Crazy Glue.
Crazy glue mixed with certain powders such as baking soda creates
instant plastic with a somewhat translucent quality similar to marbles
and alabasters.
Mix in some powdered stone the color of the stone you are filling for a
color match....

Particularly effective for alabasters and onyx.

I just got certified by Cathedral Stone to use Jahn mortars, and have
patched a marble statue, but their formula is porous -on purpose- and does
not look like marble closeup. They made me aware how a stone patch must be
just as porous and no harder than the original stone with the same
coefficient of expansion. So your goop would have to be tested. Yes. I am
interested, but doubtful, for outdoor uses, for now.

Dan

Thats the trouble of marketing this stuff. You have to make a M.S.D.S.
sheet - and then everyone knows exactly what is in your product. In the case
of jahn mortar - no big deal- high alumina portland cement and quartz - I
can copy their marble 120 formula exactly, with the added benefit of 100's
of pigments and aggregates I have collected over the years, to color match
etc. The porosity is a big problem if you are trying to patch - there is no
way you can build a gloss, the mortar just collects dirt and grime. At 2,000
psi, Jahn is too weak for floor or heavy wear areas. Mine is up to 12,000
psi - up near granite - doesn't need the aggregate to beef it up.
Coefficients of expansion are an issue however, I can backfill with a foamed
cement to make as weak or strong as I like, with a hard veneer on the
surface. The material has seen plenty of outdoor use - many Paris subway
stations, Maginot line, and is still considered the very best material for
decking on battleships, and the very best industrial flooring available.
There was a stage when false teeth were made from this cement!Much of what
is considered 'terra cotta' in China, including the 'warriors' is actually
magnesium cement.

Jahn was a big deal in the '80's, and the patches do hold up structurally -
its just that they always stand out as a smudge. Restorers are often
reluctant to specify Jahn - they would rather have something which looks
exactly like the background material - even if it has to be replaced every
twenty years or so. This is only if the replacement does not permanently
damage the background. It would be like getting white fillings for your
teeth, even if they don't last as long, versus the black/gray metal.

The only problem with magnesium cements is that if the bond is not perfect,
there is potential to leach chlorides, which are damaging to metals. The
alkalinity of portland cement protects the steel, and as the concrete
finally cures (50 years?) it no longer protects the steel. Luckily, there
are so many superior products to steel reinforcement now which do not decay
in magnesium cement - ever. The cement can also be bound with organic
binders, such as wood, straw, etc., - which is not possible with highly
alkaline portland cement.

One of my old books recommends ground rice 'flour' with plaster to make a
translucent, alabaster like material. I have never tried it - but rice flour
is cheap enough.

Jahn says I can paint some of their fine crack mortar on the marble for
slickness, and also can paint it with a waterproofer for some gloss. Really,
it looks fine from a distance. Glad to hear you are so conversant with these
materials.
Marble has no porosity compared with say limestone and sandstone, so only
coefficient is important.
Dan