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[quote="Gary"]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.[/quote]

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