In reply to johncook:
Tell me the same about the surface of an aluminium bottle, and I would agree: The metal will rapidly be coated by a µm thick but chemically highly inert and scratch resistant Al2O3, i.e. a proper oxide.
The CaO in cement, which is generated by heating limestone in a kiln will react quickly with water to form highly basic Ca(OH)2, which is also extremely water soluble. This will then react with CO2 from the air to form the solid carbonate again, which AFAIK is how concrete sets.
If I remember any chemistry at all his reaction cannot be reversed by an excess of carbonate ions, which would in fact drive more carbonate crystalisation. Weathering of marble or limestone buildings is instead due to other acids such as sulfuorus acid (from SO2 emissions) or lactic and other organic acid (from bacteria, hand sweat, etc...) which even though weak are acidic enough to release the carbonate ions.
If the resulting salts are water soluble you will get erosion/polish. If local conditions (at the micro level) are such that some of these reactions run in the reverse (e.g. if concentrations of these acids vary) you will get redeposition of the carbonate, potentially filling gaps in the matrix and leading to stronger bonding between grains (which is how a snowball hardens, except that the crystals there are dissolved by heat and refreeze, rather than chemically).
I assume that such dissolution/reprecipitation processes drive the formation of hardened surface layers on limestone. In any case, splitting a rock and scratching the newly exposed surface is not particularly informative, as the surface will be shattered at the microscopic level, with loads of terminating cracks that in the end did not contribute to the macroscopic break, but will weaken the surface.
In contrast to the claims further up in the thread I was initially responding to none of this involves any oxidation, unlike, say, chucking a chunk of metallic calcium into water or burning it in air.
I am sure, though, that one of the resident geologists soon comes along to put me in my place!
CB