I disagree with how bad NaCl salt might be, especially with the concept 
that it crystallizes and breaks stone apart. The sulfate salts are very 
destructive because they have great changes in volume when they hydrate 
or dehydrate, but NaCl isn't really that bad becaue it forms a simple 
anhydrous crystal.  NaCl can have two effects on stone that may lead to 
deterioration.  First, it can lead to additional freeze-thaw cycles of 
water.  If the stone can deteriorate in F-T, then one may increase the 
rate of F-T and see the deterioration sooner.  Second, the sodium in 
the NaCl can exchange for Ca in any expansive clays in the stone, 
leading to a greater expansion of the clay.  This rarely happens with 
metamorphic marbles, as are used on steps, but is fairly common with 
the limestones, especially the limestones with stylolites used for 
floor tiles in many buildings.  But the issue with the expansive clays 
is the effect of water on the clay, not the effect of NaCl on the clay.

Portland cement rarely penetrates the pores in building stone. The 
grain size is generally larger than the pore size.  Portland cement is 
a component in some, actually most, mortars, regardless of the best 
efforts of U. S. Lime.  There can be deterioration of stone laid with 
these mortars, as there can be with stone and other masonry materials 
laid with other non-Portland mortars.  Enough said for now; this is 
getting off topic.

Bleach on tombstones, especially marble tombstones with organic 
growths, tends to release chlorine gas, which forms hydrochloric acid 
(muriatic acid) that dissolves the stone.  That's one of the reasons 
why using bleach may not be a good idea.  In addition, the bleach tends 
to leave little black and brown dead roots on the stone,  They are 
unsightly, and also may expand when wet, and shrink when dry, weakening 
the fabric of the stone.

The salt crystals that one grows in Kindergarden tend to be sulfates 
such as Epsomite (MgSulfate x H2O).  It forms very dramatic crystals.  
One potential downside with having Epsom salts around little kids is 
that it is a laxative.  We also grew sugar crystals.  The kids can eat 
those without getting diarrhea.  NaCl tends to form very small cubic 
crystals that are not usually dramatic enough for teaching kids, unless 
one mixes in some ammonia and bluing.  One can also use a large variety 
of other chemical substances for growing crystals in classroom 
experiments.

I completely agree that the lack of maintenance results in problems.  
But, if the steps need to be deiced, then they are apparently already 
exposed to the rain, sleet, and snow, and may be somewhat 
self-cleaning.  Using NaCl is an inexpensive method of solving an ice 
safety problem.  Melting the ice with NaCl may even prevent ice from 
freezing between the individual marble block that probably comprise the 
steps, therby wedging them apart.

Steve Stokowski
Stone Products Consultants



-----Original Message-----
From: Bowman, Camille (DHR) <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Mar 3, 2010 4:40 pm
Subject: Re: [BP] Salty Steve makes piquantly marbled waves on 
ConsDistList ...

I have to add that salt is not good for stone -- any stone -- as it 
crystallizes down in the little pores and breaks the stone apart. This 
is one (of many) reasons that Portland is bad for building stones; it's 
why you dont use Chlorox bleach to clean tombstones; it why you dont 
want to apply salt to stone steps or near stone or brick buildings. 
It's very damaging to stone. Remember the kindergarten experiments 
where you grew salt crystals? That's what happens when salt gets inside 
stone...it grows and displaces the aggregates and binders.
 
It may be that you could use it during the incidence of snow but it'd 
need to be washed off thoroughly after that (and who's going to go to 
that much trouble -- lack of maintenance being the biggest enemy of 
building materials). I doubt there's any coating that would weather at 
the same rate as marble, allowing it to breathe and dry out as it needs 
to. This is a good question of a stone conservator. There was an 
excellent one at Prosoco labs. Maybe they've replaced her with an 
equally excellent one. But ask at Columbia or U Penn or other programs 
where architectural conservation studies are ongoing.
 

Camille Agricola Bowman


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