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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