On 18/02/2011 00:36, Janet wrote:
In ,
says...
On Thu, 17 Feb 2011 10:45:41 -0000, "anon and off"
wrote:
During the recent cold spell, I kept a footpath's width along
the driveway clear by copious amounts of cooking salt.
Now, although the rest of the concrete has 56 years' of
grey drabness and growth (which you could "liken" to a moss)
the bit that I kept clear is yellowish and crumbling.
same here.
I wonder if anyone here has the chemical reaction info, or
similar experiences?
I had a similar experience, many years ago. Not sure of the
explanation, but sea water will attack certain cements, and
salt-water-resistant cements are recommended for marine exposure.
We're on the coast with plenty of marine exposure, but that never
seemed to have any effect on our concrete before. Unusually for here,
this winter we needed to scatter road salt (free from local council) on
frosty concrete paths and steps. We're certainly seeing the effects
:-( lots of erosion and bits breaking off.
Very high local concentrations of sodium chloride will drive the reverse
reaction to make soluble calcium chloride (and sodium
carbonate/silicate) from your concrete. This weakens the surface
structure of the concrete and then freeze thaw action makes it worse. It
has destroyed the surfaces of high pressure cast paving slabs in a local
market town which surprised me (maybe a bad batch).
It is only usually a problem if you use a vast excess of finely divided
rapidly soluble table salt. Rock salt normally dissolves slowly enough
that it doesn't get to really damaging levels before washing away in
mel****er. But salt in excess isn't very good for concrete and will
attack any minor imperfections or weak lines.
I am amazed that any councils are giving away road salt these days. In
North Yorkshire they have taken away as many public road salt bins as
they think they could get away with! To qualify for a salt bin now you
need a blind bend with a 1:6 gradient leading to a busy junction.
Regards,
Martin Brown