[Greenbuilding] Calcium chloride admixutres (was re: concrete & frost)

[Robert Tom]

On Mon, 05 Nov 2007 12:56:09 -0500, LarenCorie <LarenCorie at axilar.net>  
wrote:

> From: "Robert Tom" <ArchiLogic at yahoo.ca>
>
>> The admixture that is typically used to make concrete
>> workable at  below-freezing temperatures is calcium chloride
>> -- a salt.
>
>> The salt, as in the oceans, simply lowers the freezing point
>> slightly( only a couple or few  degrees F)
>
> Actually, it does quite a bit more than simply lowering the
> freezing point.   When calcium chloride is mixed with water
> it creates a powerful exothermic reaction, that is just about
> hot enough to burn your hand.


Okay, okay.
"Yes" the CaCl2 actually does a bit more than simply lowering the freezing  
point a couple or few degrees. It is the active ingredient in most  
accelerating admixtures used to speed up initial setting and early  
strength development.

But the point I was trying to get across was that the addition of calcium  
chloride should not be viewed as an anti-freeze agent to provide insurance  
against freezing when placing concrete in cold weather, which is too often  
the case.

I've seen contractors place the concrete for a full (house) basement when  
there's a blizzard blowing and then take off after the pour without  
placing any insulation other than a sprinkling of loose straw over the  
exposed concrete at the top of the forms. And if that were't bad enough,  
then they come back the next day to remove the forms, leaving the green  
concrete fully exposed.  Bad Idea.

Also as mentioned, too often the

   "If a little bit is good, then a LOT should be better. Right ?"

mentality that one sees when people are stuffing loose insulation into  
small gaps seems to happen with cold weather concreters and they add too  
much CaCl2 thinking that it will given them more frost resistance. Another  
Bad Idea.

The amount of CaCl2 used should not exceed 2% (by weight) of the cement in  
the mix and should be pre-mixed with the mixing water to ensure complete  
dissolution . Otherwise $#!+ can and will happen.

I've  mixed concrete using CaCl2 in the prescribed manner and have not  
noticed any "powerful exothermic reaction ... hot enough to burn your  
hand".

Then again, since it was probably cold enough to be wearing work gloves  
(although, that's no assurance that gloves were being worn) and since I  
probably didn't stick my bare arms into a spinning mixer full of concrete  
glop, I can't comment one way or the other on that.

Maybe if it ever gets cold again up here in this age of Global Warming,  
I'll mix myself a nice cuppa Calcium Chloride tea just before heading out  
for a stroll in a blizzard with the  mutts to see if in fact, there is any  
noticeable warming. Come to think of it, sometimes they mix calcium  
chloride (aka designer road salt) in with the sand (but not very often  
because it costs more than NaCl) that they spread on icy roads here in  
winter. So maybe I should just get nekkid, get the mutts to lick me all  
over and then roll around on the freshly-salted road ?


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at chaffY a h o o  dot  c a >
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