What is concrete?

Kevin Kelley wrote:
> As for affecting the water chemistry, they may, depending on the
> original chemistry of your water supply.  Cement/concrete is essentially
> calcium carbonate (crushed up oyster shells), is moderately basic, and
> will attenuate water pH towards 8.5+. 

What a blanket statement!  Cement is the dry powder and is chiefly
made from lime.  The calcium in the final form is bound up in calcium
silicates, which are not very soluble in water.  Concrete is the final
product, obtained from mixing cement, water, and aggregate (sand and
gravel, usually).  

Not all concrete radically alters pH.  Many municipal
holding ponds are made of concrete, both on the supply and the water
treatment side.  Large changes in pH would not be cool in this case.  What
determines concrete's chemical behavior after setting is the chemical
composition of the aggregate.  If there is a lot of calcareous material in
the aggregate (e.g. limestone, dolomite), then immersing concrete blocks
will be like immersing dolomite chips.  Crushed shells have also been used
in the past, but are not used now in most applications.  The CO2 which is
liberated from the shells will attack reinforcing steel with a vengeance.
Cinder blocks are often used in conjunction with reinforcing steel, so I
would not expect to find a lot of CaCO3 in available form in the blocks.

To really settle the issue, I would call the manufacturer and ask if your
projected application is ok.  They should be able to tell you the
composition of the aggregate and whether the blocks are ok for use in a

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