The Importance of pH

 

Why pH is important

pH is a critical factor in the chemistry of concrete. The components of concrete are

Portland Cement, rock (aggregates), and water. Portland Cement is the “binding”
component in concrete and has a pH approaching 11, which is very alkaline. If you
remember your high school chemistry, neutral pH is 7. Above 7 is alkaline, and below 7
is acidic. In order for the cement to hold together the other components, it is important
for it to remain at or near a pH of 11.
 
When salt (pH of roughly 6 to 7) or other acids, such as acid rain, are introduced onto the
concrete, they enter into the small pores and micro-cracks of the concrete and attack the
surrounding materials, lowering their pH. As the pH is lowered, the cement’s ability to
hold things together is compromised. When the concrete is exposed to acidic
environments for a long enough period of time, all that will be left is sand and rock.
 
Even concrete that is not subjected to de-icing salts is subjected to a process that lowers
pH. The process is called “Carbonation.” Carbonation is a reaction between the cement
in concrete and carbon dioxide in the air. Carbonation progressively lowers the pH in
concrete, though the process is somewhat slow. Carbonation takes about 6 years to
progress 50mm (2 inches). When carbonation (lowered pH) reaches the level of the steel
reinforcement, it attacks the thin protective layer of iron oxide surrounding the
reinforcement and initiates corrosion. Since steel can expand up to 6 times its size when
corroding, the resulting pressure can cause the surrounding concrete to crack and break.
In structural concrete, this can result in structural failure.
 
A good indicator of the level of degradation in your concrete is a pH test. We use a
digital pH meter, which instantly reads the concrete’s pH. If your concrete’s pH is
between 9 and 11, you are in pretty good shape (and your concrete probably looks pretty
good as well). If the pH is between 7 and 9, the concrete is starting to break down. You
may notice some surface damage at this level – but not always. pH around 6, and you
certainly have visible damage. pH readings below 6, and you have severely deteriorated
concrete. If your pH reading is around 4, you will need to replace the concrete, for there
is little left to hold it together.
 
Another way to test the pH level is to take a core sample and spray it with
Phenolphthalein. Phenolphthalein is a pH indicator that turns magenta when the pH is
over 9.6. If the core turns completely magenta when sprayed, the pH is still in the safe
range. If it does not change color, the pH is below the safe range for concrete. This test is
a good one to see how close to the reinforcement carbonation has reached.
 
CreteDefender P2 penetrates and fill the pores and micro-cracks of concrete with a
solution that forms a gel and hardens. The pH of this gel is roughly 11, the same as the
surrounding concrete. This reaction also creates alkali metal molecules in the concrete,
which raises and stabilizes the pH in the concrete, and the alkali metal molecules absorb
carbon dioxide. The hardened gel prevents the intrusion of water, salts, acid, or other
chemicals.
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