How Salt Damages Concrete


Salt Damage:


Salt damage is most commonly due to exposure to de-icing salts. However, any chemicalcontaining chlorides, including sodium chloride, potassium chloride, or calcium chloride, found in “safe de-icing chemicals,” fertilizers, ocean water, marine air, etc., presents a danger to the concrete. All are mildly acidic and attack the bonds that hold concrete together.

 While the exact mechanism of salt damage is not known, a number of well understood,
related factors instigated by exposure to salt contribute to the deterioration of concrete.

* Salt is a mild acid and lowers the pH in the concrete. The acidic reaction attacks
the concrete paste and aggregate, weakening the structure and strength of the
concrete. It also increases the pore size, allowing additional water and chemicals
into the concrete, which can exacerbate freeze/thaw cycle damage.
* Salt is hygroscopic, meaning that salt attracts and retains water. When salts are
applied to concrete, they attract up to 10% more water into the pore structure of
the concrete. This process leaves less room for expansion in the pore structure
which, in turn, creates more pressure inside the concrete when it freezes, causing
the surface to chip, flake, and pop (typically called “spalling”).
* Salt accelerates carbonation, a process that slowly reduces the pH levels in
concrete through contact with Carbon Dioxide (air) and contributes to the
corrosion of reinforcing steel. Most salts are neutral to mildly acidic (pH of 4.5 –
7), and absorption of the salts (brine) by the concrete will accelerate the pH
reduction of the concrete.
* Salts are the primary source of chlorides introduced into concrete. Chlorides are a
major player in the corrosion of reinforcement, as they pierce the thin, protective
iron oxide layer surrounding the reinforcement and initiate a corrosive reaction in
the steel.


Sulfate Attack:

Sulfates are, quite simply, a salt of sulfuric acid. They are found in all natural waters, and
are a major dissolved component of rain. Concrete is exposed to sulfates in two ways,
externally sourced and internally sourced. Most commonly, sulfate attack occurs where
concrete is exposed to high sulfate content in the soil, in areas of run-off, and in
wastewater. Secondly, portland cement contains a small percentage of gypsum (calcium
sulfate dihydrate). The sulfate in the portland cement is an internal source, and is
activated when the internal temperature of the curing concrete reaches 160 degrees
fahrenheit, a condition which can and should be avoided.

Sulfates react in the alkaline environment of the concrete paste and create highly
expansive crystals called Ettringites. Ettringite is calcium aluminum sulfate. Aluminum is
also frequently found as a component of portland cement in trace amounts. The formation
of these expansive crystals creates a great deal of pressure inside the concrete and causes
the concrete to crack and spall.

In addition, sulfates are acidic, break down the alkaline environment in the concrete and
deteriorate the concrete paste through acidic attack. This acidic reaction causes the
concrete to weaken and crumble.


Reactive Silicates:

Reactive silicates have been demonstrated to nearly completely inhibit water penetration
and chloride ion infiltration, significantly reduce carbonation due to their high pH, and
provide excellent protection from acidic attacks, including salts and sulfates. These
products are highly effective at preventing freeze/thaw damage, and do not have any
negative impact on air-entrained concrete.

A unique characteristic of reactive sodium silicates is that, as they penetrate the pore
structure, they raise the pH of the concrete and can force residual salts and other
impurities to the surface of the concrete. Concrete is a highly alkaline substance that is
stable at a pH between 10 and 13. Raising and maintaining the pH in concrete has the
effect of arresting the damage occurring in the concrete, which gives sodium silicates a
distinct advantage over other products. While a sodium silicate won’t repair large voids
caused by either poor placement or acidic deterioration, and won’t stop corrosion that is
already in the propagation phase, it will have an extremely positive effect on sound
concrete, even if deterioration has been initiated.

CreteDefender P2 is this type of formulation… A reactive sodium silicate permanent
concrete sealer that can extend the useful life of your concrete, prevent freeze thaw
damage, and protect against salt damage, sulfate attack and chloride penetration… Call or
email us to find out how CreteDefender products can make your business dollar go farther and
keep your assets looking sharp longer!

 

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