Sulfate Resistance

There are multiple ways sulfate attack can occur. First, sulfates react with the calcium hydroxide from the free lime in the cement to form calcium sulfate (gypsum). Second, the aluminates in cement react with sulfates and calcium to form ettringite. Both the gypsum and ettringite are expansive compounds that cause fractures in the concrete.

ProAsh® reduces the sulfate attack in the following ways:

  • ProAsh® reacts with the free lime in the cement to form more cementitious material and reduces the risk of free lime reacting with sulfates.
  • ProAsh® reduces the permeability of the concrete and reduces the penetration of the sulfates.
  • Additional ProAsh® translates to less cement and results in fewer aluminates for sulfate attack.

The reduction in permeability is related to the particle size and shape of fly ash. ProAsh®, when introduced to the concrete mix, actually fills voids within the concrete matrix and lowers the permeability of the material. One test for permeability is the rapid Chloride Permeability test. The test involves measuring a small electric current through a cured concrete cylinder that has been stored in a saltwater bath.

There are multiple success stories involving ProAsh® and sulfate remediation, including the two Highway 90 bridges in Biloxi, MS, and the I-10 bridge in New Orleans, destroyed during Hurricane Katrina and since rebuilt using 20% ProAsh®.


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