Sulfate
attack is a two-phased process. Sulfates combine with
calcium hydroxide
generated during cement hydration to form
calcium sulfate (gypsum).
The volume
of this gypsum is greater than the sum of its components causing internal
pressure and expansion, which fractures the
concrete.
Then
aluminate compounds
from portland cement react chemically with sulfates and calcium to form
a
compound called ettringite
(calcium sulphoaluminate). Ettringite
formation
destroys the concrete in the same manner as gypsum formation.
Fly
Ash in concrete can reduce sulfate attack in three ways:
-
Fly
ash reduces calcium
hydroxide, which combines with sulfates to produce gypsum.
Gypsum is a material that has greater volume than the calcium hydroxide
and
sulfates that combine to form it, causing damaging expansion. Fly ash
chemically binds free lime in cementitious compounds, rendering it
unavailable
for sulfate reaction.
-
Aluminates
in the cement also combine with sulfates to form expansive compounds. Replacing
a portion of portland cement with fly ash reduces the amount of
reactive
aluminates (tricalcium aluminate) available for sulfate
reaction, thereby
lowering the potential for this type of expansive reaction.
-
Fly
ash activity reduces
concrete permeability, keeping sulfates from penetrating
concrete.
At
this point a distinction
between Class C and Class F fly ashes needs to be made.
While
both improve the permeability and general durability of concrete, the
chemistry of Class F ashes has proven to be more effective in
mitigating
sulfate and alkali-silica expansion and deterioration in concrete. Some
Class C
fly ashes have been used to mitigate these reactions, but must be used
at
higher rates of cement replacement.
Studies
by the United States Bureau of Reclamation (USBR) show that properly
proportioned concrete utilizing up to 35 percent Class F fly ash will
withstand
sulfate attack far better than conventional portland cement.
The
Portland Cement Association (PCA)
reports the use of
Class F fly ash improves
sulfate resistance, while Class C fly ash is less effective and may
even
accelerate deterioration.
ACI
232.2R-96 (Use of Fly Ash in Concrete) reports that fly ash with CaO content
less than 15% will generally improve sulfate resistance.
