What is Sulfate Attack?
Sulfate attack is a common form of
concrete deterioration. It occurs when
concrete comes in contact with water
containing sulfates (SO4). Sulfates can
be found in some soils (especially when
arid conditions exist), in seawater, and in
wastewater treatment plants.
Waterborne sulfates react with hydration
products of the tri-calcium aluminate
(C3A) phase of portland cement, and with
calcium hydroxide (Ca(OH)2) to form an
expansive crystalline product called
ettringite. Expansion due to ettringite
formation causes tensile stresses to develop
in the concrete. When these stresses
become greater than the concrete’s tensile
capacity, the concrete begins to crack.
These cracks allow easy ingress for more
sulfates into the concrete and the deterioration
accelerates. Sulfates also cause
chemical disintegration of some of the
cement hydration products.
Principal factors that affect the rate and
severity of sulfate attack are:
- Permeability of the concrete.
- Concentration of sulfates in the
- C3A content.
- Ca(OH)2 content.
Mitigating Sulfate Attack
One of the most common ways of protecting
against sulfate attack is to reduce
the alumina content by limiting the C3A
in portland cement. Historically, Type II
portland cement (with C3A between 5 and
8 percent) and Type V portland cement
(with C3A less than 5 percent) have been
specified for moderate and severe sulfate
environments, respectively. The use of
slag cement is also an extremely effective
way of reducing the potential for sulfate
How does Slag Cement Mitigate Sulfate Attack?
The use of slag cement reduces the likelihood of sulfate attack in three ways:
- Slag cement does not contain C3A, so
its addition in concrete dilutes the
total amount of C3A in the system.
- Slag cement reduces concrete permeability,
making it harder for sulfates to
penetrate into concrete.
- Slag cement reacts with excess
Ca(OH)2 to form additional calciumsilicate
hydrate gel (the “glue” that
provides strength and holds the concrete
together). This decreases the total
amount of Ca(OH)2 in the system.
Used in the proper proportions, slag
cement can give a Type I cement the sulfate
resisting properties of a Type II
cement (usually 25 to 50 percent slag
cement replacement for portland), and it
can give a Type I or a Type II cement the
sulfate resisting properties of a Type V
cement (usually 50 to 65 percent slag
cement replacement for portland).
The sulfate resistance of Type I, II and V
portland cements compared with a portland-
slag cement combinations, tested in
accordance to ASTM C1012, is shown
in Figure 2. For this combination of
materials, 15 and 25 percent slag cement
replacement achieved moderate sulfate
resistance, and 35 and 50 percent
achieved high sulfate resistance, based on
ASTM C989 six month expansion limits.
- ACI 201.2R-92, Guide to Durable Concrete; American Concrete Institute, Farmington Hills, Michigan,
- ASTM C1012-95a, Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a
Sulfate Solution, American Society for Testing and Materials, West Conshohocken, PA, 2001.
- ASTM C989-99, Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete
and Mortars, American Society for Testing and Materials, West Conshohocken, PA, 2001.