Sulfate Attack - Proactive Avoidance

There are 5 factors that influence sulfate attack:

• the amount and nature of the sulfate present
• the level of the water table and its seasonal variation
• the flow of groundwater and soil porosity
• the form of construction, and
• the quality of concrete

A survey of environmental conditions should be performed before building a structure, however, if the sulfate water cannot be prevented from reaching the concrete, the only defense against attack lies in the quality of the concrete. Concrete durability is largely effected by its water/cementitious materials (w/cm) ratio. If the w/cm ratio is decreased, the porosity decreases and the concrete becomes more impermeable. The permeability of concrete is important because it controls the amount of water migration through concrete or the ability of concrete to resist penetration of aggressive chemicals. A lower w/cm ratio also increases the compressive strength of concrete, which improves its resistance to cracking. To protect against sulfate attack, ACI Committee 201 recommends the use of dense, quality concrete with a low w/cm ratio. Air entrainment is suggested because it reduces the w/cm ratio and therefore the permeability.

A stronger relationship exists between the sulfate resistance of concrete and its tricalcium aluminate (C₃A) content. The higher the C₃A content, the more prone the concrete is to sulfate attack. To improve the sulfate resistance of concrete, lower C₃A cements are available. ASTM C 150 Type II cement (MSR) with <8% C₃A, and Type V cement (HSR) with <5% C₃A are typically specified in sulfate environments. This partial replacement of portland cement with a pozzolan such as low calcium fly ash, ground granulated blast furnace slag, or silica fume equally reduce the potential for sulfate attack. These pozzolans consume the calcium in the pore water, reduce the total mass of C₃A and decrease the permeability. When deciding which pozzolan to choose, it is important to consider its CaO content. A high percentage of CaO in fly ash may accelerate the sulfate problem substantially. For instance, ASTM Class F fly ash with <10% CaO will certainly improve the resistance of concrete to sulfate attack. Similarly, silica fume, metakaolin and natural pozzolans consume Ca to improve sulfate resistance.

The following table from ACI 201 gives recommendations for the type of cement and w/cm ratio for normal-weight concrete, which will be exposed to sulfates in soil, groundwater, or seawater.

Recommendations for normal weight concrete subject to sulfate attack