What is Sulfate Attack on Concrete?

Sulfate attack is a chemical reaction between sulfate and the cement paste in concrete that forms expansive minerals and cracks the concrete. FHWA names ettringite, gypsum and thaumasite as the products. The British spelling is sulphate attack. It is the same process.

What reacts

Sulfate ions enter the pore water of the concrete, either from outside or from inside. The permeability of the concrete controls how fast they enter. FHWA describes the reaction in three forms.

• Ettringite is a calcium sulfoaluminate mineral. It forms when sulfate reacts with the aluminate phases of the paste. FHWA says that ettringite causes most of the disruption in sulfate attack. Many researchers link the sulfate resistance of a cement to its tricalcium aluminate (C3A) content, because C3A sets how much ettringite can form.
• Gypsum forms when sulfate combines with calcium. Its expansion may not be destructive, but gypsum has little cementing power, so the concrete loses integrity. FHWA cites a limit: gypsum corrosion adds to the damage directly above 1,000 ppm of sulfate, and it matters little until sulfate exceeds 4,000 ppm.
• Thaumasite is a mineral like ettringite, with silica in place of alumina. It depends on the calcium silicate hydrate (CSH) of the paste, not on the C3A content. So a low-C3A cement does not limit it. FHWA says that it forms more readily at low temperatures, and that it is often misidentified as ettringite. It can continue until no binding agent is left.

External and internal attack

In external sulfate attack, sulfate from outside the concrete enters it and reacts with the aluminate phases of the paste. FHWA gives groundwater and deicing chemicals as examples. Soil is the most common source. The damage depends on the source, the sulfate concentration, the permeability of the concrete and the make-up of the paste.

Deicing salt is a second route. FHWA reports that standard rock salt can be as little as 92 percent pure. The impurities are often gypsum and anhydrite. One study found as much as 4 percent calcium sulfate in natural rock salt. FHWA calls the damage from these impurities deicer distress.

Internal sulfate attack takes the sulfate from inside the concrete. FHWA names three forms:

• Delayed ettringite formation (DEF) is ettringite that forms in concrete after the concrete has hardened and gained its strength. High temperatures during early curing stop primary ettringite from forming properly. After the concrete cools, the sulfate and aluminate react and make expansive ettringite. FHWA cites research on a minimum temperature of 65 to 80 degrees C, with many sources at 70 degrees C. It notes that steam curing is the usual link, and that it is not certain that cast-in-place pavements reach such temperatures.
• Excess sulfate comes from slowly soluble sulfate in the clinker, fly ash or aggregate. It gives paste expansion like external attack. FHWA says that it is almost impossible to separate it from DEF in a pavement.
• Secondary ettringite is ettringite that dissolves and then crystallizes again in voids and cracks. Most experts agree that it does not generate enough pressure to fracture sound paste. It can fill air voids, and this may reduce freeze-thaw resistance.

How to recognize it

FHWA lists fine cracking near joints and slab edges, or map cracking over the whole slab, for both types. External attack usually first appears as cracking near joints and slab edges, within a few years of construction. In advanced cases, FHWA says that the paste may turn to a non-cohesive powder. FHWA says that internal attack in many structures shows as closely spaced, tight map cracks with wide cracks at regular intervals.

Other mechanisms give the same pattern. FHWA lists the same cracking for alkali-silica reactivity (ASR) and alkali-carbonate reactivity (ACR), so cracking alone does not show the cause. The time of first appearance helps a little:

A second FHWA brief (HIF-15-018) gives 1 to 5 years for external attack. The ranges overlap, so the time does not settle the cause either.

Ettringite in a core is not proof. FHWA says that it is very common to see ettringite deposits in air voids and cracks of distressed concrete. The ettringite may be a symptom of water movement and not the cause of the damage. FHWA asks for a petrographer to section the core and judge whether the ettringite caused the cracking or filled cracks that another mechanism made. FHWA also says that cores should cover the full thickness of the concrete. It names ASTM C856 as the framework for the petrographic examination. Under the microscope, DEF shows as gaps in the paste around aggregate particles, in proportion to the particle size.

ACI 318 exposure classes

ACI 318-19 sorts sulfate exposure into four classes by the water-soluble sulfate in soil or water. The soil value is in percent by mass and comes from ASTM C1580. The water value is in ppm and comes from ASTM D516 or ASTM D4130. Seawater is S1.

MS and HS mean the moderate and high sulfate-resistant types of blended cement. S3 has two options. The first is 0.45 and 4,500 psi with a pozzolan or slag. The second is 0.40 and 5,000 psi with Type V alone. A calcium chloride admixture is not permitted in S2 and S3. Type I or Type III cement may serve in S1 if its C3A content is below 8 percent. It may serve in S2 if its C3A content is below 5 percent.

Prevention

FHWA focuses on two steps. The first is to lower the permeability of the concrete with a lower w/cm and with accepted supplementary cementitious materials (SCMs). The second is to use a cementitious system that resists sulfate. FHWA lists Type II or Type V portland cement, and moderate or high sulfate-resistant blended cement, with a pozzolan or slag cement added. FHWA also names Class F fly ash and slag cement as effective SCMs. FHWA says that a w/cm below 0.45 helps against external attack.

A mix can also prove itself by test. ACI sets limits on the expansion of mortar in the ASTM C1012 test, which exposes the mortar to a sulfate solution:

• S1: 0.10 percent at 6 months.
• S2: 0.05 percent at 6 months, or 0.10 percent at 12 months if the 6-month limit is not met.
• S3, option 1: 0.10 percent at 18 months.

FHWA’s table also lets a cement of any type serve in S2 in place of Type V. Its expansion at 14 days in ASTM C452 must be no more than 0.040 percent.

Airfield pavement

FHWA says that sulfate attack is not a common problem for pavements. It is a concern for concrete in contact with soil in many areas of the western United States. If the soil test shows class S1 to S3, mitigation is required.

FAA AC 150/5320-6G tells the designer to check the soluble sulfate in the soil and the mixing water before chemical stabilization with lime or cement. It advises caution above 3,000 ppm and control of expansion above 5,000 ppm. The stabilization items of AC 150/5370-10H (P-154 to P-157) carry the same warning. The Engineer checks the soluble sulfate during design to see whether the stabilization can react and cause heave. Item P-219 does not allow recycled concrete aggregate base in locations with high-sulfate soil (no more than 0.5 percent).

For the slab itself, FAA AC 150/5370-10H gives guidance in Item P-501. The cementitious content should be at least 517 pounds per cubic yard where severe freeze-thaw, deicer or sulfate exposure is expected. The Engineer specifies a maximum w/cm of 0.45, or a lower value for sulfate exposure. For lean concrete base (Item P-306), it says to specify Type II cement where the selected aggregate has a history of sulfate reaction.

For an inspector, sulfate attack is one possible cause of joint and surface cracking in concrete. A drone survey records where the cracking is. The cause needs a soil or water sulfate test and a laboratory examination of a core.