Why Sulphate-Resisting Cement Is Not Suitable for Marine Concrete Structures

When engineers specify concrete for marine environments, they face a paradox: sulphate-resisting cement (SRC), designed to resist chemical attack from sulphates, can accelerate deterioration in seawater. SRC reduces tricalcium aluminate (C3A) to prevent sulphate attack, but this reduction creates a critical vulnerability. Understanding this trade-off is essential for coastal and offshore construction. For a broader view of how materials interact within concrete, refer to Embedments in Concrete and When It Is Used in reinforced concrete construction.

Understanding Sulphate-Resisting Cement and Its Mechanism

Sulphate-resisting cement is a specialized type of Portland cement formulated to withstand attack from sulphates present in soil, groundwater, and seawater. Its defining characteristic is a significantly reduced proportion of tricalcium aluminate (C3A), typically below 3.5 percent compared to 5 to 14 percent in ordinary Portland cement (OPC). This reduction is the key to its sulphate resistance, but it also fundamentally alters how the cement interacts with the marine environment.

The Composition of Portland Cement

Portland cement consists of four main compounds:

• Tricalcium silicate (C3S) – responsible for early strength development
• Dicalcium silicate (C2S) – contributes to later strength gain
• Tricalcium aluminate (C3A) – influences early hydration and sulphate susceptibility
• Tetracalcium aluminoferrite (C4AF) – affects colour and heat of hydration

Of these, tricalcium aluminate is the most chemically reactive with sulphate ions, making it the focal point of sulphate-resisting cement design. The reduction of C3A in SRC is achieved by adjusting the raw materials during clinker production, typically by using raw mixes with lower alumina content.

How Sulphate Attack Damages Concrete

Sulphate attack proceeds through two primary reactions:

1. Sulphate ions react with calcium hydroxide to form gypsum, causing expansion and softening of the cement matrix
2. Sulphate ions react with tricalcium aluminate hydrate to form calcium sulphoaluminate (ettringite), which occupies a larger volume and causes internal cracking

As ettringite crystals grow within concrete pores, they generate internal tensile stresses that cause cracking, spalling, and eventual structural disintegration. By reducing C3A content, SRC minimizes the material available to form expansive ettringite, providing effective protection against sulphate attack in sulphate-rich environments. This makes SRC the preferred choice for foundations, retaining walls, and other structures in sulphate-bearing soils.

The Chloride Resistance Trade-Off in Marine Environments

Seawater contains chloride ions at approximately 19,000 to 20,000 ppm. Chloride ingress is the primary threat to reinforced concrete in marine structures, and the low-C3A formulation of SRC becomes a liability rather than an advantage.

The Role of Tricalcium Aluminate in Chloride Binding

Tricalcium aluminate has a high chemical affinity for chloride ions. When chlorides penetrate concrete, they react with C3A products to form calcium chloroaluminate hydrate (Friedel’s salt). This chemical binding process immobilizes chloride ions within the cement matrix, preventing them from reaching the steel reinforcement. The reaction can be represented as:

C3A + CaCl2 + 10H2O → C3A·CaCl2·10H2O (Friedel’s salt)

As noted by researcher P. Kumar Mehta in 1991, the reduction of C3A in SRC removes this natural chloride-binding capacity. With less C3A available to form Friedel’s salt, free chloride ions remain mobile in the pore solution and migrate deeper into the concrete toward the reinforcement.

A Quantitative Comparison of Chloride Binding

The table below compares chloride binding and related properties:

The property that makes SRC effective against sulphate attack (low C3A) compromises its performance in chloride-rich environments. For a deeper understanding of how sulphate attack can be controlled in concrete, read about Sulphate Attack On Concrete Process and Control of its damaging effects.

The Chloride Threshold for Corrosion

Steel reinforcement in concrete is normally protected by a passive oxide layer that forms in the alkaline environment of concrete (pH 12.5 to 13.5). Chloride ions break down this passive layer when their concentration at the steel surface exceeds a threshold value, typically 0.4 to 1.0 percent by weight of cement for OPC. In SRC, the reduced chloride binding means that free chloride concentrations at the steel surface reach this threshold much faster, leading to earlier corrosion initiation. This is a critical concern for the long-term durability of marine structures, which are expected to last 50 to 100 years or more.

Corrosion of Steel Reinforcement in Marine Concrete

Corrosion of steel reinforcement is the most widespread cause of premature deterioration in marine concrete. When SRC replaces OPC, corrosion accelerates due to higher free chlorides in the pore solution.

The Electrochemical Corrosion Process

Steel corrosion in concrete requires four elements: an anode, a cathode, an electrolyte, and an electrical connection. When chloride ions reach the steel surface, they initiate the following sequence:

1. Chloride ions break down the passive oxide layer at localized sites on the steel surface, creating anodic areas
2. Iron atoms at the anode lose electrons and dissolve into the pore solution as ferrous ions
3. Electrons travel through the steel to cathodic areas where they combine with oxygen and water to form hydroxide ions
4. Ferrous ions react with chloride and hydroxide ions to form various corrosion products, including rust
5. Corrosion products occupy 2 to 6 times the volume of the original steel, generating tensile stresses that crack and spall the concrete cover

Once cracking occurs, chloride ingress becomes even more direct, accelerating deterioration. The global cost of repairing corrosion-damaged marine structures runs into billions annually, making material selection a critical economic decision.

Consequences of Using SRC in Marine Structures

The real-world consequences of specifying sulphate-resisting cement for marine concrete include:

• Reduced service life due to earlier onset of reinforcement corrosion
• Increased maintenance and repair costs for jetties, piers, and sea walls
• Potential structural failure in critical elements such as piles and deck beams
• Higher lifecycle costs despite the marginal price difference between SRC and OPC
• Difficult and expensive remedial work, often requiring cathodic protection or electrochemical chloride extraction

These consequences underscore why major marine concrete specifications, including those from the British Standards Institute and the American Concrete Institute, do not recommend SRC for direct seawater exposure. For a practical overview of achieving durable concrete, explore Concrete Durability Inhibitor Admixed Cement for additional strategies.

Case Studies and Research Findings

Research by Mehta and others has consistently shown that concrete with higher C3A content outperforms low-C3A cement in marine exposure tests. Long-term field studies at the Treat Island marine exposure site in Maine demonstrate that concretes with C3A between 8 and 12 percent exhibit better corrosion resistance than those with C3A below 5 percent, provided the concrete has adequate quality and cover depth. This confirms that the chloride-binding capacity of C3A provides a genuine durability benefit in marine environments that outweighs the theoretical risk of sulphate attack from seawater sulphates.

Best Practices for Marine Concrete Construction

Specifying the right cement for marine concrete requires balancing multiple durability considerations. Sulphate-resisting cement may still have a role in certain limited applications, but for general marine construction, the following practices are recommended.

Cement Selection for Marine Environments

For marine concrete exposed to seawater, the preferred options include:

• Ordinary Portland Cement with C3A of 5 to 10 percent – provides adequate chloride binding while maintaining reasonable sulphate resistance
• Portland cement blended with supplementary cementitious materials – fly ash, ground granulated blast-furnace slag (GGBS), and silica fume improve both chloride resistance and sulphate resistance
• Blended cements conforming to standards such as BS EN 197-1 CEM II, CEM III, or CEM V – these incorporate mineral additions that refine the pore structure and enhance durability

The combination of OPC with 30 to 50 percent GGBS or 15 to 25 percent fly ash has proven particularly effective in marine environments. These supplementary materials not only provide additional chloride binding through their aluminate phases but also densify the concrete microstructure, reducing overall permeability. For more detailed guidance on cement concrete construction practices, visit Cement Concrete Construction.

Additional Durability Measures

Beyond cement selection, several other measures are critical for durable marine concrete:

1. Adequate concrete cover – Minimum cover depth should be increased to 50 to 75 mm for marine structures, depending on the exposure zone (splash zone is most severe)
2. Low water-cement ratio – A w/c ratio of 0.40 or less reduces permeability and limits ingress of both chloride and sulphate ions
3. Proper curing – Extended moist curing for at least 7 to 14 days ensures complete hydration and reduces permeability
4. Use of corrosion inhibitors – Calcium nitrite and other inhibitors can provide additional protection for reinforcement
5. Surface treatments – Silane or siloxane-based water repellents can reduce chloride penetration in the tidal and splash zones
6. Cathodic protection – Impressed current or sacrificial anode systems protect reinforcement in critical structures

When Is SRC Still Appropriate?

Sulphate-resisting cement is not entirely without merit in coastal construction. It remains a suitable choice in specific scenarios:

• For unreinforced mass concrete elements where corrosion of steel is not a concern
• In structures exposed to sulphate-rich groundwater on land, away from direct seawater contact
• As part of a blended cement system where supplementary cementitious materials compensate for the reduced chloride binding
• In temporary works or non-critical elements with limited design life

For most reinforced concrete in marine environments, however, the chloride-binding benefit of moderate C3A levels outweighs the sulphate resistance advantage of SRC. The key is to understand that sulphate resistance and chloride resistance are not the same thing and that optimizing for one can compromise the other.

Quality Control and Testing

Key quality control tests for marine concrete include:

Regular testing during construction and periodic inspections during the service life help ensure that the concrete performs as intended and that any deterioration is detected early before it compromises structural safety.

In summary, sulphate-resisting cement should not be used in marine concrete because reducing C3A removes the cement’s natural ability to bind chloride ions. Without this capacity, free chlorides penetrate to the reinforcement faster, initiating earlier corrosion. As established by P. Kumar Mehta, C3A’s affinity for chloride ions makes it valuable in seawater-exposed concrete. For durable marine structures, engineers should specify ordinary Portland cement with moderate C3A content, preferably blended with supplementary cementitious materials, combined with adequate cover, low water-cement ratio, and proper quality control.