Can Sea Water Be Used for Making Concrete? Effects on Strength and Durability

In concrete construction, the quality of mixing water plays a critical role in determining the long-term performance of the final structure. Clean water fit for drinking has always been the recommended standard for producing cement concrete. However, in coastal regions and islands where potable water is scarce or expensive, engineers have explored the possibility of using sea water as an alternative. The question of whether sea water can be used for making concrete is not a simple yes or no, it requires careful examination of how the dissolved salts in sea water affect both the strength of the concrete and the durability of embedded steel reinforcement. Understanding these effects helps professionals make informed decisions, especially when considering techniques such as concrete staining techniques for decorative concrete finishes where surface quality matters just as much as structural integrity.

This article examines the chemical composition of sea water, its impact on concrete strength, the corrosion behaviour of reinforcement in saline environments, and the practical applications where sea water concrete remains a viable option.

Chemical Composition of Sea Water and Its Impact on Concrete

Sea water contains approximately 3.50 percent dissolved salts by weight, and this saline content is what fundamentally alters its behaviour when used as mixing water for concrete. The dissolved solids consist primarily of sodium chloride, along with significant proportions of magnesium chloride, magnesium sulphate, calcium sulphate, and potassium sulphate. The table below presents the typical distribution of these constituents in average sea water.

The distribution of these salts in percentage terms is as follows:

• Sodium chloride (NaCl) makes up approximately 78% of the total dissolved salts
• Magnesium chloride (MgCl₂) and magnesium sulphate (MgSO₄) together account for about 15%
• The remaining 7% consists of calcium sulphate (CaSO₄), potassium sulphate (K₂SO₄), and trace elements

What makes this composition particularly important for concrete technology is that chlorides and sulphates have opposing effects on cement hydration. Chlorides tend to accelerate the setting of cement and improve early strength development. Sulphates, on the other hand, retard the setting process and reduce early strength gain. The net outcome of these two contradictory chemical actions determines whether sea water concrete achieves acceptable performance. For structures that require protection against moisture ingress, implementing proper waterproofing methods for concrete structures becomes especially important when sea water is involved.

Strength Reduction When Using Sea Water in Concrete Mixes

Laboratory studies and field experience consistently show that the net effect of the accelerated setting caused by chlorides and the retarding effect caused by sulphates is an overall reduction in concrete strength. The recorded decrease in compressive strength ranges from 8 to 20 percent compared to concrete mixed with fresh potable water. This reduction is significant enough that it cannot be ignored in structural design, and it must be factored into the mix design calculations.

The extent of strength loss depends on several variables:

1. The salinity level of the specific sea water source, which varies by location and season
2. The type and grade of cement used in the mix
3. The water-cement ratio adopted in the design
4. The curing conditions and ambient temperature during hydration
5. The presence of any chemical or mineral admixtures in the concrete

Despite this reduction, sea water can still be used for making concrete in situations where the 8 to 20 percent loss in strength is within acceptable limits for the intended application. Engineers can compensate for the reduction by adjusting the water-cement ratio or increasing the cement content in the mix design. For example, if a target strength of 25 MPa is required and sea water is expected to cause a 15 percent reduction, the mix can be designed for approximately 29.5 MPa to compensate. This approach requires careful laboratory trials with the actual sea water source to determine the precise correction factor. Engineers familiar with standard concrete cube testing procedures in compression tests should note that the same quality control methods apply when verifying the corrected mix design.

It is worth noting that the early strength gain can actually be higher with sea water due to the accelerating effect of chlorides. The strength deficit becomes more apparent at later ages, typically beyond 28 days, when the retarding influence of sulphates becomes dominant. This behaviour means that relying on early test results alone can be misleading when approving sea water concrete for structural use.

Corrosion of Reinforcement and Protective Cover Requirements

The most significant concern with using sea water in reinforced concrete is not the strength reduction but the potential for corrosion of the embedded steel reinforcement. Chloride ions are well known for their ability to break down the passive oxide layer that protects steel in the alkaline environment of concrete, leading to active corrosion. However, research indicates that sea water does not necessarily lead to corrosion of reinforcement provided two critical conditions are met.

The first condition is that the concrete must be dense and of low permeability. A well-compacted concrete with a low water-cement ratio prevents the ingress of oxygen and moisture that are necessary for the corrosion reaction to proceed. The second condition is that there must be sufficient concrete cover over the reinforcement to provide a physical barrier against the external environment.

For concrete structures that will be permanently submerged in sea water, the minimum recommended cover over the reinforcement is 75 millimetres. This is substantially higher than the typical cover of 20 to 40 millimetres used in normal inland structures. The additional cover provides a longer path for chloride ions to travel before reaching the steel surface, thereby extending the service life of the structure. When designing reinforcement layouts, engineers should also account for embedments in reinforced concrete and their structural requirements to ensure that cover requirements are maintained throughout the element.

There is one category of concrete for which sea water must never be used regardless of the precautions taken: prestressed concrete. Prestressed concrete relies on high-tensile steel wires or strands that are under constant tensile stress. These highly stressed steel elements are extremely susceptible to stress corrosion cracking and hydrogen embrittlement in the presence of chloride ions. The risk of catastrophic failure without warning makes the use of sea water in prestressed concrete entirely unacceptable.

Surface Quality and Aesthetic Considerations

Beyond structural strength and reinforcement corrosion, sea water concrete presents challenges related to surface finish and appearance. The dissolved salts in sea water tend to promote efflorescence, which is the unsightly white crystalline deposit that forms on concrete surfaces as moisture evaporates and leaves salt residues behind. Sea water concrete also tends to develop persistent dampness because hygroscopic salts in the concrete matrix continue to attract moisture from the air.

These surface quality issues mean that sea water concrete is generally unsuitable for architectural concrete, exposed interior surfaces, or any application where the finished appearance is important. The dampness can also cause problems with paints, coatings, and floor finishes that may not adhere properly to a constantly moist substrate.

However, there are many concrete applications where surface finish is not a primary concern. Massive foundations, underwater concrete works, and temporary construction elements are examples where the aesthetic drawbacks of sea water concrete can be accepted. Attending industry trade shows for concrete contractors can provide practical insights into how professionals in coastal regions manage these surface quality challenges in real-world projects.

Practical Guidelines for Using Sea Water in Concrete

When the decision is made to proceed with sea water concrete, certain practical guidelines should be followed to maximise the chances of satisfactory performance.

• Cement selection: Sulphate-resisting Portland cement (SRC) is preferred because it is formulated to resist sulphate attack, which is a risk given the magnesium and calcium sulphate content in sea water
• Water-cement ratio: Keep the water-cement ratio as low as possible, ideally below 0.45, to ensure dense concrete with minimal permeability
• Compaction: Thorough mechanical vibration is essential to eliminate voids and honeycombing that would otherwise create pathways for chloride ingress
• Curing: Extended wet curing for at least 14 days is recommended to ensure complete hydration and the development of a dense microstructure
• Cover: Maintain the minimum 75 millimetres cover for submerged elements and increase it further for elements in the splash and tidal zones where exposure is most severe

It is also advisable to use supplementary cementitious materials such as fly ash, ground granulated blast furnace slag (GGBS), or silica fume in the mix. These materials react with the calcium hydroxide produced during cement hydration to form additional calcium silicate hydrate, which densifies the concrete matrix and reduces its permeability. The selection of appropriate water-based concrete sealers as protective coatings can further reduce moisture ingress and extend the service life of the structure.

The following checklist summarises the key factors to evaluate before approving sea water for concrete production on any given project:

1. Confirm that the strength reduction is acceptable for the structural design
2. Verify that the cover requirements can be physically achieved in the reinforcement layout
3. Ensure the concrete will not be subjected to wetting and drying cycles that accelerate chloride concentration
4. Confirm that no prestressed concrete elements are part of the design
5. Check that surface finish requirements do not conflict with efflorescence and dampness risks
6. Conduct trial mixes with the actual sea water source before production begins

Conclusion

The use of sea water for making concrete is a practical solution in coastal regions where fresh water is scarce, but it comes with well-defined limitations that must be respected. The 8 to 20 percent reduction in compressive strength can be managed through mix design adjustments, but the corrosion protection requirements for reinforcement demand careful attention to concrete quality, cover depth, and structural detailing. Sea water should never be used in prestressed concrete under any circumstances. For plain concrete, mass concrete works, and certain reinforced elements with adequate cover and dense concrete, sea water remains a viable alternative. Understanding the behaviour of reinforced concrete water tank joints and their design is one example of the specialised knowledge required when working with concrete in water-retaining and marine environments. The key to success lies in recognising the limitations of sea water concrete and compensating for them through sound engineering practices rather than ignoring them.