Why seawater Is no longer the enemy of strong, durable concrete

For more than a century, one of the unspoken rules of concrete production has been simple - keep seawater out of the mix. Chlorides cause corrosion, salt damages reinforcement and durability suffers. That assumption has shaped standards, specifications and site practice across the world. But as construction faces mounting pressure to cut carbon, reduce freshwater use and build more resilient infrastructure, that long-held rule is being challenged, writes John Ridgeway.

Today, researchers and engineers are revisiting a provocative question - can seawater be used in concrete without compromising strength or longevity? In certain contexts, the answer is increasingly yes - provided the material is designed properly and used intelligently.

The motivation is hard to ignore. Concrete production already accounts for around eight per cent of global CO₂ emissions and its environmental footprint extends beyond cement alone. Freshwater extraction for construction is a growing issue, particularly in coastal regions and arid countries where demand is high and supply is limited. For island nations, ports, coastal infrastructure and marine projects, importing potable water solely for concrete mixing can be both costly and unsustainable. Seawater, by contrast, is abundant, local and effectively limitless.

Historically, the problem with seawater has never been compressive strength. In fact, concrete mixed with seawater can achieve comparable and in some cases slightly higher, early strength than concrete mixed with freshwater. The real issue lies elsewhere with durability and reinforcement corrosion. Chloride ions in seawater accelerate corrosion in steel reinforcement, leading to cracking, spalling and reduced service life. This risk made seawater unsuitable for conventional reinforced concrete and standards rightly took a conservative approach.

What has changed is the way concrete itself is evolving?

One major shift has been the growth of unreinforced and non-steel-reinforced concrete applications. In mass concrete, pavement layers, gravity structures, marine blocks, breakwaters and precast units, built without steel reinforcement, the corrosion risk is largely irrelevant. In these applications, seawater can be used without introducing the traditional durability concerns, provided the mix is properly proportioned and cured.

Another significant development is the rise of alternative reinforcement strategies. Fibre-reinforced concretes, using synthetic, glass or basalt fibres, eliminate steel entirely. In these systems, seawater becomes far less problematic. Even where reinforcement is required, corrosion-resistant options such as stainless steel, epoxy-coated bars or fibre-reinforced polymer (FRP) reinforcement are increasingly viable. When steel corrosion is no longer the limiting factor, seawater use becomes technically feasible.

Material science has also moved on. Modern supplementary cementitious materials, such as ground granulated blast furnace slag, fly ash, calcined clays and silica fume, play a crucial role in mitigating chloride penetration. These materials refine the pore structure of concrete, reducing permeability and slowing the movement of aggressive ions. A denser microstructure means chlorides struggle to reach reinforcement in the first place, extending service life even in aggressive environments.

Slag rich concrete

Research has shown that slag-rich concrete, in particular, performs well when mixed with seawater. The chemistry of slag reduces chloride mobility and can improve resistance to chemical attack. In some cases, long-term strength development is actually enhanced, making the concrete more durable than conventional mixes.

Curing practices also matter. Proper curing allows cement hydration to progress fully, sealing capillary pores and improving resistance to ingress. Poor curing will undermine durability regardless of water source. When seawater concrete is treated with the same care as high-performance concrete - rather than commodity mixes - its performance improves dramatically.

There are already real-world examples demonstrating that this approach works. In Japan, seawater-mixed concrete has been used for decades in unreinforced marine structures. In the Middle East, where freshwater scarcity is acute, controlled use of seawater in concrete has been explored for coastal infrastructure. More recently, academic studies have demonstrated that seawater-mixed concrete can meet structural performance requirements when paired with appropriate materials and design strategies.

However, perhaps the most compelling argument for seawater concrete is resilience. Coastal infrastructure is exposed to salt spray, tidal inundation and aggressive marine conditions regardless of how it is mixed. Designing concrete that is inherently tolerant of chlorides, rather than trying to exclude them entirely, can result in structures that are better suited to their environment. This mindset aligns with modern resilience-based design, which accepts exposure and manages it intelligently rather than pretending it does not exist.

That said, seawater is not a universal solution and caution is essential. It is not appropriate for conventional reinforced concrete using carbon steel reinforcement without additional protective measures. It also requires careful control of mix design, testing and quality assurance. Standards in many countries still prohibit its use outright, which can limit adoption even where technical justification exists. Moving forward will require regulators, designers and material suppliers to work together to update guidance based on evidence rather than assumption.

Logistical considerations

There are also logistical considerations. Seawater quality varies depending on location, pollution levels and biological content. Pre-treatment or filtration may be required to remove organic matter or debris. Consistency is also critical because concrete performance depends on predictable chemistry, not just availability.

The broader significance of seawater concrete lies in what it represents. Construction is entering an era where resource efficiency matters as much as structural performance. Using seawater challenges the industry to rethink default practices and embrace context-specific solutions. In regions where freshwater is scarce and marine exposure unavoidable, insisting on traditional rules can be environmentally and economically counterproductive.

As the industry pushes toward lower-carbon binders, circular materials and climate-resilient design, seawater concrete fits naturally into the conversation. It is not about cutting corners or lowering standards. It is about applying science, evidence and engineering judgment to deliver structures that perform well, last longer and place less strain on fragile resources.

The question is no longer whether seawater can be used in concrete at all. That debate has largely been settled. The more important question is where, when and how it should be used responsibly. As standards evolve and confidence grows, seawater may move from being a last-resort material to a deliberate design choice in the right applications.

In a world where construction must do more with less, even something as fundamental as mixing water deserves a second look.