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The world will have a lot more buildings in the years ahead. Any construction means use of concrete, which, in turn, means sucking out more water from reservoirs and putting out more carbon dioxide. The attention, therefore, is on ways to build with minimal harm to the world.
Globally, there are more and more scientific papers on the use of seawater and sea sand in concrete. The logic is obvious. Concrete-making involves huge quantities of freshwater — nearly 16 billion cubic metres globally. This will increase as cities expand and more buildings come up. Furthermore, urbanisation often happens in areas already stressed for water.
Researchers say that using seawater in concrete making has one immediate advantage (other than saving freshwater, of course) — the concrete sets much faster. Seawater has more sodium, chlorine and sulphates. If the concrete also contains ‘supplementary cementous materials’ such as coal fly ash or (even better) sugarcane bagasse fly ash, it shows improved properties. Further, seawater reduces the porosity of the concrete, making it denser.
A 2019 paper in Advances in Structural Engineering notes that using seawater and sea sand in concrete production saves freshwater, river sand and coarse aggregates, particularly in marine construction projects. “When used together with corrosion-resistant fibre-reinforced polymer composites, the durability of the structures in a harsh environment can be expected to be outstanding,” the paper says. Such concrete boasts high strengths (150-180 MPa).
Another recent paper in Cement and Concrete Research notes that concerns over seawater corroding steel reinforcement are perhaps overblown. “UHPC [ultra high-performance concrete] has a dense microstructure, which impedes the diffusion of oxygen, water, and chloride into the concrete,” it says. Studies have shown that corrosion may occur on a thin layer of steel fibre close to the surface of the specimens, but this is, by and large, not an issue. Indeed, it is possible to make concrete without steel, using seawater.
Seawater has been shown to increase the density of concrete, though only by 2-3 per cent, compared with freshwater. Seawater does somewhat reduce the workability of concrete. This is said to be due to the acceleration of cement hydration by ions in seawater. However, the use of ‘superplasticisers’ is a simple answer to it.
Yet another study by Chinese researchers, published earlier this month, developed a life-cycle assessment model for ‘seawater, sea sand, self-compacting concrete’ (SWSS-SCC), and ran experiments by varying factors such as concrete curing method, fly ash incorporation, sea sand transportation, energy cost, and cost of carbon dioxide emissions. The research determined that using seawater and sea sand “slightly increases” the concrete’s compressive and tensile strengths. Replacing some of the cement with fly ash “notably decreases the early-age strengths, but curing at 60 degrees C can offset that effect”.
Most of the research reviewed by Quantum, however, stressed that more investigations are needed to bring the sea into buildings.
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