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Mineral that makes concrete grow stronger found in walls of Japanese nuclear plant

Image credit: Dreamstime

A rare mineral that has allowed Roman concrete marine barriers to survive for more than 2,000 years has been found in the thick concrete walls of a decommissioned nuclear power plant in Japan.

According to researchers at Nagoya University, the formation of aluminous tobermorite increased the strength of the walls of the plant more than three times their design strength, following years of full operation. Such findings could help scientists develop stronger and more eco-friendly concrete.

“We found that cement hydrates and rock-forming minerals reacted in a way similar to what happens in Roman concrete, significantly increasing the strength of the nuclear plant walls,” said Nagoya University environmental engineer Ippei Maruyama.

Research has shown that Roman concrete used in the construction of marine barriers has managed to survive for more than two millennia because seawater dissolves volcanic ash in the mixture, leading to the formation of aluminous tobermorite. Since aluminous tobermorite is a crystal, it makes the concrete more chemically stable and stronger.

However, experts said it is difficult to incorporate aluminous tobermorite directly into modern-day concrete. While scientists have generated the mineral in the lab, it still needs to be tested at temperatures above 70°C. On the other hand, laboratory experiments have shown that hot environments are detrimental to concrete strength, which has led to regulations that limit its use to temperatures below 65°C.

Maruyama and his colleagues found that aluminous tobermorite formed in a nuclear reactor’s concrete walls when temperatures of 40-55°C were maintained for 16.5 years. Samples of aluminous tobermorite were taken from the Hamaoka Nuclear Power Plant in Japan, which operated from 1976 to 2009.

In-depth analyses showed that the reactor’s very thick walls were able to retain moisture. Minerals used to make the concrete reacted in the presence of this water, increasing the availability of silicon and aluminium ions and the alkali content of the wall. This ultimately led to the formation of aluminous tobermorite.

The scientists found aluminous tobermorite formed inside the walls due to reactions between minerals in the concrete mixture in the presence of water and moderately high temperatures over a prolonged period.

The scientists found aluminous tobermorite formed inside the walls due to reactions between minerals in the concrete mixture in the presence of water and moderately high temperatures over a prolonged period.

Image credit: Ippei Maruyama, Nagoya University, and Chubu Electric Power Co.

“Our understanding of concrete is based on short-term experiments conducted at lab time scales,” Maruyama explained. “But real concrete structures give us more insights for long-term use.”

Maruyama and his colleagues seek to make concrete more durable and environmentally friendly. Cement used in concrete manufacturing produces nearly 10 per cent of human-made carbon dioxide emissions, so the team is looking to produce more eco-friendly mixtures that still meet standardised requirements for strong concrete structures.

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