Japanese researcher Ippei Maruyama discovered that during the operation of the Hamaoka nuclear power plant, the concrete walls of its reactor chamber became three times stronger than was laid down in the original design. The object has long been decommissioned, which allows various studies to be carried out there. In the course of his work, Maruyama discovered a direct connection between the phenomenon and the mystery of concrete in Ancient Rome.
In modern Italy there are concrete marinas over 2000 years old, which are much more durable than modern ones. Current technologies do not allow making structures from this material that will last in seawater for more than a couple of decades. Even more interesting, concrete from the builders of Ancient Rome only becomes stronger over time.
Now the secret of ancient concrete is revealed – in its structure the mineral “alumina tobermorite” is formed, which was not there initially. It is believed that the salt water gradually dissolved volcanic ash in ancient Roman concrete, resulting in very hard crystals. This process took hundreds of years, it is impossible to repeat it in laboratory conditions, except to heat the initial mixture above 70 ℃. But this is already dangerous for the concrete itself.
The Hamaoka NPP reactor operated from 1976 to 2009, of which 16.5 years at full capacity. The walls of the reactor chamber practically did not cool down, their temperature ranged from 40 to 55 ℃. Maruyama believes that it was this long but gentle heating that triggered the reaction of formation of alumina tobermorite in the concrete, which made it stronger. The discovery gives a chance to develop a new method of passive strengthening of concrete in a long-term perspective.