In recent years, nuclear energy has attracted significant attention as countries worldwide look for cleaner energy sources to tackle climate change. While nuclear power produces electricity without releasing large amounts of greenhouse gases, it creates another challenge: dealing with radioactive waste. Nations are exploring solutions to responsibly store and manage these leftover materials.
In India, where there is a growing demand for both energy and construction materials, researchers from the Bhabha Atomic Research Centre (BARC) have been examining an unusual idea: using leftover uranium mill tailings (UMT) as a component to make building blocks known as geopolymers.
Their main goal was to see if it was possible to turn uranium mill tailings into strong and safe geopolymer blocks that could potentially be used in construction. Uranium mill tailings are the metallic, muddy materials left behind after valuable uranium has been extracted from the ore. Globally, vast amounts of these tailings exist, posing a long-term challenge for safe storage and preventing environmental contamination. The scientists collected samples of these uranium mill tailings (UMT) from an Indian uranium processing facility.
Geopolymerization is a chemical process where materials rich in silica (SiO₂) and alumina (Al₂O₃), like the uranium mill tailings and fly ash used in this study, react with a strong alkaline solution. This reaction causes these elements to dissolve and then reform into a new, strong, three-dimensional network of aluminosilicate polymers called a geopolymer. For this study, the researchers mixed the UMT with fly ash (FA), a byproduct from coal-fired power plants, and bed material (BM), another industrial waste from iron ore processing, to form the geopolymer.
The researchers tested three different recipes or compositions of geopolymer blocks. Each recipe combined UMT, fly ash (FA), and bed material (BM) in varying proportions. They also used sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solutions to activate (or harden) these powders into solid blocks. Before mixing, the scientists examined the size and shape of the raw materials, such as the tailings and ash, to ensure they were fine enough for good bonding.
After mixing everything into a thick paste, they poured it into moulds to create blocks. These moulds were left overnight at room temperature and then placed in a warm oven (60°C) for another night to help the geopolymerization reaction happen. Once the geopolymer blocks were formed, they were left to harden for about four weeks. Afterwards, they were tested using standardised methods from the Bureau of Indian Standards. All three geopolymer mixes exceeded the minimum required strength of 3.5 MPa. The compressive strength results were roughly 10.7 MPa for one mix, 12.0 MPa for another, and 15.6 MPa for the third, making them strong enough for particular real-world construction needs. Adding more fly ash tended to produce higher-strength blocks thanks to its chemistry and a significant share of alumina (Al₂O₃).
Because uranium mill tailings can contain radioactive elements, the scientists paid special attention to radon gas and radioactive isotopes. They measured how much radon gas was released over time (radon exhalation rates) and the levels of critical radioactive elements like uranium (²³⁸U) and radium (²²⁶Ra). Using a special gamma-ray spectrometer, they could see whether these geopolymer blocks had higher-than-acceptable radiation levels.
The release of radon gas from the geopolymer blocks was relatively low, in some cases similar to or even lower than rates reported for common building materials like concrete and bricks. The levels of different radioactive elements in the blocks met worldwide safety thresholds set by the International Atomic Energy Agency (IAEA). The scientists also calculated the radium equivalent activity (Raeq), which helps measure whether the material is safe for specific kinds of construction projects. From this calculation, the team concluded that the blocks are suitable for non-residential buildings, roads, and foundations, depending on the Raeq value.
Using computer models, the researchers imagined a typical room with walls made of these blocks. Their predictions suggested that radon levels would stay well within national and international limits.
Uranium tailings naturally contain radioactive elements, including radium (²²⁶Ra). Radium can decay to form radon gas (²²²Rn), which can then escape from the material. However, in these geopolymer blocks, the researchers believe the internal network formed through geopolymerization can help trap and hold some of these radioactive elements. When scientists measured radon release, they discovered that proper mixing and curing conditions kept the emission rates low. This combination of chemical bonding and the material’s density significantly lowers the movement of gases.
While the uranium-waste geopolymer technology is promising, its long-term environmental impact needs further investigation. A comprehensive study of the long-term effects of using such blocks for construction is needed. According to the researchers, the more significant challenge is the lack of specific regulations or standards in India for the radioactivity levels in general construction materials. This makes it challenging to definitively say whether these geopolymer blocks would be approved for widespread use. Developing such standards is crucial.
Worldwide, millions of tons of uranium mill tailings accumulate. Reducing huge piles of radioactive waste into geopolymers and other construction materials that can lock away harmful radiation could help avoid long-term environmental and health issues. However, future efforts must address standardisation, public confidence, and long-term durability to ensure widespread acceptance of such technologies.
This research article was written with the help of generative AI and edited by an editor at Research Matters.