Effects of inactive mineral fillers and fine aggregate on the strength and crack resistance of cementitious immobilization matrices

When low-level radioactive waste is generated, it must be locked away in a solid form that will not crumble, crack, or leak harmful substances for thousands of years. One of the most common ways to do this is to mix the waste with cement, creating a solid block that traps the dangerous material inside. This process, known as immobilization, relies on the cement hardening into a durable matrix. However, cement has a natural tendency to shrink as it dries and reacts with water. If this shrinkage is not held back, the material develops cracks. For a radioactive waste container, even a tiny, invisible crack is a failure, because it creates a path for water to enter and carry radioactive particles out into the environment. Engineers have long known that adding sand to cement helps prevent this shrinking and cracking, but for many types of waste, the mixture is kept as a smooth paste or fine mortar without coarse stones. This study asked a critical question: if we cannot use sand, can we simply add other fine powders to make the cement strong enough to survive without cracking?

Researchers at FNC Technology Co., Ltd. and the Korea Atomic Energy Research Institute set out to test this idea by creating ten different cement mixtures designed to hold radioactive waste. They focused on two specific types of fine powders that do not react chemically with the cement but are often added to change its properties: ultrafine quartz, which is essentially very fine sand, and magnetite, a heavy iron mineral often used for radiation shielding. They also included standard sand in some mixtures to see how it compared to these fine powders. The goal was to see which mixtures could meet strict safety standards: they needed to be strong enough to withstand pressure, resist water leaking through them, and, most importantly, remain completely free of visible cracks after drying and enduring extreme temperature changes.

The team created mixtures using only cement, mixtures with just the fine powders, and mixtures that combined the powders with standard sand. They added a small amount of cobalt powder to each block to act as a safe stand-in for radioactive cobalt-60, allowing them to test how well the cement held the material inside. After letting the blocks cure for 28 days, they subjected them to a series of harsh tests. The blocks were dried out, then cycled through temperatures ranging from freezing cold to hot heat thirty times, and finally soaked in water for 90 days. Throughout this process, the researchers watched closely for any signs of cracking and measured the strength of the blocks at every stage.

The results revealed a surprising disconnect between strength and safety. The mixture containing only ultrafine quartz turned out to be the strongest of all, reaching a compressive strength of 46.3 megapascals after 28 days. This was significantly higher than the strength of the plain cement block, even though the quartz mixture had a higher water content and appeared to have more tiny holes inside it. The researchers found that the quartz particles acted as a physical filler, helping the cement structure form more tightly without creating new chemical bonds. However, despite this impressive strength, the quartz-only block failed a crucial test: it developed visible cracks within a day or two of drying. The magnetite-only block also cracked, though not as quickly as the quartz one. In contrast, the plain cement block remained crack-free during the initial drying but eventually developed cracks after the temperature cycles and water soaking.

The story changed completely when standard sand was added to the mixtures. Every single block that contained sand remained perfectly smooth and free of visible cracks, even after drying, the extreme temperature cycles, and the long soak in water. These sand-containing blocks maintained high strength levels, well above the minimum requirement of 3.44 megapascals, and showed no signs of structural failure. The researchers observed that the sand particles acted like a skeleton, physically holding the cement paste in place and preventing it from shrinking enough to tear itself apart. This physical restraint was far more effective at preventing cracks than the chemical or physical properties of the fine powders alone.

Perhaps the most revealing finding concerned the safety of the radioactive material inside. The researchers measured how much cobalt leaked out of the blocks over five days. All the mixtures, including the ones that had cracked, held the cobalt extremely well, with leakage rates far below the safety limit. This means that even the cracked blocks were technically good at trapping the radioactive material during this specific test. However, the study concluded that relying on this high leakage resistance or high strength alone is dangerous. A block can be strong and leak-free in a lab test but still fail in the real world if it has visible cracks, because those cracks represent a loss of structural integrity that could lead to failure under different conditions.

The study ultimately demonstrated that while fine mineral powders like quartz and magnetite can make cement very strong, they cannot replace the role of sand in preventing cracks. The fine powders improved the internal structure of the cement, but they could not stop the material from shrinking and breaking as it dried and cooled. The addition of standard sand provided the necessary physical restraint to keep the blocks intact. For the safe disposal of radioactive waste, the researchers found that a mixture must not only be strong and leak-resistant but also physically unbroken. The most reliable solution identified was to include fine aggregate, ensuring that the waste form remains a single, solid piece capable of withstanding the stresses of the environment without developing the pathways that could eventually lead to a release.