Multi-source solid wastes synergy guiding design of high-density low-carbon binders for nuclear protection engineering

The rapid growth of nuclear energy has significantly increased the production of nuclear waste, posing challenges for its safe disposal. The most common approach is to use ordinary Portland cement (OPC) to solidify waste containing radioactive nuclides, preventing leakage. However, OPC has low density and insufficient heavy nuclide content, serving mainly to encapsulate waste, without effectively shielding the radiation continuously released during decay. This study develops high-density low-carbon supersulfate cement (HDLSC) by utilizing steel slag powder (SSP) with high iron content and alkalinity as a functional activator, enhancing its gamma-ray shielding performance. A comprehensive investigation is conducted on the hydration kinetics, mechanical properties, microstructure, radiation shielding performance and environmental benefits of the designed HDLSC. The high-temperature resistance of HDLSC is also characterized to evaluate its structural stability under simulated service temperatures for nuclear waste disposal engineering. SSP exhibits significantly better compatibility with the supersulfated cement (SSC) system than the OPC system, because the SSC system continuously consumes portlandite generated from SSP hydration. This superior compatibility contributes to the excellent mechanical performance of HDLSC: its 28-day compressive strength reaches 35.2 MPa, which is 62.9% higher than that of single SSP-blended cement (21.6 MPa). Furthermore, the synergistic activation of SSP and phosphogypsum accelerates slag dissolution, promoting the abundant formation of iron-rich C-A-S-H gels and ettringite in the matrix. On one hand, the formation of amorphous and crystalline hydration products effectively refines the pore structure, reducing the critical pore diameter and cumulative pore volume to 263.5 μm and 0.229 mL/g, respectively. On the other hand, the densely compacted matrix increases the bulk density of HDLSC to 2.46 g/cm³ , which is 9.8% higher than the 2.24 g/cm³ bulk density of OPC. Due to the consumption of portlandite via pozzolanic reactions and the thermal stability of SSP, HDLSC exhibits an extremely low mass loss (15.9%) and a high compressive strength (11.6 MPa) after exposure to 800 °C. Additionally, the uniform distribution of heavy nuclides within the matrix significantly enhances the gamma-ray radiation shielding performance of HDLSC, with the linear attenuation coefficient improved by 9.6% compared to OPC, and the tenth value layer reduced to 16.88 cm. Moreover, HDLSC offers substantial environmental benefits by recycling large amounts of industrial solid waste, reducing carbon emissions by 85.4% compared to the OPC. Overall, the developed HDLSC demonstrates excellent mechanical and radiation shielding properties, providing a microstructurally optimized, eco-efficient solution for nuclear protection engineering.