NaOH-activated slow-release energy material agents for rock preconditioning: Enhanced expansive pressure and a mechanistic framework

With the increasing global demand for critical minerals and low-impact subsurface energy systems, efficient and sustainable rock preconditioning methods are urgently needed. Slow-release energy material agents (SREMAs) are promising next-generation materials for enhancing rock permeability, with potential applications in mineral recovery, geothermal stimulation, and underground hydrogen storage. However, its broader adoption has been limited by suboptimal expansive pressure and a lack of mechanistic understanding. This study addresses both challenges by incorporating sodium hydroxide (NaOH) as an inorganic accelerator to enhance the performance and reveal the underlying expansion mechanism. Seven SREMAs formulations with varying NaOH contents were evaluated using the outer pipe method, and three representative samples were characterized using inductively coupled plasma mass spectrometry, scanning electron microscopy, and X-ray diffraction. The results show that NaOH accelerates hydration reactions and increases 24-h expansive pressure by 15.03% to 52.62%, with a peak performance at 0.15wt% NaOH. NaOH addition promoted the formation of calcium hydroxide and ettringite and optimized the microporous structure of calcium silicate hydrate, the latter playing a critical role in transforming internal expansion into effective outward pressure. A novel mechanistic framework linking mineralogical transformations to enhanced macroscopic performance is proposed. This study provides new insights into sustainable rock preconditioning and offers a foundation for optimizing SREMAs for applications in which low-carbon, high-efficiency subsurface engineering is essential.