Synergistic enhancement and mechanism of mullite whiskers and halloysite nanotubes (HNTs) on the mechanical properties of oil well cement

Oil-well cement stone is inherently brittle and prone to cracking, posing serious threats to construction safety and long-term zonal isolation. To improve wellbore sealing integrity and service life, mullite whiskers were synthesized via a molten-salt method. A novel synergistic toughening system combining mullite whiskers and halloysite nanotubes (HNTs) is proposed to reduce cement brittleness and cracking. Mechanical tests reveal an optimal blend of 0.7% mullite whiskers plus 1% HNTs (labeled M7-H1). Compared with the neat reference, this formulation increases 3-day and 7-day compressive strength by 49.39% ± 3.5% and 58.37% ± 3.5%, respectively, and raises tensile strength by 32.23% ± 3.1% and 29.30% ± 2.9%. XRD, TGA, and MIP analyses demonstrate that both mullite whiskers and HNTs act as nucleation sites for hydration products, accelerating secondary hydration and promoting calcium–silicate–hydrate (C-S-H) gel formation. A “bridging–nucleation–filling” tri-modal reinforcement mechanism is thereby established. The concurrent refinement of the pore structure reduces total porosity from 33.38% ± 0.42% to 24.34% ± 0.25% and cuts the fraction of detrimental pores (>50nm) from 59.9% to 36.2%. Owing to the facile, low-cost synthesis of the mullite whiskers, the proposed approach offers a readily deployable, high-performance solution for achieving durable cement-sheath sealing and life extension in deep and shale oil/gas wells.