Effects of autoclave holding duration and nano-SiO2/TiO2 replacement on hydration products and pore structure of pressed reactive powder concrete

This study investigated the effects of autoclave holding duration and nano-inorganic powder modification on the mechanical and microstructural development of pressure-formed reactive powder concrete (RPC). Nano-silica (NS) and nano-TiO 2 (NT) were incorporated at replacement levels of 1% and 3% by partially replacing silica fume, and specimens were autoclaved for 5, 8, and 12 h. Compressive strength, TGA/DTG, XRD/QXRD, MIP, and SEM were collectively analyzed to clarify the relationships among nanopowder type, hydrothermal phase evolution, pore structure, and strength. The Plain mixture continuously increased in strength to 243.3 MPa at 12 h, whereas the nano-modified mixtures generally showed their most pronounced relative strength enhancement at 8 h. At this duration, 1% NS achieved 226.7 MPa, 19.4% higher than the Plain mixture. NS more effectively promoted C–S–H-related and tobermorite-type products together with capillary-pore refinement, whereas NT influenced hydration predominantly through micro-filling and heterogeneous nucleation effects. The results demonstrate that nano-modification effectiveness depends on the combined effects of nanopowder type and autoclave holding duration rather than on a single hydration or crystalline phase. The principal contribution of this study is the integrated comparison of reactive NS and non-siliceous NT under identical pressure-forming and autoclave-curing conditions, linking hydrothermal phase development and pore refinement with strength evolution.