Effect of Nanobubbles on the Microstructural and Mechanical Performance of Strain-Hardening Cementitious Composites

Carbon dioxide (CO 2 )-consuming strain-hardening cementitious composites (CC-SHCCs) are sustainable and highly ductile materials characterized by outstanding tensile properties and carbon consumption capacity. This study proposes the use of nanobubble water (NBW) for CC-SHCCs based on the durable physical properties and element transport capabilities of nanobubbles. The analysis of porosity characteristics of NBW and CO 2 -capturing NBW (NBW + C) shows that nanobubbles reduce micro-sized porosity, while significantly enhancing nanosized porosity. The use of NBW led to an improvement in compressive strength, and NBW + C highlighted the benefits of CO 2 capture by achieving a maximum of 128.5 MPa. In the direct tensile test, the specimens using NBW and NBW + C as the mixing water exhibited improved load distribution and extended strain-hardening regions. The specimen utilized NBW + C both in the mixing and curing stages and achieves a strain capacity of 7.79% and an energy absorption capacity ( g -value) of 1023 kJ·m −3 , which represents exceptional tensile characteristics. Chemical analysis showed that the introduction of nanobubbles and increased CO 2 concentration promoted the transfer and accumulation of calcium and hydroxide ions, accelerating the formation of calcium silicate hydrate (C-S-H) gels and calcium carbonate (CC). In particular, an exceptionally high net CO 2 consumption capacity ( C C O 2 ) of 2.333 was achieved when NBW + C was used as mixing and curing water.