Seawater-mixed recycled micropowder as a supplementary cementitious material: Evaluation of strength, hydration, microstructure, and chloride binding

This study utilized recycled micropowder (RMP) recovered from seawater-mixed concrete (termed as seawater-mixed RMP [SRMP]) as a supplementary cementitious material. Metakaolin (MK) was used as a modifier in SRMP-containing systems, and the performance of the SRMP–MK system was systematically evaluated. The results showed that seawater ions in SRMP accelerated hydration and mitigated strength loss caused by cement dilution. MK effectively enhanced the strength of SRMP-containing systems, with the optimal mixture (S30/M10) showing compressive strengths of 22.2 and 45.2 MPa at 3 and 28 days, respectively. Although the addition of 10% MK reduced fluidity to 146 mm, it lowered 28-d drying shrinkage to a minimum of 1411 με. MK shortened the final setting time and the duration of the phase-boundary-controlled reaction stage in the hydration kinetics of the SRMP–MK system, and the overall hydration mechanism remained nucleation and crystal growth–phase-boundary-controlled reaction–diffusion. Owing to its high-reactivity alumina phase and pozzolanic reaction, MK promoted the formation of Friedel’s salt and the C–(A)–S–H gel, leading to microstructure densification. The free-Cl − content in S30/M10 was 0.045%, which is below the Cl − limit specified in Chinese standards for reinforced concrete structural members. The SRMP–MK system shows considerable environmental benefits, with energy consumption and CO 2 emission values of 101.8 MJ/t·MPa −1 and 13.26 kg/t·MPa −1 , respectively. These findings provide new insights into the reuse of construction waste and support the feasibility of its practical application.