Incorporating supplementary cementitious materials into ultra-high-performance seawater sea-sand concrete (UHPSSC): Hydration, microstructure, and mechanical performance

Ultra-high-performance seawater sea-sand concrete (UHPSSC) demonstrates exceptional mechanical and durability performance comparable to traditional UHPC. This study investigates the effects of supplementary cementitious materials (SCMs) on its hydration, microstructural development and mechanical behavior. Fly ash (FA), ground granulated blast- furnace slag (GGBS) and metakaolin (MK) were used to partially substitute cement clinkers in UHPSSC. A series of tests were conducted, including flowability test, mechanical strength test, calorimetry test, TGA, XRD, and SEM. Results indicated that FA and GGBS could substantially enhance the flowability of UHPSSC, which even surpassed that of reference UHPC. UHPSSC without SCMs exhibited higher compressive strength than UHPC from 3 to 56 days due to accelerated cement hydration facilitated by chlorides and microstructural refinement from Friedel's salt. Both FA and GGBS were detrimental for UHPSSC's strength, but MK improved the strength at dosages below 20 % due to its strong pozzolanic reaction and filler effect. Moreover, MK also reduced the exothermic peak of hydration and produced the most compact microstructure of UHPSSC. GGBS could advance the hydration peak and yield relatively dense microstructure for UHPSSC. Conversely, FA significantly retarded C3S hydration in UHPSSC due to its low reactivity, resulting in the least compact matrix.