Understanding the role of C-S-H seeds and sulfate in the lightweight cementitious composites containing fly ash cenospheres

Matrix strength and interfacial bonding between aggregate and matrix are critical factors influencing the performance of lightweight cementitious composites (LCC). This study proposes an environmentally friendly and efficient strategy for developing high-performance fly ash cenospheres (FAC)-containing LCC by combining sodium sulfate (SS) and calcium-silicate-hydrate (C-S-H) seeds. Moreover, the roles of SS and C-S-H seeds on achieving superior strength of LCC were elaborated by characterizing the hydration, microstructure, pore structure, and interfacial zone. Results show that the combined use of C-S-H seeds and SS accelerated silicate and aluminate hydration, shortened the setting time of LCC, and refined the pore size. FAC particles provided a nucleation site for the precipitation of ettringite due to the increased aluminum concentration and formation of gypsum on interface. Furthermore, C-S-H seeds and SS significantly improved the interfacial zone between FAC and matrix due to the enhanced pozzolanic reactivity of FAC. C-S-H seeds and SS primarily functioned through a physical nucleation and chemical activation to improve the matrix strength and interfacial zone, respectively, resulting in a high-performance LCC. Consequently, remarkable increases of 131.4 ± 19.8 % and 29.0 ± 5.1 % in early and late compressive strength of LCC were achieved. These findings present a novel approach for developing high-strength lightweight concrete with enhanced early strength under ambient curing.