Impact of ettringite seeding on hydration, strength and shrinkage of Na2SO4 activated slag

Na2SO4 activated slag (NSAS) is accepted as one of the most promising low carbon cementitious materials, while its slow strength development seemed the most prominent defect. In this study, nanoscale ettringite (NE) was prepared, and used as nucleation seed to stimulate the hydration of NSAS, thereby promoting the strength development. Compressive strength, hydration process, pore structure, and shrinkage were studied, and the underlying mechanism was revealed. Results indicated that 5% NE enhanced the compressive strength of NSAS paste by 74% at 7 d and by 98% at 28 d, and the reason was that NE significantly facilitated the hydration of slag and the formation of hydrates, including ettringite and C-(A)-S-H gel, especially in the early stage. Furthermore, the presence of NE hastened the dissolution of aluminum phases, and these aluminum phases were inclined to precipitate as ettringite rather than C-(A)-S-H gel. Meanwhile, NE caused the size miniaturization of ettringite crystal formed in hydration. The reason might be that NE provided more nucleation sites, which led to the dispersed growth of ettringite. The small ettringite was harmless to pore structure, and compensated for autogenous shrinkage. In the presence of 1% NE, the porosity and autogenous shrinkage at 28 d decreased from 9.62% and 5209 μm/m to 8.24% and 3992 μm/m, respectively. In addition, NE-NSAS showed great potential in low-carbon cementitious materials.