Enhancing the Performance of Supersulfated Cement: A Comparison of Nano C–S–H Seeds and Nanosilica

To advance the application of nanomaterials in low-carbon supersulfated cement (SSC), this study systematically compares the improvement efficiency and mechanisms of nano C–S–H seeds and nanosilica (NS) in modifying the hydration kinetics and microstructure of SSC. Two series of SSC, formulated with varying mass ratios of ground granulated blast-furnace slag (GGBFS) to phosphogypsum (PG), were evaluated using a multitechnique approach encompassing compressive strength tests, pH value monitoring, hydration heat release, inductively coupled plasma optical emission spectroscopy, X-ray diffraction, thermal gravimetry, nuclear magnetic resonance, SEM, and mercury intrusion porosimetry. Results indicate that the incorporation of C–S–H seeds accelerated the dissolution of both GGBFS and PG with lower OH^– consumption, thereby elevating the saturation index (SI) for C–S–H, C-A–S–H, and ettringite. In contrast, NS inhibited PG dissolution and decreased the SI for ettringite. The C–S–H seed exhibited superior acceleration of the nucleation and growth (NG), phase–boundary interaction (I), and solid-state diffusion (D) (NG-I-D) processes compared to NS at low PG content. However, NS adversely affected the I-D process in SSC containing 30% PG, primarily due to its pozzolanic reaction consuming OH^–. Moreover, the C–S–H seed promoted the formation of Q² (1Al) units in C–(A)–S–H, increased the Al/Si molar ratio, and enhanced the ettringite content, leading to denser packing between these hydrates, which resulted in a refined pore structure and denser microstructure. Consequently, the early age compressive strength of SSC with C–S–H seeds and NS was increased by 312.0% and 104.0%, respectively. This work provides a promising approach for improving the performance of SSC by expediting hydration kinetics and promoting the formation of hydrates.