Monitoring of nucleation and growth of C-S-H phases by analytical ultracentrifugation and ICP-OES

Abstract

The pathway of nucleation and growth of C-S-H is important for understanding the hydration kinetics of Portland cement and subject of the present work. Therefore, analytical ultracentrifugation (AUC) is used to characterize the nucleation of C-S-H in aqueous environments with a detection capability ranging from ions to sub-micron particles. Complementary, the chemical composition of the aqueous phase is investigated by means of inductively coupled plasma-optical emission spectroscopy (ICP-OES). Combination of AUC and ICP-OES allows a clear distinction between formation of nanoscale particles and development of ion concentration. The experiments were designed with the aim at producing C-S-H from C3S hydration without portlandite and calcite. Therefore, the C3S hydration was conducted in diluted continuously stirred suspensions with special designed reaction control. Results from the aqueous phase composition prove that both pH and calcium ion concentrations are nearly constant, signaling that portlandite has not formed, and that silicon concentration varies within 24 hours of hydration with known assignments of C-S-H precipitation. Thus, the used experimental setup is suitable for the investigation of C-S-H crystallization from C3S hydration without formation of portlandite. In parallel to ICP-OES, the aliquots were examined by AUC regarding the sedimentation coefficient, which refers to the velocity of sedimentation; larger values correlate with increased species size or density. The experimental data prove that the aqueous phase contains in the majority ions, but also clusters with variable sizes/densities, and species that are larger than atom clusters. These are assumed to be nano-scaled particles. The sedimentation coefficient of the clusters varies broadly, which may reflect variation in composition / water content. In conclusion, the present work shows that the nucleation of CS-H originated from hydrating C3S occurs via pre-nucleation clusters.