Hydration Characteristics and Products of β-C₂S Doping with Na and Al in the Presence of NaAlO₂ and NaOH

Abstract

The hydration behaviors of Na and Al co-doped β-C₂S in NaAlO₂ and NaOH were compared with that in water by performing isothermal calorimetry. The microstructure development of C-S-H gel obtained from β-C₂S hydrated pastes was characterized with XRD, TGA/DTG, ²⁹Si, ²⁷Al and ²³Na NMR, and SEM/BSE-EDS. Results indicated that both alkalis accelerated the hydration process of β-C₂S and stimulated early and later ages mechanical strength of β-C₂S pastes and NaAlO₂ had the better effect. ²⁷Al and ²³Na NMR confirmed the incorporation of aluminum and uptake of sodium in C-(N)-(A)-S-H structure. The presence of sodium ions promoted the incorporation of tetrahedrally coordinated Al in C-(N)-(A)-S-H structure. Alkalis enhanced the precipitation C-(N)-(A)-S-H gel with higher percentage of Q¹ units and shorter silica chains in the structure. The morphology and chemical composition of C-(N)-(A)-S-H gel in the pastes depend on the hydration medium used for the hydration.