Microstructure of chemically activated gamma-dicalcium silicate paste

This paper presents new insights on the microstructure of chemically activated gamma-dicalcium silicate (ɣ-C2S) pastes. Effects of Na2CO3, NaHCO3, Na2SiO3, and NaOH activators on the hydration reaction of ɣ-C2S paste was monitored using isothermal calorimeter, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and 29Si solid-state nuclear magnetic resonance (NMR) spectra. It was observed that this low hydraulically active (often addressed as 'non-hydraulic') ɣ-C2S phase exhibits higher reactivity in the presence of the alkaline activators. For all of the activation agents, the primary reaction products were identified to be C(N)SH, calcite, gyrolite, and gaylussite. The average calcium to silica (Ca/Si) and sodium to silica (Na/Si) atomic ratios of the C(N)SH gel were in the range of 0.8-1.0 and 0.3-0.4, respectively. NaOH was found to be the least effective among all of the activators used in this study for ɣ-C2S activation at the tested concentrations. The maximum degree of reaction of NaOH activated ɣ-C2S paste was around 50% after 56 days of curing. Whereas, more than 90% degree of reaction of the ɣ-C2S phase was achieved for all other activators after same hydration duration.