Microstructure of chemically activated gamma-dicalcium silicate paste

This paper presents new insights on the microstructure of chemically activated gamma-dicalcium silicate (gamma-C2S) pastes. Effects of Na2CO3, NaHCO3, Na2SiO3, and NaOH activators on the hydration reaction of gamma-C2S paste was monitored using isothermal calorimeter, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Si-29 solid-state nuclear magnetic resonance (NMR) spectra. It was observed that this low hydraulically active (often addressed as 'non-hydraulic') gamma-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)-S-H, calcite, gyrolite, and gaylussite. The average calcium to silica (Ca/Si) and sodium to silica (Na/Si) atomic ratios of the C-(N)-S-H 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 gamma-C2S activation at the tested concentrations. The maximum degree of reaction of NaOH activated gamma-C2S paste was around 50% after 56 days of curing. Whereas, more than 90% degree of reaction of the gamma-C2S phase was achieved for all other activators after same hydration duration. (C) 2018 Elsevier Ltd. All rights reserved.