Hydration kinetics and phase stability of dicalcium silicate synthesized by the Pechini process

Reactive dicalcium silicate (Ca2SiO4) has been synthesized by the Pechini process, and hydration kinetics studied. With increasing calcination temperature, the amorphous product first crystallizes to alpha'(L)-phase and subsequently to the beta- and gamma-phases, The specific surface area, ranging from 40 to 1 m(2)/g, strongly depends on the calcination temperature of 700 degrees-1200 degrees C for 1 h, Samples with a high surface area have a high water demand; a water/cement ratio >2.0 is required to produce formable pastes in some instances. Hydration kinetics are determined by XRD, Si-29 magic-angle spinning nuclear magnetic resonance (MAS NMR), and differential scanning calorimetry/thermogravimetry (DSG/TG). The hydration rate depends only on the surface area, not on the polymorph. Complete hydration occurs in as early as 7 d, Very little calcium hydroxide (Ca(OH)(2)) is formed in the most reactive specimens (calcined at 700 degrees and 800 degrees C), which indicates the Ca/Si ratio in C-S-H gels is similar to 2.0, but more Ca(OH)(2) forms from samples calcined at higher temperature. The silicate structure of the hydrated Ca2SiO4 pastes is investigated using Si-29 MAS NMR spectroscopy and trimethylsilylation analysis.