13C, 27Al and 29Si NMR Investigation of the Hydration Kinetics of Portland-Limestone Cement Pastes Containing CH3-COO--R+ (R=H or Na) Additives

The hydration kinetics of Portland-limestone cement pastes with organic additives in the form of acetic acid and sodium acetate were studied by using solid-state C-13, Al-27 and Si-29 NMR spectroscopy. The evolution of the relative content of various phases was monitored over the period of one month: amorphous and crystalline calcite (in C-13 spectra), ettringite, aluminum in C-S-H gel, calcium aluminates and calcium hydroaluminates (in Al-27 spectra), as well as alite, belite and silicon in C-S-H gel (in Si-29 spectra). The retarding effect of the additives on cement hydration at early age was demonstrated. We show that the kinetics of phase assemblage formation is influenced by the acetate ion adsorption on the surface of the anhydrous cement components and hydrated phases. The kinetics of formation of ettringite in the cement paste, depending on the addition of acetic and or sodium acetate, is discussed in the context of potential thaumasite sulfate attack.