A Fourier transform infrared spectroscopic investigation of the early hydration of Portland cement and the influence of sodium lignosulfonate

The early stages of hydration of Portland cement Type I in the presence and absence of sodium lignosulfonate (a superplasticizer) have been investigated by Fourier transform infrared spectroscopic technique. Sequential experiments were performed at various times ranging from 2 min to 24 h. A number of discrete OH bands in the water stretching region and the dynamics of changes in the sulfate region during early hydration of Portland cement have been characterized. The OH stretching region on the spectra provided information on the nature of chemically bound water and the effects of sodium lignosulfonate on these entities. The results of these studies indicate that the superplasticizer prolongs the induction period and inhibits the hydration reaction. The reaction of the CSH with carbon dioxide is demonstrated by enhanced intensities of the carbonate bands. The hydration reaction seems to be controlled by dispersions of various charges present in hyperalkaline solution in cement paste. The induction period seems to be associated with an amorphous ettringite-like phase. The mechanism of inhibition is discussed in the light of a Charge-Controlled-Reaction-Model. According to this model, the Ca2+ ions from initial hydration go into the solution to form a bilayer of counterions with the negatively charged surfaces. Consequent to this intrinsic process, a trilayer consisting of superplasticizer anions is immediately formed, which inhibits further hydration.