Surface Area Measurement of C3A/CaSO4/H2O/Superplasticizers System

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Title: Surface Area Measurement of C3A/CaSO4/H2O/Superplasticizers System

Author(s): Florent Dalas, Sylvie Pourchet, André Nonat, David Rinaldi, Martin Mosquet, and Jean-Pierre Korb

Publication: Special Publication

Volume: 288

Issue:

Appears on pages(s): 1-15

Keywords: BET method; comb-type superplasticizers; NMR relaxation; specific surface area; tricalcium aluminate.

Date: 9/14/2012

Abstract:
Hydration leads to hydrated phases precipitation which are more or less covered by the adsorbed admixture depending on the added amount and the nature of the surface. To predict the optimum dosage and to understand the fluidity evolution at the early age, it is interesting to study the specific surface area development of cement during this period. It was shown that the workability evolution is mainly due to the tricalcium aluminate (C3A) hydration. Here we choose a simplified system: C3A/CaSO4/calcite/superplasticizer. We measured the specific surface area by two techniques: the BET method using nitrogen as reference and the nuclear magnetic spin-spin relaxation of 1H water. We highlighted a good correlation between the surface measured by NMR relaxometry and by the BET method. Continuous NMR measurement confirms the point-by-point BET measurements that are often subject to question. Thanks to both methods, we studied the influence of the dosage in superplasticizer and of the structure in terms of anionic group density on the specific surface area. In the development of the surface area, we demonstrated that the step mainly impacted by superplasticizers is the nucleation and not the growth. When the dosage of superplasticizer increases, the nucleation is promoted and the growth of ettringite is limited. As consequence, the specific surface area of ettringite increases with both the number of charges added and the backbone charge density. The surface area goes up in presence of superplasticizer as consequence of a dispersing effect (hydrates are not attached to anhydrous phases) and of a reduction of hydrates size.