A New Mechanism for Cement Hydration Inhibition: Solid-State Chemistry of Calcium Nitrilotris(methylene)triphosphonate

Abstract

The reaction of the cement retarder nitrilo-tris(methylene)phosphonic acid, N[CH2PO(OH)2]3 (H6ntmp) with calcium oxide, tricalcium silicate (C3S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF) has been studied individually, and in the case of C3A in the presence of gypsum, to gain an understanding of the effect on the individual minerals prior to studying a typical sample of Portland cement. The reaction of H6ntmp with calcium oxide results in the initial formation of soluble [Ca(Hnntmp)](4-n)-, which precipitates over time as [Ca(H4ntmp)(H2O)]∞, whose sheetlike structure has been confirmed by single-crystal X-ray diffraction. The study of the hydration of C3S in the presence of H6ntmp indicates that no C−S−H forms, and the surface changes from silicon-rich to calcium-rich associated with the formation of various calcium phosphonates. The hydration of C3A is severely inhibited in the presence of H6ntmp, with the phosphonic acid reacting primarily with calcium as opposed to aluminum to form a Ca−P-rich layer at the surface of C3A. The H6ntmp enhances calcium solubility, promoting the dissolution of calcium from C3A and promoting, in the presence of gypsum, the formation of ettringite. In the presence of H6ntmp the surface of hydrated Portland cement grains is rich in calcium and phosphorus and deficient in silicon and aluminum, consistent with the formation of a calcium phosphonate coating spectroscopically related to [Ca(H4ntmp)(H2O)]∞. We have proposed a new mechanism by which phosphonic acids inhibit cement hydration. Dissolution, of calcium by extraction with the phosphonic acid, exposes the aluminum-rich surface to enhance hydration, followed by precipitation of a layered calcium phosphonate that binds to the surface of the cement grains, inhibiting further hydration by acting as a diffusion barrier to water as well as a nucleation inhibitor. Samples were characterized by 31P, 27Al, and 29Si MAS NMR spectroscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.