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[CH 2 PO-(OH) 2 ] 3 (H 6 ntmp) 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 H 6 ntmp with calcium oxide results in the initial formation of soluble [Ca(H n ntmp)] (4-n)-, which precipitates over time as [Ca(H 4 ntmp)(H 2 O)] ∞ , whose sheetlike structure has been confirmed by single-crystal X-ray diffraction. The study of the hydration of C3S in the presence of H 6 ntmp indicates that no C-S-H forms, and the surface changes from silicon-rich to calciumrich associated with the formation of various calcium phosphonates. The hydration of C3A is severely inhibited in the presence of H 6 ntmp, 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 H 6 ntmp 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 H 6ntmp 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(H 4 ntmp)(H 2 O)] ∞ . 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 31 P, 27 Al, and 29 Si MAS NMR spectroscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

Tailored setting times with high compressive strengths in bassanite calcium sulfoaluminate eco-cements

“…For example, Bishop and Barron [20] proposed the mechanism of "dissolution-precipitation" for phosphonic acids as an special case of the first mechanism (surface adsorption of retarders directly onto anhydrous surface), and consists of two 3 steps: i) dissolution (calcium is extracted from the surface of the cement grains and then the aluminum-rich surface is exposed), and ii) precipitation (the soluble calcium phosphonate oligomerizes on the particle surface or in the solution and, in the former, it will inhibit/retard further hydration).…”

Analysis of cubic and orthorhombic C3A hydration in presence of gypsum and lime

“…Crystalline hydrates, such as C 3 AH 6 , C 4 AH 19 , and C 2 AH 8 , form quickly, generating large amounts of heat [3 and references therein]. This happens even in the presence of retarders, as observed by means of 27 Al MAS NMR [4]. To slow down the hydration reaction, calcium sulfate is added to the mix, which causes ettringite to form.…”

Influence of Mineral Admixtures on the Mechanical Properties of Fresh and Hardened Concrete

“…Therefore, the enhancement in the compressive strength of concrete when mixed with UFG is due to the smaller particles of the materials that act as filler in the concrete, and also due to the reaction between the sulphate and the C-S-H of the cement. This reaction seems to increase the concrete strength as more C-S-H is produced, which, in turn, accelerates the concrete strength [11].…”