The reaction product of boric acid and the polysaccharide guaran (the major component of guar gum) has been investigated by 11B NMR spectroscopy. By comparison with the 11B NMR of boric acid and phenylboronic acid complexes of 1,2-diols (HOCMe2CMe2OH, cis-C6H10(OH)2, trans-C6H10(OH)2, o-C6H4(OH)2), 1,3-diols (neol-H2), monosaccharides (L-fucose, mannose and galactose) and disaccharides (cellobiose and sucrose) it is found that the guaran polymer is cross-linked via a borate complex of two 1,2-diols both forming chelate 5-membered ring cycles ([B5(2)]), this contrasts with previous proposals. Based upon steric constraints we propose that preferential cross-linking the guaran polymer occurs via the 3,4-diols of the galactose side chain. The DeltaH and DeltaS for complexation of boric acid to cis- and trans-1,2-cyclohexanediol have been determined, from the temperature dependence of the appropriate equilibrium constants, and used in conjunction with ab initio calculations on model compounds, to understand prior conflicting proposals for guaran-boric acid interactions. 11B NMR derived pH dependent equilibrium constants and ab initio calculations have been used to understand the reasons for the inefficiency of boric acid to cross-link guaran (almost 2 borate ions per 3 monosaccharide repeat units are required for a viscous gel suitable as a fracturing fluid): the most reactive sites on the component saccharides (mannose and galactose) are precluded from reaction by the nature of the guar structure; the comparable acidity (pKa) of the remaining guaran alcohol substituents and the water solvent, results in a competition between cross-linking and borate formation; a significant fraction of the boric acid is ineffective in cross-linking guar due to the modest equilibrium (Keq). In contrast to prior work, we present evidence for the reaction of alcohols with boric acid, rather than the borate anion. Based upon the results obtained for phenylboronic acid, alternative cross-linking agents are proposed.

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.