375033 Influences of Dilute Organic Additives on Hydration at Low-Surface-Area Silicate Particles
Dilute quantities of additive or contaminant species influence hydration and crystallization processes, which correspondingly affect macroscopic material properties. A technologically important application is the use of very low absolute concentrations (<0.1 wt%) of organic additives, for example, saccharides or phosphonates, that adsorb at inorganic (e.g., silicate, aluminate) particle surfaces and thereby inhibit hydration reactions in aluminosilicate cement-water mixtures. The competitive adsorption of additive molecules and water at silicate particle surfaces prevents the formation of cross-linked silicate hydration products, which are central to the development of mechanical strength. Consequently, the presence of additive molecules maintains the fluidity of cement-water mixtures, which is desirable for example, in oilwell cementing applications.
To establish the effects of organic additives on hydration processes at silicate surfaces, it is essential to identify and understand the compositions, distributions, and interactions of water, additive molecules, and silicate surface species. However, detection of such surface species has been exceedingly challenging by conventional characterization methods due to their low concentrations, lack of long-range structural order, heterogeneous molecular environments associated with surfaces, and low surface areas (<1 m2/g) of the silicate particles. By comparison, detailed molecular-level information can be obtained by using solid-state nuclear magnetic resonance (NMR) spectroscopy, which is sensitive to local atomic environments of surface species. Specifically, dynamic nuclear polarization (DNP)-enhanced NMR techniques provide significantly improved (~100) signal sensitivity, which enables detection of dilute species, for example, adsorbed additive molecules and surface silicate moieties. The DNP-NMR results establish that the adsorption of saccharides occur through hydrogen bonding at hydroxylated (Q0) and hydrated (Q1, Q2) silicate surface sites, with a propensity to adsorb preferentially at the hydrated sites. By comparison, phosphonate molecules adsorb electrostatically as Ca2+-phosphonate complexes at hydroxylated and hydrated silicate sites to inhibit silicate hydration. The analyses contribute new insights regarding the hydration processes and site-specific interactions of different molecular surface species in cement-water mixtures, under industrially relevant conditions and compositions.
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