Synchrotron X-ray microtomography, electron probe microanalysis, and NMR of toluene waste in cement

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Synchrotron X-ray microtomography shows vesicular structures for toluene/cement mixtures, prepared with 1.22-3.58 wt % toluene. Three-dimensional imaging of the cured samples shows spherical vesicles, with diameters ranging from 20 to 250/μm; a search with EPMA for vesicles in the range of 1-20/μm proved negative. However, the total vesicle volume, as computed from the microtomography images, accounts for less than 10% of initial toluene. Since the cements were cured in sealed bottles, the larger portion of toluene must be dispersed within the cement matrix. Evidence for toluene in the cement matrix comes from 29Si MAS NMR spectroscopy, which shows a reduction in chain silicates with added toluene. Also 2H NMR of toluene-d8/cement samples shows high mobility for all toluene and thus no toluene/cement binding. A model that accounts for all observations follows: For loadings below about 3 wt %, most toluene is dispersed in the cement matrix, with a small fraction of the initial toluene phase separating from the cement paste and forming vesicular structures that are preserved in the cured cement and quantitatively imaged with tomography. Furthermore, at loadings above 3 wt %, the abundance of vesicles formed during toluene/cement paste mixing leads to macroscopic phase separation (most toluene floats to the surface of the cement paste).

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Environmental Science and Technology

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