Investigations on cement pastes by small-angle X-ray scattering and BET: the relevance of fractal geometry

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Abstract
The microstructural properties of Portland cement (PC) were studied and compared by small-angle X-ray scattering (SAXS) and the Brunauer—Emmett—Teller (BET) method. SAXS measurements have shown that the microstructure of PC samples cannot be described in terms of a classical porous medium with a well-defined specific inner surface, but rather obeys the formalism of fractal geometry. The application and potential of the SAXS technique for randomly disordered and fractal structures are outlined. Measurements have been taken on powdered and size-fractionated cement samples with different water—cement ratios (w/c) and different amounts of added chloride salts, and on different types of cements. The analysis of the data obtained by SAXS showed that inner surface and volume structure are fractal; however, only the mass fractal dimension Dm (2·3-3·0) could be evaluated with sufficient precision and reproducibility. By comparing these Dm values and the specific surface areas obtained by BET (40-70 m2/g), a correlation between Dm and BET surface was found when only one parameter (w/c) was varied. The SAXS measurements showed that drying procedures (which are necessary for BET measurements) give rise to a different microstructure of the cement samples, as revealed by different Dm values for these samples.

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