Reaction Mechanism of Blended Cements: A 29Si NMR Study


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Title: Reaction Mechanism of Blended Cements: A 29Si NMR Study

Author(s): Hans S. Pietersen

Publication: Special Publication

Volume: 132


Appears on pages(s): 795-812

Keywords: blast furnace slag; blended cements; calcium silicates; compressive strength; fly ash; hydration; kaolinite; metakaolinite; silica fume; Materials Research

Date: 5/1/1992

29Si NMR has been employed as a tool to characterize the reaction mechanism of hydration in several blended cements up to 6 to 9.5 months. The cements investigated were blends with silica fume, fly ash, activated kaolinite, and blast furnace slag. Spectra deconvolution indicated that, in the silica fume as well as in the activated kaolinite blend, the reaction of the anhydrous calcium-silicates is initially accelerated with respect to the ordinary portland cement. In the fly ash blends, this effect is smaller. Both in the silica fume and fly ash blends, an increase in the amount of silica middle groups (Qý-type) at - 84 ppm, relative to the amount of silica end groups (Q1-type) at - 79 ppm, is notable, which indicates an increased tendency to form longer CSH chains. The size distribution and glass content of the fly ashes used seem to influence the hydration reaction, which is reflected by somewhat higher Qý/Qý ratios and an increased initial hydration. In the blends with activated kaolinite, it was not possible to deconvolute the Q1 and Qý chemical shifts at all ages, due to changes in the shift maxima Q1 and/or Qý. This may be due to the formation of amorphous noncrystalline alumina-containing reaction products. The chemical shift of the blast furnace slag appeared too broad for a successful deconvolution. In general, both the total (Q1 + Qý) as well as the Qý/Q1 ratio correlate with compressive strength data, Qý species contributing markedly. Paper contains a general overview of the application of NMR spectroscopy in cement and concrete research.