An electron microprobe study of a mature cement paste

A portland cement paste 23 years old, and essentially fully hydrated, was studied by electron probe microanalysis. X-ray images indicated that the shapes of the original, largely polymineralic cement grains, and those of the regions occupied by the individual phases within them, are substantially preserved in the hydrated material. This was shown especially clearly by the Mg and Fe images, probably because these elements do not readily migrate in the alkaline medium. Estimation of individual phase compositions is rendered uncertain because of possible admixture of phases on or below a micrometer scale, but the atomic ratios relative to Ca are, approximately: Mg 0.03, Al 0.08, Si 0.60, S 0.03 and Fe 0.015 (Ca:Si 1.67) for the C-S-H, and Al 0.40, Si 0.33 and Fe 0.27 for the hydrogarnet.