The Hydration of Portland cement

The hydration of Portland cement (a mixture of calcium silicates and aluminates) results in solidification. Christensen et al used neutron diffraction to investigate the reactions between water and the constituent oxides of cement. They showed that the hardening process for Ca3SiO5 consists of three stages:

During the first stage there is a rapid dissolution of Ca3SiO5. The induction period may be due to the formation of a protective layer surrounding the grains until it to is dissolved. Precipitation of Ca(OH)2 the only crystalline product, marks the beginning of the final reaction stage, where all dissolved silicon goes into producing an amorphous gel, with a neutron diffraction pattern quite different from the crystalline phases.

Since both hydrogen and oxygen scatter neutrons strongly, the detection of water is relatively easy. Chemical reactions which proceed through hydrated (or hydroxylated) precursors are then ideal for study by neutron diffraction. For example, the figure below illustrates the thermal dehydration of Fe2F5.2(H2O). At low temperature (160° Centigrade) the high background level is due to incoherent scattering from water: as the temperature in increased, the decrease in water content can be measured directly from the decrease in the background, which is accompanied by the transformation of the peaks due to crystalline Fe2F5.2(H2O) into the different set of peaks due to the anhydrous product.

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Last updated by Andrew Crowe on 01/02/1996