On the hydration of Portland cement

Abstract
When Portland cement is contacted with water, calcium ion is rapidly leached from the solid to form calcium hydroxide solution but only traces of silica are found in the aqueous phase. It is proposed that the hydrated, calcium-depleted surface of grains consists of low molecular mass silicic acids and that these interact with dissolved hydroxylated calcium species (principally Ca(OH)$_{2}$) to produce a semi-permeable membrane of 'calcium silicate hydrate' at the hydrated grain surface. Osmotic pressure within this membrane causes its rupture and hence the growth of excrescences from the grain as the contents are extruded into the surrounding calcium hydroxide solution. The interstitial solid material is best regarded as a coagulum resulting from the combination of low molecular mass silicate anions with dissolved calcium hydroxide. It is proposed that the low tensile strength of Portland cement paste results from microstructural features consequent upon this mechanism of hydration and setting.

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