Abstract
The total surface energy (or surface enthalpy) of a calcium silicate hydrate, tobermorite, having the composition of Ca3Si2O7•2H2O, was determined at 23.5 °C. The tobermorite was obtained from the room-temperature hydration of tricalcium silicate, Ca3SiO5, or β-dicalcium silicate, β-Ca2SiO4, two of the most important constituents of portland cements. The hydration reactions were carried out in three different ways, and 14 preparations were obtained. For each preparation the heats of solution in a mixture of nitric acid and hydrofluoric acid were measured at 23.5 °C, and the surface areas were determined by the B.E.T. method, using water vapor at 25 °C as the adsorbate. The cross-sectional area of the adsorbed water molecule was taken to be 11.4 Å2. The surface energy of tobermorite at 23.5 °C was found to be 386 ± 20 ergs/cm2. It is close to the geometric mean of the surface energies of calcium hydroxide and hydrous amorphous silica, previously reported.Nitrogen adsorption did not measure the true surface area of most tobermorite preparations. This was indicated by negative surface energy values in a number of instances.
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