Analysis of some adsorption isotherms on calcium silicate hydrates

Tricalcium silicate was hydrated in suspension at a water/solid ratio (w/s) of 9 and as a paste at a w/s of 0.7. Water contents after outgassing at room temperature were 23–25% for the suspension hydrates and 19–20% for the pastes. Outgassing was standardised so that subsequent increases in BET nitrogen surface area were small even when the pumping time was doubled. Nitrogen adsorption isotherms were similar for both types of product except for the greater volume of mesopores in the suspension hydrates filled at relative pressures greater than ca 0.9_p0(r_p > 100 ǎ). Electron microscopy suggested that the additional uptake of nitrogen involved capillary condensation between bundles of fibrils. Adsorption at low relative pressures would have occurred principally between the fibrils which were of the order of 20 ǎ thick. Pore sizes could be seen to extend continuously from the mesopore to micropore range and the fact that all the isotherms contained a hysteresis loop closing with an abrupt desorption at ca0.44p0 was regarded as a property of liquid nitrogen rather than as an indication of the pore size distribution. A more quantitative indication of the pore size range present was obtained using the α_s method with standard data for Fransil (silica). Plots for pastes were similar to those for suspension hydrates indicating a similar texture accessible to nitrogen in the pastes at the high w/s used. In order to characterise further the surface of the hydrates argon and carbon dioxide adsorptions were determined and the data compared using the Frenkel–Halsey–Hill equation.