Water sorption isotherms and hysteresis of cement paste at moderately high temperature, up to 80 °C

The constitutive models of concrete often consider water desorption isotherms to be near-equilibrium and significantly affected by moderately high temperature, 40-80 degrees C, typically through microstructural changes. However literature data suggest that adsorption, not desorption, is near-equilibrium and moderate temperatures do not cause microstructural changes. This work supports the latter theory, through dynamic vapor sorption experiments on cement paste at 20-80 degrees C. Samples were pre-conditioned at 60% relative humidity and 20 degrees C, and isotherms were measured for several humidity ranges and testing rates. The results, corroborated by classical DFT simulations, indicate that adsorption is near-equilibrium and mostly unaffected by temperature, whereas desorption is out-of-equilibrium due to the ink-bottle effect at high humidity, and interlayer water at low humidity. Starting from the second cycle, desorption at higher temperatures features a shift of the cavitation pressure and overall a smaller hysteresis. A conceptual model of pore-specific temperature-dependent hysteresis is proposed to qualitatively explain the results.

Automated summary

The constitutive models of concrete often assume that water desorption is near-equilibrium and affected by moderate temperature through microstructural changes. However, literature data and experiments on cement paste suggest that adsorption, not desorption, is near-equilibrium and temperature has limited effect. Desorption is out-of-equilibrium due to ink-bottle effect and interlayer water. A conceptual model of temperature-dependent hysteresis is proposed to explain the results.