Characterizing pore structure of cement blend pastes using water vapor sorption analysis

Pore structure is an essential factor that influences the mechanical behavior and durability of cement-based porous materials with or without added binders. An empirical model for water vapor sorption isotherms was employed to evaluate the pore structure of hardened cement pastes incorporating granulated blast furnace slag and silica fume. The model is an extension of the Brunauer–Emmett–Teller multilayer adsorption theory. Assuming cylindrical-shaped pores and an adsorbed liquid-like layer between the pore surface and gas phase, pore size distributions of the blended cement pastes were estimated. Calculated pore size distribution curves were compared with those measured by mercury intrusion porosimetry. Added granulated blast furnace slag and silica fume had minor effects on the monolayer adsorption capacity, but reduced the energy of the first and subsequent adsorption layers. The adsorbed liquid-like layer generated sharper pore size distribution peaks that were shifted to the mesoporous region. The pore size distributions were comparable with those determined by the mercury extrusion branch, but differed from those obtained by the mercury intrusion branch. Hysteresis of the water vapor adsorption–desorption isotherms and mercury intrusion–extrusion curves was due to the entrapment of a non-wetting phase in the porous system, further promoted by residual mercury in the pores following mercury extrusion.