Atomistic and mesoscale simulation of sodium and potassium adsorption in cement paste.

Abstract

An atomistic and mesoscopic assessment of the effect of alkali uptake in cement paste is performed. Semi-grand canonical Monte Carlo simulations indicate that Na and K not only adsorb at the pore surface of calcium silicate hydrates (C-S-H) but also adsorb in the C-S-H hydrated interlayer up to concentrations of the order of 0.05 and 0.1 mol/kg, respectively. Sorption of alkali is favored as the Ca/Si ratio of C-S-H is reduced. Long timescale simulations using the Activation Relaxation Technique indicate that characteristic diffusion times of Na and K in the C-S-H interlayer are of the order of a few hours. At the level of individual grains, Na and K adsorption leads to a reduction of roughly 5% of the elastic moduli and to volume expansion of about 0.25%. Simulations using the so-called primitive model indicate that adsorption of alkali ions at the pore surface can reduce the binding between C-S-H grains by up to 6%. Using a mesoscopic model of cement paste, the combination of individual grain swelling and changes in inter-granular cohesion was estimated to lead to overall expansive pressures of up to 4 MPa-and typically of less than 1 MPa-for typical alkali concentrations observed at the proximity of gel veins caused by the alkali-silica reaction.