Water balance and pore structure development in cementitious materials in internal curing with modified superabsorbent polymer studied by NMR

In this contribution, the water balance between modified superabsorbent polymer particles of various grain sizes and a hydrating cement matrix is studied both for ordinary and white Portland cements by means of NMR relaxometry. Different approaches for the evaluation of the experimental data are compared: inverse Laplace transforms and several multicomponent fitting models. Both evaluations provide well-comparable results with respect to the overall water quantities absorbed and re-released by the superabsorber. The experimental data indicate fast water uptake within less than 5 min after watering the superabsorber/cement mixture. The re-release of the water begins along with the onset of the hydration reaction and is finished after 1 day for superabsorber powders with a dry grain size below 125 μm and takes about 2 days for larger dry grain size. The water uptake of the superabsorber inside the cement matrix is quantified to about 2250% of its dry weight.