Water exchange between superabsorbent polymers and the cementitious matrix unraveled by nuclear magnetic resonance

Superabsorbent polymers are cross-linked hydrogel networks possessing a large fluid uptake capacity. When added to a cementitious mix they absorb part of the mixing water. During cement hydration, they provide this absorbed water as internal curing water to the cementitious matrix, which allows to maintain the internal relative humidity. In the ideal case, this permits to compensate for self-desiccation and to mitigate autogenous shrinkage. The kinetics of water release are hereby of utmost importance, and are studied by Nuclear Magnetic Resonance (NMR). Free water and SAP-entrained water could be clearly distinguished in the T2 relaxation spectra. The measured free and entrained water in function of the degree of hydration corresponded well to the Powers and Brownyard model, showing, however, a more S-shaped curve. Through NMR measurements a distinction could be made between SAPs that gradually released their stored water, maintaining effectively the internal relative humidity, and SAPs that released the entrained water prematurely