The effectiveness of superabsorbent polymers for internal curing as studied by NMR

Due to their high swelling capacity and retention, SuperAbsorbent Polymers (SAPs) can be applied as an admixture in cementitious materials for mitigating autogenous shrinkage. By releasing the stored water for internal curing, the self-desiccation is counteracted. However, not all SAPs completely mitigate autogenous shrinkage and it is very important to use a SAP with the ideal properties. If the water is released too soon (i.e. before setting), the microstructure will be completely different and the effective water-to-cement ratio changes. If the water is released too late, the purpose of internal curing vanishes. The kinetics and water release by the SAPs during cement hydration are studied nondestructively by means of Nuclear Magnetic Resonance (NMR) as a function of time and degree of hydration. The results are linked to the Powers and Brownyard model. Two different SAPs were studied; SAP A and SAP B. SAP A showed a partial and gradual release of entrained water for internal curing after final set. This mixture showed complete mitigation of autogenous shrinkage and fits the model of Powers and Brownyard. SAP B seemed to release the stored water prematurely till initial setting, to re-absorb part of the free water before final setting and again release it more quickly before the more steady decrease in free water for cement hydration. As part of the entrained water is quickly released during hydration, there was less mitigation of autogenous shrinkage. NMR proved to be an effective tool to study the water release by SAPs towards the cementitious matrix