Introduction
Super Absorbent Polymers (SAPs) can be applied as an admixture in cementitious materials. As the polymers are able to swell, they will absorb part of the mixing water and can then release that water back towards the cementitious matrix for internal curing. They can be  used to mitigate autogenous shrinkage in systems with a low water-to-binder ratio.  As, upon hydration of cement, the capillary water will be consumed followed by a reaction with the more strongly bound gel water, there will be self-desiccation leading to autogenous shrinkage. The SAPs can retain water, act as water-filled inclusions (macro pores) and release the water to reduce self-desiccation shrinkage during hardening. This internal curing will maintain the internal relative humidity. Due to the release of water by the SAPs, the SAPs decrease in size and afterwards a macro pore remains with the dry remnant of the SAP. When (external) water is available again, the SAP is able to swell to full extent.

    Results
Using NMR the porewater distribution can be measured non-destructively by  looking at the relaxation distribution, which can provide information on the kinetics of SAP entrained water release for internal curing. As an example in figure 1 the relaxation is given as function of time for a cement paste with SAP.



Fig 1. Signal intensity [a.u.] as a function of the relaxation time T2 [s] for a SAP cement paste system.
The time step in between curves is approximately 10 min up to an age of 2-3 days, followed by a measurement every two days up to a week,
 every week up to a month and every two weeks up to two months.


 In this SAP systems, additional peaks in comparison to just cement paste are allocated to SAP-entrained water in the 10−2 s to 100 s range. This water is stored in the SAP particles and can be released for internal curing. In that way, as the T2 is different, the water kinetics from the SAPs towards the cementitious matrix could be studied in detail as a function of time and as function of the degree of hydration. For this mixture (Ae), it is clear that water is still present in the SAP inclusions at later ages (after final setting); i.e. at higher values of the degree of hydration in the lower positioned curves. In figure 2 we have given the corresponding initial signal intensity.


Fig 2. Signal fraction [−] of the free water peak and the SAP peaks as a function of the logarithmic time
[days] showing the cement reaction


The largest peak from the free water has the largest portion of the total signal fraction. In time, this water is used and consumed for cement hydration causing the total signal intensity to drop in time. Initial setting occurred at approximately 8 h of age and final setting was at  approximately 11 h of age as determined with the Vicat needle test. The time of initial setting corresponds to the drop in NMR intensity signal. From this point onwards, the free water was used more visibly in NMR in all studied samples. After final setting, the SAP-entrained water was visibly consumed, i.e. for internal curing. The signal intensities for SAP A decreased more steadily and hence SAP A is ideal to mitigate autogenous shrinkage.
 

    Conclusions
Based on the findings of this research on internal curing by SAPs as studied by means of NMR following some general conclusions could be drawn:
The free water and the entrained water by the SAPs could clearly be distinguished in the T2 relaxation spectra. The SAP signals relate to the exchange of their entrained water molecules with water molecules at the interface with the cementitious matrix..
The amount of mixing water absorbed by the SAP obtained from comparing the flow tests and microscopic analysis is the same as the amount determined by means of NMR.


     

D. Snoeck, L. Pel and N. De Belie, The water kinetics of superabsorbent polymers during cement hydration and internal curing visualized and studied by NMR, Scientific reports 2017


D. Snoeck, L. Pel, N. De Belie, Superabsorbent polymers to mitigate plastic drying shrinkage in a cement paste as studied by NMR, Cement and Concrete Composites 93, 54-62 (2018).
 

D. Snoeck,  L.Pel, N. De Belie, Comparison of different techniques to study the nanostructure and the microstructure of cementitious materials with and without superabsorbent polymers, Construction & Building Materials 223, 244-253 (2019).

Didier Snoeck, Leo Pel, Nele De Belie, Kinetics of SAPs During Hardening, Drying and Healing in Cementitious Materials Studied by NMR,    January 2020,  DOI: 10.1007/978-3-030-33342-3_15,   In book: 3rd International Conference on the Application of Superabsorbent Polymers (SAP) and Other New Admixtures Towards Smart Concrete.

Yanliang Ji, Zhenping Sun, Chao Chen, Leo Pel, Ahmed Barakat, Setting characteristics, mechanical properties and microstructure of cement pastes containing accelerators mixed with Superabsorbent polymers (SAPs): an NMR study combined with additional methods,  Materials 12, 315 (2019)