If the slag initially produces mobile magnesium ions rather than magnesium hydroxide, then the previous theory would not work. Silica gel works in a similar fashion. These extremely small reactive silica particles also generate silica gel, but on a scale too small to cause cracking of the pcc matrix. This is also the reason why proponents of the proposition that available alkalis in fly ash, when added to a concrete mix, can compound alkali related problems, may be wrong. Most likely, the silica gel generated by the highly reactive, amorphous, exremely small silica spheres in fly ash can not only mop the alkalis from the fly ash, but some of the alkalis from the cement as well.
Economic considerations are another matter.
Apparently the test model strays too far from what actually occurs with PCCP in the field. This situation would rarely occur in field concrete. In field concrete, one or more sides are unfrozen as the freeze-front moves through the concrete. Water should be able to exit through the pore system, ahead of the freeze-front. It is also known that bulk ice will attract water through a capillary sized pore system.