  The proneness of water to crystallize is a major obstacle to understanding
its putative exotic behavior in the supercooled state. It also represents a
strong practical limitation to cryopreservation of biological systems. Adding
some concentration of glycerol, which has a cryoprotective effect preventing to
some degree water crystallization, has been proposed as a possible way out,
provided the concentration is small enough for water to retain some of its bulk
character and/or for limiting the damage caused by glycerol on living
organisms. Contrary to previous expectations, we show that in the ``marginal''
glycerol molar concentration $\approx18\%$, at which vitrification is possible
with no crystallization on rapid cooling, water crystallizes upon isothermal
annealing even below the calorimetric glass transition of the solution. Through
a time-resolved polarized neutron scattering investigation, we extract key
parameters, size and shape of the ice crystallites, fraction of water that
crystallizes, crystallization time, which are important for cryoprotection, as
a function of the annealing temperature. We also characterize the nature of the
out-of-equilibrium liquid phases that are present at low temperature, providing
more arguments against the presence of an iso-compositional liquid-liquid
transition. Finally, we propose a rule-of-thumb to estimate the lower
temperature limit below which water crystallization does not occur in aqueous
solutions.
