  The nature of Calcium--Silicate--Hydrate (C--S--H), the binding phase of
cement, remains a controversial question. In particular, contrary to the former
crystalline model, it was recently proposed that its nanoscale structure was
actually amorphous. To elucidate this issue, we analyzed the structure of a
realistic simulation of C--S--H, and compared the latter to crystalline
tobermorite, a natural analogue to cement, and to an artificial ideal glass.
Results clearly support that C--S--H is amorphous. However, its structure shows
an intermediate degree of order, retaining some characteristics of the crystal
while acquiring an overall glass-like disorder. Thanks to a detailed
quantification of order and disorder, we show that its amorphous state mainly
arises from its hydration.
