mike wrote:

Postcrete is an expensive way to go if you're doing a long run. Much
cheaper to mix concrete and add a dash of rapid hardener.


That's a valid point, however concrete is not as good at filling small
gaps. The aggregate tends to bridge them and prevent the concrete
falling into them.

Since postcrete is essentially a gap filler, the absence of aggregate
is actually a good thing. However, for almost all other applications,
the inclusion of aggregate makes for a much better job.