More bonding stucco
to cast concrete.

One method of bonding mortar to concrete is a bonding coat made of acrylic and portland cement painted on with a stiff brush or broom, followed by a  finish coat.

Here is my method:

Reggie's method of bonding stucco to concrete.

Mix cement mortar with clean sand, not too coarse,
with a 50-50 mixture of Flex-con and water.

A good mix for this a mix that is a little rich (less sand, more cement),
1 bag portland
1/4 bag lime
15 -18 shovels of sand.

Trowel mortar on 1/4-1/2" thick on clean concrete.
If the concrete  is straight this coat can be
evened out with a trowel, and roughed up by rubbing with
a rubber float when it starts to set. It also can be roughed
up with a brush, but care should be used not to use a brush
that is too wet. A wet brush tends to leave the surface powdery
when it dries. 
 The most important thing is to leave the surface rough with
open pores to provide a good bond for the finish coat.

This is similar to the method that is widely used for plastering
swimming pools, but I like my way better.
          
Here, acrylic bonding admixture is used in the scratch coat over
these flagstones. Does it work ? I guarantee it.