I want to build a two brick high “wall” along the edge of the walkway in my lawn. Can I use hydrallic cement for the job? I ask, because I have some in my basement and I am a lazy man. Thanks in advance.
You probably could, but hydraulic cement usually sets up very quickly, so you’d have to work fast.
Mortar is so cheap that I’d just as soon use the “correct” material, personally.
Mortar is also softer and more plastic, due to a higher lime content. In most cases it it is less likely to damage bricks when the masonry shifts, though I guess that it won’t matter much in a two brick high wall.
Thanks for the replies. Part of the reason I want to use the hydrallic cement is that I hate for it to go to waste and I don’t want to buy a bag of mortar and then have it sit in my basement for the next 20 years going to waste. Again, thanks.
If you don’t purchase/use mortar spec’d for job and materials at hand, you will experience cracked joints, cracked brick and all sorts of problems related to the incompatibility of the hydraulic cement and the type of brick you are using (mostly from different abilities to hold/resist moisture and different am’ts of flexing/expansion driven by temp and moisture).
The result is a crumbly, short-lived structure.
*Why do we never have time or money to do it right the first time, but we always have twice as much money and twice as much time to do it right the second time? * 
That’s why my motto is: measure twice, cut once, swear three times, go back to store to buy more lumber, re-measure twice, try and do the cut right again…
you can give away the unused material to friends.
you can make more projects; plant pedestals, bench, stepping stones.
Similar to my motto.
Measure it with a micrometer. Mark it with a piece of chalk. Cut it with an axe.