Short answer no, at least not with best results.
Morter is a mixture of sand, lime, and cement, and is primarily used for laying stone, block and brick as well as
being used for stucco/plastering.
Whereas concrete is sand, gravel, and cement.
Certainly the mortar mix would harden and for surfaces not
walked on would last for a considerable legnth of time.
Unfortunatly, he wont know how well it worked for a considerable length of time. Could be a couple years till
he notices it wasent the right choice. If its going to be
exposed to the elements of weather or abuse, use concrete mix not mortar mix. In fact it would be best to buy the highest strength mix available.
Well guys I really need to go with the real thing because this footer will be outside and eventually support an old reconditioned wrought iron section of fence weighing about 250 pounds. I will have several steel supports in-bedded in the concrete and the fence itself will be attached to these supports. The whole shebang will be abutted to my patio and once in place will have have vines growing on it to block the setting sun. I need to make sure the concrete base is very substantial and there for the long haul. It's been a fun project resurrecting that old rusty fence and making it into a sun screen. I might take the bag of mortar I have and spread its contents in the concrete mix as I go along and just a bit at a time. Thanks to all.
Concrete is a 3 part mix. 3-Aggregate (stone), 2-Sand, 1-Portland. It can be measured by any volume you wish.... shovel full, bucket full, etc..... Do it the right way. Using mortar will be weak and you are begging for cracks.
I wouldn't necessarily make it into mud pies. If this project is that critical then going with standard concrete may be your best bet but that's not to say you couldn't add aggregate to the mortar and use it for smaller projects. Concrete is portland cement, lime and sand. Concrete is portland cement, sand and aggregate. Sometime lime is added to cement depending upon temp so adding aggregate to mortar will in fact give you a close proximity to concrete. Yes, the purists will argue that they use "different sand" and stuff like that but in the end, it's the same ingredients for the binder in each. If you want a higher compression strength just add some more portland to the mortar as mortar typically has a higher sand to cement ratio.