Here's one recipe I use to
make rocks. good for starting out I think.
But mainly, how is mortar mix different from Portland cement?
Portland cement is the main ingredient in all other cement type products like concrete, mortar, stucco, tile thin set, grout. Think of it as four in baking. Cake and most cookies are mostly flour. By adding a few other things in different amounts you get different bake goods.
Mortar is a general term that can mean anything from clay based to very high tech materials. When a package says something like "mortar cement" they're telling you it isn't mud mortar, it isn't lime mortar, instead they're saying this mortar is made with Portland cement. Cement mortars generally also have lime as an ingredient to make them more flexible.
As others have said, the difference between concrete and mortar is the size of the rock added. Mortar uses sand and smaller rock down to dust size. Concrete uses some sand, but also pea size and larger rock. Rock is stronger than cement so using larger rock (of the right kind) yields a stronger result. There are many recipes for concrete, just like there are for mortar. We use the terms mortar and concrete as a way to differentiate between the two basic differences in recipes and uses. It's like saying cake vs cookie. We all understand the difference between a cake and a cookie. A cookie (most) can be put into a baggie and placed in a lunch bag, but a piece of cake won't hold up.
Most premixed mortars will say Type M, Type S, Type N, Type O to describe the ingredient recipe. Each type is used for a specific application. For example, Type M has the highest percentage of Portland cement and is the strongest. Everyone new to working with cement products always want the strongest, right? Why would there be any other kinds? There's a down side, the more Portland the more shrinkage, the more cracking. Especially a problem when making rocks if you're forming the entire rock from mortar. Less of a problem if the mortar is a final coat over a cured base.
For making rocks you generally want a softer product. I like Type S for most cases. If the rocks are to be walked on I'll use a Type M and deal with the cracking by making a substructure first. For the easiest to work and best detail I use Type S and add a product called "mortar clay", but it doesn't wear great. When walked on it will wear fast, although that's a nice look too.
I have a bag of P.cement that I've been mixing with mortar mix and sand (not sure where I got that recipe...). Do I even need to be mixing the 2 or can I use one in place of the other??
Adding more Portland to premixed mortar is generally a bad idea unless you know what your doing. Result is harder, but more brittle, and more prone to cracking when curing.
And where can I get the fiber (fiberglass)? Do places like HOME DEPOT carry it? How does it come: bag, box, by the pound, etc?
Fiber is generally a really bad idea when making rocks as you will soon find out. The little bits of fiber will poke out and be visible. You can use them in building a base structure. For example, start with some mesh or chicken wire and spread a little mortar with fiber over the wire and let that cure. Now you can apply a layer of mortar without fiber over that and form the final rock surface. But fiber use is normally far, far from being needed.
The same is true for using concrete to make rocks. Concrete is stronger and wears better than mortar, but the large rock chunks making forming and smoothing just about impossible to make good looking rocks.
People new to working with cement products are obsessed with strength...but that's inexperience. Glass is very strong, but makes a poor baseball bat. Rubber is very soft and flexible but also make a poor bat, though you'd never break a rubber bat. Wood was the best material for bats for a long time, just enough strength, but also flexible enough to not break too often. Now they have even more high tech bats. Stronger can mean softer and more flexible.
Resist the typical novice approach thinking you can improve on thousands of years of material use and process with no experience. Instead start with standard practices and learn. Then, after you know something, you can change recipes and methods if you like.
Never ask a person working at a store anything about their products. Generally they don't even know where products are located in the store, much less how they're used. They will just BS you and no info is better than BS info.
