Traditional Dry-Stack Stone Walls:
Stone masonry originated with dry-stacked stonework where the walls are carefully layed up without mortar. Gravity serves as the glue that holds everything together. Free-standing dry-stack stone walls are usually made larger at the base and then taper in slowly as the height increases. For absolutely no expense but the labour, farmers built miles upon miles of stone fences this way in Ireland and in the northeastern states.
Many old Irish houses were built in a similar way. Where “mortar” was used, it was often merely mud or limestone plasters with little strength. The mortar functioned as caulking to stop the flow of air, rather than as cement to bond the stones together. Short, dry-stacked stone walls are especially ideal for landscaping projects. Taller walls require more skill and time.
Traditional Mortared Stone Walls:
Mortared stone walls evolved out of dry-stack stonework with the emergence of cement mortars. The first types of cement were made of burnt gypsum or lime mixed with water to make a paste with slight bonding capability. Stone walls still had to be built as carefully as they were without mortar. The cement paste just filled the gaps between the stones and cured to form a soft, rock-like substance.
The basic formula for modern cement originated in England in 1824. It is called “Portland cement” because the colour is similar to the rocks on the English island of Portland. It is still called Portland cement everywhere in the world it is manufactured. This cement is made with calcium from limestone or chalk, plus alumina and silica from clay and shale. The ingredients are ground, mixed in the right proportions and burnt in a kiln at a temperature of about 2500 degrees F (1350 C) to drive out water bound up in the raw materials. In the kiln, it fuses into chunks called clinker. It is cooled and powdered, and gypsum is added to control how fast it sets up. Portland cement is mixed with sand and water, and often lime to make a smooth mortar for stone and brickwork. Adding the lime makes the mortar softer and more flexible.
With the aid of Portland cement, it is possible to build a taller stone wall that does not taper inward like a dry-stacked wall. The cement has some ability to “glue” a stone wall together with less care, but proper stone working techniques are still important. Building a free-standing stone wall is a true art and requires a lot of time and skill to do it well.
Veneered Stone Walls: Most stonework today consists of a non-structural veneer of stone against a structural wall of concrete or cinderblock. Concrete consists of Portland cement mixed with sand, gravel and water. The larger particles of gravel interlock like little fingers to make the concrete resistant to cracking. A steel reinforcing bar can be added to serve as much longer “fingers” to make a wall that is very resistant to cracking. Concrete is a fast and relatively inexpensive way to put up a structural wall, so few people take the time for labour-intensive traditional mortared stone walls anymore.
Instead, the structural wall is put up first, and thin, flat stones are essentially glued onto the face of the wall with cement mortar. Metal tabs in the structural wall are mortared in between the stones to tie everything together; otherwise, the stonework would just peel right off the wall. The structural wall serves as a form on one side of the wall to make it really easy to lay up the stonework, provided the rocks have good flat edges to work with.
Slip form Stone Walls: A slip formed wall might be described as a cross between the traditional mortared stone wall and a veneered stone wall. This is the method of stone masonry we have used the most. Short forms, up to two feet tall, are placed on both sides of the wall to serve as a guide for the stonework. You place stones inside the forms with the good faces against the formwork and pour concrete in behind the rocks. Rebar is added for strength, to make a wall that is approximately half concrete and rebar and half stonework. The wall can be faced with stone on one side or both sides. With slip forms, it is easy even for the novice to build free-standing stone walls.
Reference: http://www.dirtcheapbuilder.com/Home_Building/Stone_Masonry.htm