Concrete is an excellent building material. In consists of a binder (clay, lime or Portland cement, aggregate (sand and gravel) and reinforcement (rebar, wire or fibers). It is a relatively low-tech material and can be made very durable and strong, even by the beginner. Concrete has several methods of implementation, depending on what you are using it for, and in many countries, entire buildings, from the floors to the roofs, are made out of concrete, and will last for centuries.

Concrete is considered a “plastic” material in that initially it is a liquid that sets into a hard shape. To say that it dries is not entirely accurate, because actually, a chemical reaction takes place when concrete sets. The binder reacts with water and air to produce a rock-like substance. This reaction is not reversible if you use Portland cement as a binder. Other forms of concrete, like Roman concrete, use a mixture of silica, lime and clay to create a binder that reacts at room temperature, instead of the 3000 or so degrees needed for Portland's initial reaction. Roman concrete has the added advantage of turning into limestone over the years, increasing its durability. Some folks have even suggested that the pyramids in Egypt may have been constructed in this way. Those stones may have been a primitive concrete initially that slowly turned to stone over a thousand years.

Ferro-cement
Ferro-cement is concrete that is heavily reinforced with both wire mesh and rebar. This type of construction is extremely flexible, which seems strange for something as hard and dense as concrete. Because of the reinforcement, ferro-cement does not need to be as thick as concrete, and designers consider it more as a tensile skin that can be made in just about any shape. It is relatively inexpensive, very durable, very strong and requires a bit of skill to get it right. The process is to create an armature out of rebar, then cover it with lathing and wire mesh, and stucco thin layers of a sand/cement mix. Additives are used to give the concrete different added benefits, like quick or slow curing times, added strength or water proofing. As far as concrete is concerned, ferro-cement is one of the most versatile methods available.

Percrete and EPS crete
These are very similar in that they use a lightweight aggregate instead of sand or gravel. The aggregate is either perlite, an expanded volcanic clay, or Styrofoam. These materials give the resulting concrete added insulation value and a greatly reduced weight. They are not as strong as regular concrete with sand, but they can be made strong enough for any home application. They are most often used as a filling between two concrete walls to increase the insulation value of the wall without a decrease in strength. An advantage of using Styrofoam is that is a good use of an otherwise non-recyclable material.

Concrete is very useful and relatively cheap. It is most often used as foundations and bond beams, and sometimes as walls. There is no better material for a roof, as it will literally outlive your grandchildren.