Concrete, in its most basic form, is a combination of paste and aggregates (sand and rock). The paste, made of cement and water, covers the surface of the fine (sand) and coarse (rocks) aggregates and binds them together to form concrete, a rock-like mass. Cement plays two main roles in this process: it gives the concrete its strength, and it allows it to set into stone when dried. Concrete can be used in many different forms. It can be poured into a mold and allowed to set into a solid shape, like a block or pillar. Or it can be mixed with sand and rocks added incrementally, until the right mixture is reached. This kind of concrete is flexible and can be shaped into various forms such as pipes or windows. Finally, it can also be molded after it has set into a hard stone-like substance called mortar. Mortar is used between layers of concrete for additional support or to keep it stable while it cures. It's made of lime, which comes from limestone, and water. The lime reacts with the moisture in the air to create an acid that makes the mortar more resistant to heat and bacteria. Limestone is used because it contains a large amount of calcium carbonate, which is what makes up much of the Earth's crust. Concrete is by far the most common type of aggregate found in stone walls.