Mortar is the material placed between bricks (or stones) to provide a cushion and evenly spread the load of the wall and the building. This is important because if we just stacked the bricks, small unevenness would “point-load”–or concentrate–the weight from above and cause the bricks to crack and shift. Mortar spreads the weight evenly over the bricks below. It makes individual units into a cohesive, unified wall that is evenly carrying the building’s weight down to the ground.
Mortar is just sand that is mixed with a “binder” to hold it together. The binder is just a sticky material that holds the sand together for a long time. The sand is what is bearing the weight. The binder just holds the sand together. Examples of binders are lime and cement.
So the sand is the most important structural component in mortar, and those sand particles have to be sized and shaped properly to fit together and be strong.
Limestone makes up about 8% of Earth’s crust. It is mined all over the world and used to make building materials, pharmaceuticals, food, steel, and much, much more. Throughout written history it has been used to make mortar and plaster.
When limestone is burned in a kiln (large structure for burning at high temperatures), we get lime. After burning, it is called quicklime until water is added to it, and then it’s called “slaked” or “hydrated” lime. The term “hydrated” is used to describe mason’s lime, agricultural lime, and any lime that has water added to it after burning.
But not all lime is the same. Limestone deposits can be virtually pure calcium, which makes very special mortars and plasters, and they can also have other minerals mixed in that give them very different properties when they are burned.
Besides variables in the limestone deposit, there are variables in how it is processed. Different temperatures, different burn times, different additives, and many more variables in the recipe can create a variety of properties in the lime.
Hydraulic lime is made from limestone that has a clay content, and the clay turns into cement when it is burned, making it get hard under water.
Natural cement has an even higher clay content in it’s parent deposit, although not as high as that used for making lime. It is mostly clay. It is a cement that is more breathable than portland.
Portland cement is a carefully engineered product that is made from combining limestone and other minerals in precise amounts, rather than being made from a single mined deposit. However, it is burned at much higher temperatures making it much harder than the limes for mortar. (See “Why Lime Mortar?” for a discussion of hardness and other qualities of long lasting mortar.)