Basics of Binders in Mortar


Above: Matching a red mortar

Now that we understand the basics of mortar and the basics of how sand works within the mortar, we are ready to cover the basics of lime

A quick review of the basics of mortar and the basics of sand:

  • Mortar is the stuff between the masonry units.  It is a mixture of a binder (usually a type of lime or cement) and sand, both of which are critical components.  Its job is to spread the load evenly across the masonry unit below.
  • The three critical qualities of mortar are compressive strength, flexibility, and moisture permeability.
  • The sand’s job is to provide the compressive strength and resist compaction.  Sands are highly variable and should be examined for superior performance.
  • The binder’s job is to hold the sand together, to contribute flexibility for building movement, and to contribute moisture permeability so that water can escape rapidly.
  • The two critical qualities of sand that contribute compressive strength are particle size and particle shape.  These can be determined with simple tests.
  • The ratio of binder to sand is critical because too much or too little of the binder will weaken the mortar.
  • The void space in a sand can be determined with a simple test, and will determine the amount of binder to use for that sand.

Categories of Binders

As was mentioned in the Basics of Mortar above, man has been building masonry structures for a long, long time.  Over time, several categories of binders emerged as superior in their performance for mortar, plaster, and stucco.  These categories are natural lime, natural hydraulic lime, natural cement, and Portland cement.

Let’s get clear on the terms:

Hydraulic means that it hardens under water.  Of the four types of binders, all of them are hydraulic except for the natural limes. 

Portland Cement is the newest kind of binder and the most widely understood because it is scientifically formulated.  Instead of burning a natural mineral deposit, amounts of different minerals are brought in according to a recipe. 

Its advantages are clear: predictable, fast-setting and hard.  It is quite poor on flexibility or vapor permeability. It is made mostly from limestone, but other chemicals and minerals are added in a complex manufacturing process to create a standard composition. The final product is only 60-67% calcium.  It is the most energy-intensive to manufacture and has the shortest life as a finished product.

Natural Cements are made by burning limestone that has a relatively high clay content (around 20%).  The result is hydraulic cement that is very durable, fast-setting, and more vapor permeable than Portland.

Natural Hydraulic Lime is also made by burning impure limestone that contains some clay.  It has a higher calcium content than cement, but still sets up relatively quickly.  The hardness and flexibility depends on the amount and type of clay in the natural deposit.

Natural Lime is made by burning high-calcium limestone.  Limestone that is 95-98% calcium makes the best lime for mortar.

Natural lime hardens with a very different chemical process than hydraulic binders. The hydraulic binders react with water to harden. Natural lime reacts with carbon dioxide to harden.  If kept away from the air, it will remain soft indefinitely. It is the most flexible and the most vapor permeable. Although it is not as quick to use as hydraulic binders, its working properties are excellent for all kinds of mortar, plaster, and stucco.

The graphic below is helpful to see the differences between the types of binders. The center circle represents natural lime. It is shown in the graphic as “pure lime” because natural lime is made from pure calcium carbonate limestone. When limestone is burned, the carbon is forced out, leaving calcium oxide. Once water is reintroduced in the slaking process, it becomes calcium hydroxide, or lime putty.


By adding sand to the putty and exposing it to the air, the carbon comes back in from the air as the water leaves.  This chemical process, called carbonation, actually turns it back into stone.  This time, instead of being pure limestone, it is calcareous sandstone because of the addition of sand!  Depending on the quality of the lime putty, the finished product is highly breathable, flexible and strong.

Of the four different binders, Natural Lime is the least energy-intensive to manufacture, and absorbs carbon-dioxide from the air, making it the “greenest” of the binders.  However, each of the binders has its place in construction and restoration, and becoming familiar with all four as valuable building materials will help tradesman and designers create greater longevity for the future.