Basics of Mortar Sand
A quick review of the basics of mortar:
– Mortar is the stuff between the masonry units. It is a mixture of a binder (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 mortar sand’s job is to provide the compression strength and resist compaction.
– The binder’s job is to hold the sand together, to contribute flexibility for building movement, and moisture permeability.
We cannot stress enough the importance of the sand in the making of mortar! It is generally thought by masons, contractors, and architects that all sands are basically the same. As long as it is washed, sharp and fine, it will perform well as mortar sand.
This may be true relative to the super-hard cement in almost universal use as the binder in mortars, but anyone who understands mortar must understand sand. Just because it is in a pile at the masonry supply yard does not mean it is suitable for making mortar and would make good mortar sand.
Let’s look at the qualities of the sand that contribute to its superior performance in a mortar. There are two critical qualities of the sand that affect how it contributes to strength, flexibility, and permeability. These qualities are particle size and particle shape.
The compressive strength of sand depends on the sizes and shapes of the individual particles. Sands that are stronger have a wide variety of particle sizes. Sands that have just a few particle sizes are not strong because they do not pack together well, and these sands depend on a very hard binder to keep them together. Very hard binders tend to be brittle, so mortar flexibility is no longer possible.
Sand is the same. With the right mix of particles, called particle size distribution, the sand packs together very tightly and can carry more weight. The larger grains of sand touch other smaller grains in many places, creating a very strong and tightly packed bed that will form perfectly around the masonry units (bricks or stones).
To determine a correct sand, sieve with different size sieves to see how much of each particle size are in that particular sand. These sizes can be graphed which will tell you the particle size distribution of a sand. This is a critical piece of information to know in order to know how a mortar will perform in terms of strength and flexibility. Mortar Sand should have more of the mid-range size particles, but still have fine and coarse particles.
Another factor in the sand’s strength is the shape of the particles. Sands where the grains are all round (like many sandblast sands) will not pack together well. Rounded sands will be slippery similar to walking on marbles, they will never pack well. Sands that are needle-like with sharp points, which is the case with crushed rock (sold as sand), will also not perform well. Sands should be natural, not rock crusher waste, and should have a good mix sharp, angular and sub-angular particles.
Besides the sands’ particle shape and size distribution, there is another determining critical factor in the compressive strength and flexibility. That factor is the ratio of the sand to the binder. There should be just enough of the binder to thinly coat each particle of sand.
If there is too much binder, the finished mortar will compress under load (unless the binder is very hard). Remember, very hard binders are not flexible. The balance of strength and flexibility come when the mortar has enough of the binder to fill the void spaces.
Each sand can be tested to find out what the total void space is. It is a simple test, (please do try this at home!) and gives the mason a very clear answer for how much lime or cement should be used with any given sand.