Quick and short of it…. Make sure the sand meets ASTM C-144. But you might ask yourself, what the heck does that mean? ASTM C-144 is a standard specification for aggregate for masonry mortar, brought to you by the American Society for Testing Materials (ASTM). To start off many people will use the term aggregate rather than sand. This is because aggregate is a broader term which can refer to recycled materials that can be use in lieu of sand if so desired. The specification has a lot of good guidelines to follow when choosing sand, such as cleanliness, shape, composition and grain size distribution.
Good, clean sand is important. Depending on where you, get sand you may run into some complications. If you’re purchasing your sand from a distributor, pre-bagged sands are very clean as they are typically washed and dried. However, bulk sand may have some impurities, it is unlikely but should be noted for other potential issues discussed in a moment. If you are getting sand from a nearby creek or stream for historical purposes, be careful. You should check with your local laws to see if it is even legal as there could be environmental implications. That being said, be sure that the sand is free of silt and organic matter.The shape or sharpness of sand will help make a mix denser and create overall a more durable mortar. Angular aggregates fill in void spaces better then rounded sands. A good visual for this is to try and imagine a case of bottled water with round bottles, now imagine the same bottles but square. The square bottles will be tight against each other, while the round bottles only touch at four contact points, leaving voids.
Composition of aggregate will determine long term effects of the mortar. There are aggregates out there that can cause delayed expansion and failure of mortar. One to look out for is crushed dolomite limestone. There are others but for most restoration work that is the most common. Your local supplier should know if they carry this type and will most likely not recommend it for any masonry mortar.
Grain size distribution is the most important factor, when choosing an aggregate. The ideal sand should have a wide range to sizes in its composition. Workability and durability are greatly affected with good distribution. If you can gain access to a sieve analysis, look for a bell curve when the numbers are plotted on a graph. This shows that there are a few large pieces, an increasing amount of medium sizes and a small amount of fines or powder. Excess fines in a sand result in poor workability and is often corrected by an excess amount of water, resulting in poor durability.
Good sand that is clean and dry will have a theoretical void ratio of 33%. In other words, given a certain volume of sand there will be an ideal space of air of between the grain of 33% and 66% sand. fortunately there is a simple test anyone can do to see what the void ratio of sand is.
First get two clear containers one being at least twice the size of the other, fill the small container with water and pour it into the large container and make a mark. Then fill the small container again and pour it into the large container, being sure to have left the first measure of water in the container. Mark the level of the second measure and space 4 lines evenly between the two creating a total of six marks on the large container. Empty the large container and fill the small container with water. Pour the small container of water into the large container, reaching the bottom of the six lines. Now fill the dry small container with dry sand pour it into the large container. After everything settles you will be able to see how much water was displaced. Starting with the top line representing 0%, each line moving down the large container will represent 20%. The resulting percentage will be the void ratio and represent the minimum amount of lime by volume you should use to make a good mortar.
Poor sand can have ratios above 50% and will increase the amount of lime required thus resulting in added expense for you project. So when in doubt conduct a small test on the available sands choose the one that has a ratio closest to 33% to save on the amount of lime required and sleep easier knowing that you have decent sand.