Frequently Asked Questions

What Sand Should I Use With Natural Hydraulic Lime For Repointing My Building? FAQ Friday

Sand for Natural Hydraulic Lime

How to choose sand for Natural Hydraulic Lime

Quick and short of it…. when choosing sand for Natural Hydraulic Lime, 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).

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.

The composition of aggregate will determine the 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 of sizes in its composition. Workability and durability are greatly affected by good distribution. If you can gain access to 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 number of fines or powder. Excess fines in sand result in poor workability and are 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.

Originally written by: Randy Ruth
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One thought on “What Sand Should I Use With Natural Hydraulic Lime For Repointing My Building? FAQ Friday

  1. Michael Lynch says:

    I had a hard time understanding the purpose of the placement of the water level lines and the exact method of marking the wet large container I also am confused about the importance of displacement percentages. I don’t suppose there’s a video available.

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