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It is also important to have a mix of particles sizes that fit together well, so that the voids between bigger particles are filled with smaller particles. This is true down to the very finest particles of sand. Filling the smallest voids with lime. Add the lime in just the right quantity so that the voids between the finest particles are all filled, but with no excess lime, to give you the perfect lime mortar.
For a load-bearing masonry building, not very strong at all. The guidance from the Brick Industry Association is that mortar should always be weaker than the masonry units it binds, and that you should always use the lowest compressive strength mortar that still meets performance requirements. In other words, “stronger” or higher psi-rated mortar is not better. This is especially true when you are discussing load-bearing masonry buildings.
The need for mortar to be weaker than the masonry is especially true for load-bearing masonry buildings. The term “load-bearing masonry” describes most of human construction prior to the skyscraper era. Its chief feature is the reliance on stacking masonry units (like bricks) that hold in place largely by gravity. Masons say these bricks are "in compression".
To be clear, even in an 18th century building, some elements (such as the floor joists) are under both tension and compression, because the timber is spanning and has uneven load along its length. In contrast, a load-bearing masonry building relies primarily on the compression and even distribution of pressure across each masonry unit at that level.
For this reason, it is important to recognize that mortar, and in particular lime mortar, in load-bearing masonry buildings has the chief responsibility to cushion and distribute the loads evenly, rather than act as a glue. This is why the slower setting and long-term flexibility of mortar is so important for building components in compression. If the mortar is too rigid, it will create individual points of excess loading stress (the term is “point-loading”) that will fracture the masonry units (bricks, stone, block, etc.)
It is important to recognize that load-bearing masonry walls are broader at the base. The wall narrows to receive the floor joists at each story the building climbs. In this way, the wall is itself equivalent to a modern spread footer. (Keep this in mind when there is talk about an historic building being unsound just because it doesn’t have “proper” foundations.) In order for the load to be evenly distributed, these bricks or other masonry units are not just stacked, but carefully woven together so that each depth of the wall ties into bricks in front of and behind it.
Because the load is evenly distributed over this wide footprint, the actual pounds per square foot (PSI) load at the base of the wall, even on a five-story Georgian structure including the load the floors carry into the walls as well, is easily under 200 PSI (and probably closer to 100 PSI) – with a generous safety factor added in. In other words, the load is minimal and the mortar should therefore be lightly rated.
Many masons and the general construction industry’s response to this is to provide a “weak” cement mortar like Type O with a maximum compressive strength of 350 psi. This certainly makes the mortar softer than most masonry units and historic bricks. While this type of mortar is consistent with the PSI characteristics, there are other criteria to consider.
Remember that in a load-bearing masonry building, we want the mortar to act as a cushion to evenly distribute the load. Type O mortars are hydraulic (like any portland cement-based mortar), meaning as soon as you add water, they have largely hardened to full depth within 48 hours. In contrast, non-hydraulic lime mortars resist point-loading almost indefinitely.
There are structures thousands of years old constructed of lime mortar that are still sound to this day. Could you ask for a better trial or proof of concept? It is simply a system that works for these structures.
While Type O mortars reach a similar final strength, they will not accommodate movement and allow for stress relief, especially near the exterior of the masonry envelope. There is no autogenous healing from Type O mortars, whether of Portland or “natural” cements. They won’t have the breathability of a lime putty mortar, which is important when historic masonry is often very porous. Typically, Type O mortar also doesn’t last as long, barely lasting 150 years, and often only 50.
Plus, Type O risks leaching. The type of lime added to this type of portland cement makes the mortar workable. This also assists with water retention during working. However, the result is free lime that will leach out and cause staining later; it is lime that doesn’t carbonate.
How does industry try to fix this leaching free lime problem? Usually they add pozzolans that are industrial waste products from coal and steel production. That means you may be adding huge quantities of heavy metals to your mortar. Why have such a nasty stew of “hazmat” materials in your mortar when you can have a reliable, environmentally-safe lime mortar of just lime putty and sand?
Remember OUR mortar, is a mortar with a thousands-of-years track record for working in porous, load-bearing structures that flex (and so don’t require expansion joints). Non-hydraulic lime mortar stays flexible, breathable, and strong far longer in load-bearing masonry than any of its hydraulic counterparts.