A Tale of Three Sands or What Type of Sand makes for Strong Mortars?

“I thought that large stones with curvilinear faces, bedded in common mortar, do not form so strong a walls they may when their interstices are filled with fitting stones together with the due quantity of mortar; so mortar made with sand, whose grains come near to equal in size and globular, cannot be so strong at any period or induration, as that which is made with the same mixed with as much fine sand as can be received in its interstices, in order that the lime may cement the grains by the greater number and extent of contiguous surfaces. By this notion I was excited to make provision for a new series of experiments.” — Bryan Higgins, Irish scientist

In case you don’t recognize the name, Bryan Higgins was a member of the Royal Society and studied mortars to learn what was behind serious structural failures following reconstruction after London’s Great Fire of 1666 to re-learn what had been forgotten within the building trades about making good mortar.

Higgins, Joseph Priestly, and Antoine Lavoisier (the latter credited today as the Father of Modern Chemistry) competed for the discovery of oxygen and carbon dioxide in their treatises on different kinds of air. Priestly had the connections, Higgins did the good work, so you know who gets credit.

Then there is the fact that even as other members of the British Royal Society appealed for passage for Lavoisier out of France to save his life during the French Revolution, Priestly is credited with having ensured that support for his colleague was denied. Lavoisier was guillotined. The lengths to which some will go for professional jealousy!

Anyway, Higgins deserves credit for some of the best research on mortars that from my experience stands the test of time.

The gradation of aggregate has a profound effect on the performance and working qualities of mortars, plasters and stuccoes. In this regard, let’s compare three different sands I have run across on construction sites in recent weeks.

These three aggregates have very different structural capacities for making mortar
These three aggregates have very different structural capacities for making mortar

A comparison of three sands:

At first glance, the aggregate below looks reasonably well graded. While one might be inclined to think this is sand, it is in fact crushed rock.

Crushed rock is increasingly being sold as sand, but it does not have good structural qualities
Crushed rock is increasingly being sold as sand, but it does not have good structural qualities

A growing proportion of the aggregate sold today is not natural sand from gravel pits but instead crushed rock. This is due in part to the fact we are running out of sand. That may sound unlikely, but it is a huge concern for many industries (and for those trying to keep high-value beaches each time Mother Nature tries to reclaim shoreline). But where it can be successfully used, the processing of crushed rock for concrete, road construction and other industries is big business.

This aggregate came from a contractor who was installing test patches in preparation for stuccoing a large brick building. They had hydrated the brick wall starting the day before and the lime putty used to make the stucco was of a very high quality. Yet the stucco kept cracking even after being compressed with a float.

When they asked for our input, my first thought was that the initial coat may have been a little too thin and had failed due to rapid water loss into a too-dry brick substrate. But as we applied a fresh coat of stucco a surprising thing happened: the stucco patch separated from the wall! What could have caused this?

I took a look at the aggregate by itself. At first glance it looks ordinary, but as I spread some out across my palm I realized it had an odd texture. Looking at a few grains of it through my loupe I could see this was in fact a crushed rock.

The crushing process produces mostly elongated flaky pieces. You can see the fractures in individual grains. Aggregate like this might work in mortar for laying bricks (although I would not be confident of it in instances where strength was needed), but it would not work well for stucco. It’s rough and therefore has a large surface to volume ratio making it harder to mix and tougher to adequately hydrate. The individual grains themselves have fissures that increase the void and pore space in the mix. An ideal plastering sand would be more cubical in shape.

Crushed stone will not pack or compress well, complicating attempts to get it to adhere to a vertical surface. Equally critically, it will not hold water well through the period of carbonation so that there is likely to be considerable shrinking and cracking in a lime mortar.

When a crushed rock aggregate is sieved and graphed, it often creates a double peak rather than a smooth curve.

About this same time I got a call from a homeowner making a lime-sand plaster to apply to the wood lath in his nineteenth century home. He had carried out the pore space test to determine he needed a 1:3 mix (one part lime to 3 parts sand).

He was trying to make his mortar by hand in a mortar pan and things were not going well. No matter how much he mixed it, the plaster was lumpy and not sticking to itself.

I looked at the sand he was using which was an “all-purpose” bagged mix from one of the big box building supply stores.

At first glance this sand looked fine. But looking closer, it is a blend of sand and crushed rock. While the shape of the graph of this blended aggregate creates something closer to a smooth bell curve, and even comes close to a good fineness modulus number, the inclusion of even this smaller fraction of crushed rock makes it difficult to create a good mortar for the purposes of stucco and plaster.

The first contractor had now come to me with a pit sand dug near a river that they wanted to assess for use in stucco.

Up close note that the sand is mostly cubical (as it should be), rather than flaky like the crushed rock.

The long and the short of this review is that the grading and shape of particles in aggregate can have profound effect on working properties in mortar.

Aggregates are graded in different ways for different uses and industries. But when constructing with lime mortars where the strength comes from the sand, not the lime, the goal is to find aggregate with a good distribution of particle sizes with a cubical nature. Normally this will mean natural sands.

A smooth, evenly balanced curve indicates optimal packing and therefore strength due to interlocking particles. While not all natural sands will produce a good curve, combining two sands or sieving the sand and removing or adding to one screen size is possible to improve the curve.

Here are a few things to remember when considering aggregates:

  • Commercially cleaned sands have very few fines remaining which often increases the void space in the sand and thus requires more lime.
  • Commercial grading often splits the aggregate into groups for various applications with concrete being the biggest client. Concrete buyers want larger aggregate, so the top and the bottom of the natural aggregate range of sand is stripped out of many “mason’s” and bagged sands. This leaves a narrow range of particle sizes that often will not produce a clean bell curve or create a strong lime mortar. (Again, sand is the structural element in lime mortars, whereas in cement mortars the sand is only part of the strength).
  • Sharp sand has a cubic angular shape that equates strength. But these may not be the most workable for stuccoes.  When stuccoing, I prefer a good combination of sub-angular and sub-rounded grains. More rounded aggregate creates less particle-to-particle interlocking and can be easier to work and compact.
  • Flat particles or elongated ones like you typically get in crushed rock impede compaction. Although they may be acceptable for a bricklaying mortar, they are difficult to useless for plaster and stucco.
  • Surface texture is a consideration in mixing. Smooth aggregate (low surface-to-volume ratio) will be easier to wet out and to mix whereas a coarser texture aggregate (higher surface-to-volume ratio) will take longer to mix.