“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, 18th century Irish scientist
Bryan Higgins, a member of the Royal Society, studied mortars to learn what was behind serious structural failures plaguing London’s post-Great Fire of 1666 rebuilds to re-learn what had been forgotten within the building trades about making good mortar. He and Joseph Priestly competed for the recognition of carbon dioxide in their treatises on different kinds of air.
Today I want to look at three different sands I have run across on construction sites in the last few weeks. The gradation of aggregate has a profound effect on the performance and working qualities of stuccos and renders.
A comparison of three sands:
The aggregate at left looks reasonably well graded. While one might be inclined to think this is a sand, it is in fact crushed rock.
Most of the aggregate produced in this country today is not natural sand and gravel but crushed rock. It is a huge industry supplying concrete, road construction and other industries.
This sand came from a contractor who was installing test patches in preparation of 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 wood float.
When they asked for our input, my first thought was that the initial coat may have been a little thin which can lead to rapid water loss by suction from the brick substrate. But as we pressed on the second layer 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 wouldn’t be so sure), 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 tears that must be satisfied, increasing the void and pore space in the mix. An ideal plastering sand would be more cubical in shape.
This crushed stone will not pack well and thus not compress well. That will make it hard to adhere to a surface. But as critically, it will not hold its water well through carbonation so that there is likely to be considerable shrinking and cracking.
Lets look at the sieve data on this aggregate to see what passes each sieve with this crushed rock.
About this same time I got a call from a homeowner who was 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).
At first glance this sand looked ok. But looking closer, I realized it was a blend of sand and crushed rock.
While the shape is more bell-curved, the inclusion of even this smaller fraction of crushed rock makes it difficult to create a good mortar
Back to the contractors trying to find a good sand for stucco, they sent me another sand for consideration from York PA.
They knew this was a pit sand, being dug near a river, so there would be somewhat more rounded particles, but it was a natural sand.
Up close note that the sand is mostly cubical (as it should be), rather than flaky like the crushed rock.
Grading of aggregate can have profound effects on performance and working properties.
Crushed Rock Rock and Sand Sand with Good Packing
Aggregates are graded in different ways for different uses and industries.
For most lime mortar uses dense grading is preferred. Which is shown in the graphs above. You are trying to achieve a good distribution of particle size so there is a maximum density of aggregate with low void space.
These are a few things to remember when considering aggregates:
- In open grading there are few fines which leads to more void space and therefore more binder is required.
- In a uniform grading the particles are all in a narrow size range. (This is not useful for lime mortars or renders.)
- Angular or sharp sand has a cubic angular shape that gives stiffness and strength. But these may not be the most workable for stuccoes. When stuccoing, I prefer a good combination of sub-angular and sub-rounded.
- Rounded aggregate creates less particle-to-particle interlock and can be easier to work and compact.
- Flat particles or elongated ones like you typically get in crushed rock impede compaction and, although they may be alright as a bricklaying mortar, they are difficult to use as a stucco.
- Another thing to be aware of is surface texture. A smooth aggregate with lower surface-to-volume ratio will be easier to wet out and to mix whereas a coarser texture with higher surface-to-volume ratio will take longer to mix.