A graph like the one in the previous post is only one way of understanding your sand. It’s important to look at the aggregate in several ways. Setting the aggregate retained from each fraction out in dishes side by side allows you to see the mineral types and their relationships more easily.
After completing a dozen sievings of different sands and relating the graphical display with the actual sand in each bowl you can begin to gauge the quality of sands in the field and predict the likely properties of a mortar or a render made from it even before graphing. Passing a handful of soil through just three sieves like 8, 16, and 50 can immediately give you a sense if a sand is in the ball park.
The sieving process will not give you all the information you need to make a visually compelling mortar match. The type of minerals, shape of the sand grains and particle distribution are all important. A skilled mechanic can make almost any aggregate work for most uses on historic buildings. But learning how to locate (or assemble from several sources) a better aggregate will make for better working characteristics and a more durable product. It will also help in those instances where even after harvesting local sand from the original site does not provide an exact match because the geology of the area has shifted (a common occurrence with bodies of water).
Almost anything you read on grading and selecting aggregates is aimed at modern construction. Up through the 19th century most assessment of aggregates and what worked best in each area came largely from empirical observations. Only in the 20th Century as buildings began climbing well beyond load-bearing masonry did the study of aggregates become more rigorously studied and standardized.
For high performance concrete in buildings or road bed construction the mineral type, particle shape, particle distribution, and void space today are all carefully engineered. Today’s critical construction activities are focused on factors such as high strength, precise amounts of water and cement, fatigue and frost resistance.
Although it is not unusual for the aggregate in a historic mortar to graph in a nice curve, the fineness modulus number often included in any grading of sand today is focused primarily on modern construction expectations rather than matching historic materials. However, with some additional leeway on those numbers, the fineness modulus number does point to highly workable mortars for historic buildings. But empirical knowledge and using your eyes and brain for observation is equally vital. For those of us working on historic mortars it is useful to know how all of these factors relate to the choice of aggregate.
If you are interested in better understanding 18th Century thinking regarding mortars and their aggregates, my favorite come from the Irish scientist Bryan Higgins. In his Experiments and Observations Made with the View of Improving the Art of Composing and Applying Calcareous Cements and … Higgins was seeking to relearn how to make durable mortars after buildings constructed following the Great Fire of 1666 in London began failing repeatedly. Yes, it seems humans have lost sight of good construction principles severals times in the past.
For instance, in trying to point narrow joints you will need to have aggregate that does not have pieces larger than the joint, but you still want the mortar to be structural to ensure durability.
A mostly rounded sand will have different working and strength properties than an angular or sharp sand.
Lack of fines in the aggregate will mean more binder and water needed because of more gaps between the larger particles.
A sand that consists of a narrow range of particle sizes will have a mealy texture and lack cohesion. The addition of some coarser as well as finer particles will immediately change the working properties.
And crushed rock is rarely, if ever, a suitable replacement for sand. More about crushed rock in this post comparing three different aggregates including bagged sands that are really crushed rock and therefore unworkable for plaster.
So what if you sieve a sand and the graph is far from ideal — for example a dreaded double peak? Do you necessarily have to reject it?
Let’s look at ways to fix it. What if we sieved some of the sand and selectively increased the amount of several fractions while stripping out other sieve fractions. Do a mockup and re-sieve to see how it improves the graphing. Admittedly this may not be practical for every project, but if you have a small area to repoint and want to use a readily available sand rather than look further afield, this can be a very practical solution.
As always, make some mortar and test it to learn empirically how the change in formulation alters the working properties. In time this will allow you to walk up to a mixer load that is giving bad working properties and instantly recognize what is wrong and how to fix it with a few quick tweaks.
Of course I have found that in general, despite assumptions by many to the contrary, that the best way to get a good mortar for repairing historic mortars is in fact to dig your own from the site, washing it to the degree the builders did for a perfect match. This produces perfect visual matches easily without lots of time spent searching for sands, tweaking them, and trying to tint and add other components to fake it. More on this in a future post on matching mortars and renders using locally available materials, or jump there now.