Sand is Not Sand is Not Sand!
A graph is the only 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 it.
Passing a handful of soil through just three sieves like 8, 16, 50 can immediately give you a sense if a sand mix is in the ball park for being mortar sand. More on this in a future post on matching mortars and renders using locally available materials.
The sieving process will not give you all the information you need to make a good mortar. The type of minerals, shape of the sand mix grains and particle distribution are all important. While a skilled mechanic can make almost any aggregate work for most uses on historic buildings, learning how to locate (or make) a better sand will make for better working characteristics and a more durable product. It will also help you in those instances where you are trying to match an historic mortar or render with local materials that may have shifted some over time.
Almost anything you read on grading and selecting aggregates is aimed at modern uses and cements where the chemical and physical properties are much more important to get right. There has always been interest in selecting the right aggregates going back in history for thousands of years. This information came largely from empirical observations until the late nineteenth and earliest twentieth century when aggregates were subjected to rigorous scientific investigation.
My favorite earlier scientific explanations of aggregates and their impact on mortar strength come from Bryan Higgins in his “Experiments and Observations Made with the View of Improving the Art of Composing and Applying Calcareous Cements and …” which was the result of government interest in relearning how to make durable mortars after buildings constructed following the Great Fire of 1666 in London began failing repeatedly. Yes, humans sometimes have to relearn things over and over.
For high performance concrete in buildings or road bed construction the mineral type, particle shape, particle distribution, and void space are all important and 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 etc. For this you must have the proper sand mix in the mortar.
Although it is not unusual for the aggregate in a historic mortar to graph in a nice curve, the fineness modulus number is related to critical performance rather than matching historic materials. Empirical knowledge has always been 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.
- 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 (durable).
- 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. Addition of some coarser and finer particles to the sand mix will immediately change the working properties.
- Although you can lay bricks with most any gradation of sand when it come to pointing and plastering, everything has to be just right.
- And crushed rock is rarely, if ever, a suitable replacement for sand.
One of the coolest things you can do with the sieve analysis and graphing is to play “what if” with a sand