An introduction to aggregates for mortar

Too often when people try to make plaster or stucco and find they are having application problems, it turns out the aggregate is to blame. Just like Harry Francis from the National Lime Institute used to say “Lime is not lime is not lime” let’s paraphrase to “aggregates are not aggregates are not aggregates” or “sand is not sand is not sand.” In short, it is not as simple as accepting whatever is at the local masonry supply house. Nor for that matter if the sand is good, is it enough to just accept the 3:1 sand-to-lime ratio that should only be used for rough estimating purposes. (More on the pore space ratio in the next posting).

Sieving sand and weighing it to 3 decimal places

Sieving sand and weighing to 3 decimals

So how do we determine that an aggregate will help make a good mortar? Sieving helps to determine the shape of the bell curve and to factor the fineness modulus.

While these terms may sound foreign,  these are standard procedures used in assessing aggregates right up to the current day. But the concept is the result of centuries of experience.

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Here is a simplified explanation of how the procedure works and what it means for mortar or plaster making and in-field working properties.

Graph with a decent bell curve graphically shows a proportional balance in particle sizes

Graph with a decent bell curve graphically shows a balance in particle sizes

By carrying out a mortar analysis, the results of the weight retained in grams on each sieve can be entered into our simple spreadsheet which will then chart the shape of the bell curve.

By then adding up the accumulated % retained and dividing by 100, you will find the fineness modulus. A good sand should fall into the range of 2.3-3.4, but it isn’t quite as simple as that.

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We will have more in future posts on graphing and even how sands that aren’t ideal can be tailored to meet our needs, as well as why having a good bell curve may still not mean an aggregate is appropriate for making mortars and plasters  …

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