Masonry History Integrity - National Center for Preservation ...

Chapter 2: Got Mortar?
Sand
Sand is measured by the pound or ton and delivered to the site in bags or by the truckload. There
are three basic varieties of sand: coarse, fine and medium.
Coarse sand has large grains and feels bulky. It’s used mostly for drainage ditches or gardening.
Fine sand has small grains and feels soft. It’s like beach sand or play box sand.
Medium sand is what you want for mixing mortar. It’s not too coarse, not too fine, and has
uniform grains that make for smooth working.
Water
Water for making mortar should be clean, free of debris or contaminants and drinkable or
“potable.” The word potable is from the Latin word pōtābilis meaning drinkable.
HYDRAULIC
Hydraulic refers to fluids in motion or something
controlled by water pressure. Pumps, clocks and
even pipe organs can be hydraulic. The word
hydraulic is from the Greek word hydraulikos
meaning water organ.
“Hydraulic cement” is a combination of cooked
and ground minerals that undergoes a chemical
reaction, called hydration, when mixed with sand
and water. It hardens under water. Portland cement,
used to make mortar and concrete hardens, or cures,
under water and is therefore hydraulic.
Mortar made with Portland cement
The ingredients of mortar can be combined in different proportions to give it different strengths.
The types of mortar from strongest to weakest are designated by letters: M, S, N, O and K. Those
letters were selected from the words: M-a-S-o-N w-O-r-K.
A standard rule for mixing mortar is: The amount of sand should be between a minimum of 2.25
and a maximum of 3 times the total of the cement and lime, combined.
In other words, for Type M mortar with a 3/1/12 ratio, the cement and line combined equals 4; and
the maximum amount of sand to be used would be 12, (4 x 3).
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