This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
157. The sand used in making mortar should have no mixture of clay or loam in it, but should be clean and sharp particles of quartz or other disintegrated rock. These particles enter readily into the irregularities of the surfaces of stone and brick, thus forming a more perfect bond.
Sea sand or salt water should never be used in the preparation of mortar, as the mortar will not dry properly, and the salt in the sand, when united with the carbonate of lime, forms an efflorescence or deposit on the outside of brickwork or stonework. The grains of sea sand are globular in form, owing to the constant rolling and washing of the waves, and they do not unite readily with the lime or cement.
Pulverized brick, cinders, furnace slag, and scoriae are used as substitutes for sand in making mortar, where sand is scarce. It is generally admitted that these substances are as good as, if not superior to, sand in making good mortar; the cost of pulverizing, however, adds to the expense, so that they are seldom used where good sand is obtainable.
The addition of a small quantity of brick dust to the ordinary lime-and-sand mortar seems to give it the property of setting under water. It also acts as a preventive of disintegration when the mortar is exposed to the elements.