Cement mortar should be used for all mason work below grade, or where situated in damp places, also for heavily loaded piers and in arches of large span. It should be used for setting coping stones, and wherever the mason work is especially exposed to the weather.
For use in ordinary masonry cement mortar should be mixed about as follows: Spread about half the sand required for mixing evenly over the bed of the mortar box (which should be water tight), and then spread the dry cement evenly over the sand and spread the remaining sand on top. Thoroughly mix the dry sand and cement with a hoe or shovel, as this is a very essential part of the process. The dry mixture should be shoveled to one end of the box and water poured into the other end. "Cements vary greatly in their capacity for water, freshly-ground cements requiring more than those that have become stale. An excess of water is, however, better than a deficiency, particularly when a very energetic cement is used, as the capacity of this substance for absorbing water is great." The sand and cement should then be drawn down with a hoe in small quantities and mixed with the water until enough has been added to make a good stiff mortar, care being taken not to get it too thin. This should be vigorously worked with a hoe for five minutes to get a thorough mixture. The mortar should leave the hoe clean when drawn out of it, very little sticking to the steel. But a very small quantity of cement mortar should be mixed at a time, particularly that made of Rosendale cements, as the cement soon commences to set, after which it should not be used. As a rule natural cement mortars should not be used after they have been mixed two hours, and Portland cement mortars after four hours (for best work not over one hour).
The sand and cement should not be mixed so as to stand over night, as the moisture in the sand will destroy the setting qualities of the cement.
Should be Kept Moist. - "Hydraulic cements set better and attain a greater strength under water than in the open air; in the latter, owing to the evaporation of the water, the mortar is liable to dry instead of setting. This difference is very marked in hot, dry weather. If cement mortar is to be exposed to the air it should be shielded from the direct rays of the sun and kept moist."
"A paste of good hydraulic cement hardens simultaneously and uniformly throughout the mass, and its strength is impaired by any addition of sand." As mortar is never used by itself, however, but as a binding material for brick and stone, and there can obviously be no advantage in making the strength of the mortar joints greater than that of the bricks or stones they unite, sand is always added to the cement in making mortar. As cement is much more expensive than sand, the larger the proportion of sand in the mortar the less will be its cost. The proportion of sand should vary according to the kind of cement and the kind of work for which the mortar is to be used. For natural cements the proportion of sand to cement by measurement should not exceed 3 to 1, and for piers and first-class work 2 to 1 should be used. Portland cement mortar may contain 4 parts of sand to 1 of cement for ordinary mortar, and 3 to 1 for first-class mortar. For work under water not more than 2 parts of sand to 1 of cement should be used. When cheaper mortars than these are desired it will be better to add lime to the mortar instead of more sand.
The following table shows the comparative strength of English Portland cement mortar, with different proportions of sand and at different ages:
AGE AND TIME IMMERSED.
PROPORTION OF CLEAN PIT SAND TO 1 CEMENT.
1 to 1.
2 to 1.
3 to 1.
4 to 1.
5 to 1.
P. 177, "Notes on Building Construction," Part III.
The values in the table represent the breaking strength in pounds on a sectional area of 2¼ square inches. The superintendent should see that the cement and sand for each batch of mortar are carefully measured to get the right proportions.