In large cities the building ordinances require certain mixtures of mortar. For various mixtures see [page 104]. But where there are no legal requirements, the contractor's own judgment and experience should guide him in the selection of the proper mix, as each one has certain qualities which recommend it. Portland cement mortar is stiffer and harder to work than lime mortar, but should always be used where permanence and strength are required, as in exposed or heavy bearing situations, or in cold weather when the mortar must set before it freezes. Lime mortar may do for the usual work, above the grade in dry warm climates. It should not, however, be used for basement walls, except possibly where the soil is extremely dry, nor for piers or points in the wall that carry heavy loads. It is smooth and works easily, so that more brick can be laid per day than with cement mortar, but its use should be limited to the proper situations. Cement-lime mortar has the combined qualities of both cement mortar and lime mortar. It has both strength and good working qualities, and costs less than cement mortar. It is preferred by many to cement mortar; we recommend its use in proper situations, throughout this Manual.
Mixing Cement Mortar
Cement mortar must be mixed and used immediately, as cement sets very quickly. On this account, it should not be made in large quantities and, once it has obtained a partial set, should never be re-tempered. The cement and sand for cement mortar should be thoroughly mixed while dry, by turning the material over three or four times with a shovel, when sufficient water to make a plastic mass should be added, turning again several times. It is always desirable to add a small quantity of lime, when tempering the cement mortar, in order to make it easier to work.
Mixing Lime Mortar
Lump lime should be slaked as soon as possible after delivery on the job. If it is allowed to stand for any length of time, it should be stored in an air-tight box; otherwise, it may become air slaked, reducing mostly to powder and soft, crumbly lumps. The fresh lime is always in hard lumps and should be free from cinders. Lime is slaked in a mortar box by adding water to the lump lime. On adding water the lime becomes very hot, gives off vapor, and finally burst into a powder which gradually reduces to a paste known as lime putty. The exact amount of water necessary varies with different limes, but in all cases, the right proportions used in slaking must be watched carefully. An excess of water will stop the slaking process. If not enough water is used the lime will "burn" and lose its strength. If left overnight, the mortar box should be covered with planks.
When slaking is completed, the lime is reduced to what is called "putty", which should be of uniform consistency throughout, containing no unslaked lumps. The sand should now be added, the exact amount depending upon the quality of mortar desired, and thoroughly mixed with the putty, and then shoveled out on wooden planks where it should remain for at least a week before using.
Hydrated lime
Hydrated lime is in powdered form and needs no other preparation than the addition of water. Its quality is uniform and produces satisfactory results, even when experienced labor cannot be obtained, and may be used at once on the job. Its uniformity and the facility it gives in handling the mortar make it preferable to lump lime although it costs somewhat more.
The mixing of cement-lime mortar is very simple, as all that is needed is to add the proper amount of cement to the lime mortar, at the time it is tempered for use, and very thoroughly mix into a consistent mass.
Tempering Mortar