The amount of water required for mortar will vary with the proportion of sand to cement, the character and condition of the ingredients, the weather, and the purpose which the mortar is to serve. If the water is stated as such a percentage of the combined weight of cement and sand, the amount required for a rich mortar will be greater than for a poor one, since the cement requires more water than the sand. Fine cement will require more water than coarse; the same is true of sand. Sand from absorbent rock will require a larger amount of water. On a hot, dry day, more water must be used to allow for evaporation; and again, if the mortar is to be placed in contact with brick or porous stone, the mortar must be more moist than when used in connection with metal, or with hard rocks such as granite. All of these points must be borne in mind when determining the proper consistency for a given purpose.
279. We may arrive at the approximate amount of water required in the following manner: find what proportion of water is required for the neat cement. This will vary among different samples, and especially between Portland and natural cements; the former requiring twenty to twenty-eight per cent, of water (by weight), and the latter thirty to forty per cent. Then find the amount of water required to bring the sand alone to the consistency of mortar. This will vary considerably, fine sand requiring much more water than coarse, etc., as mentioned above. Having these two quantities, we may find the amount of water required for a mortar having any given proportions of these samples of cement and sand. Thus, suppose we find that the neat cement requires twenty-five per cent, water and the sand ten per cent, water to bring them to the proper consistency. If we wish to make a one-to-three mortar from these ingredients, using one hundred pounds of cement, the required amount of water is (100 X .25) + (100 X 3 X .10) = 25 + 30 = 55 pounds.
280. However, it will usually be better to experiment directly upon the mixture which it is proposed to use, and for this purpose the following rule will be found of value. For ordinary purposes, that amount of water should be used which for given weights of the dry ingredients will give the least volume of mortar with a moderate amount of packing. In the actual use of mortars it is not practicable to state that a certain definite amount of water shall always be used with given quantities of the dry materials. It is the resulting consistency of the mortar that must be specified and insisted upon, while the amount of water required to produce this consistency will vary from day to day and must be left to the discretion of the inspector or foreman. For a discussion of the relation of consistency to the tensile strength of the mortar, see Art. 46.