DENSITY OF MORTAR. The density of a mortar is represented by the ratio of the volume of the solid particles to the total volume of the mortar.* The density is the complement of the voids, i.e., 1—d=v in which d is the density and v the ratio of the volume of the voids to the volume of the mortar. The density of a mortar is an important factor in its strength, permeability and cost; and a knowledge of the density is essential to any thorough understanding of the best method of proportioning a cement mortar and also of the laws governing its strength.
To determine the density of a mortar, weigh the amount of cement, sand, and water employed in making a given quantity of mortar, and then measure the volume of the mortar produced If the weights are determined by the metric system, the space occupied by the solid particles of the cement or of the sand is found by dividing the weight of the material used by its specific gravity,—if the weight is taken in grams the volume will be in cubic centimeters, and if the weight is in kilograms the volume is in cubic decimeters, etc. If the weights are determined in pounds, the weight of the material used divided by its specific gravity gives the weight in pounds of a volume of water equal to the volume of the solid particles; and this result divided by the weight of a cubic inch or of a cubic foot of water will give the volume in cubic inches or in cubic feet, respectively.t The percentage of each of the ingredients is found by dividing the absolute volume of each by the volume of the mortar.
The following example will make the above statements more clear. What is the density of a 1 : 3 mortar by weight, the specific gravity of the cement being 3.17 and that of the sand 2.64 ? 500 grams of cement and 1500 grams of sand were mixed with water to a normal consistency, which required 193 grams of water; and the volume of the freshly mixed mortar was 962 cubic centimeters. The volume occupied by each of the ingredients is computed as follows: The sand used in the above example has the same granulo metric composition as the "coarse" sand in § 237. With the medium
sand a 1:3 mortar of the same consistency has a density of 0.702 and contains 8 per cent of air; and with the fine sand of 4 237 the mortar has a density of 0:603 and contains 11 per cent of air. If the tamping is not very thorough, the per cent of entrained air may be twice that stated above. If the mortar is mixed wetter than standard consistency employed in testing cement, there will be less air entrained. If the mortar is mixed to standard con sistency, the volume of the freshly mixed mortar will be the same as that of the mortar after it has set; but if the mortar be mixed quite wet, the volume after setting will be considerably less than that of _the freshly mixed mortar, as the sand and cement settle and cause free water to rise to the surface. The density of the set mortar, or at least of the settled or compacted mortar, is the more important, since the only object in determining the density of the freshly mixed mortar is to find the amount of air entrained in the mixing.
The density of ,neat cement mortar varies with the cement and with the plasticity, and ranges between 0.49 and 0.59, usually between 0.51 and 0.55. The density of a sand mortar varies with the fineness of the sand, being less as the sand is finer, and also with the richness of the mortar, being slightly less for lean mixtures; and usually ranges from 0.60 for a fine sand to 0.75 for a coarse sand. Or, to state the preceding facts in another form, the voids in neat cement mortar vary from 40 to 50 per cent, and in sand mortar from 25 to 50 per cent, being greater the finer the sand.
The whole theory of the pro portioning of cement mortar is comprised in two laws, viz.: 1. For the same cement and the same sand, the strength increases with the amount of cement in a unit of volume of the mortar.
2. For the same proportion of cement in a given volume of mortar, the strongest mortar is that which has the greatest density, i.e., contains the largest proportion of solid matter.