OOST OF MORTAR. Knowing the price of the materials, it is very easy, by the use of Table 22, page 120, to compute the cost of the ingredients required for a cubic yard of mortar. The expense for labor is quite variable, depending upon the distance the materials must be moved, the quantity mixed at a time, etc. As a rough approximation, it may be assumed that the cost of mixing mortar is $1.00 per cubic yard. The following example illustrates the method of computing the cost. The cost of a cubic yard of mortar composed of 1 part portland cement and 2 parts sand, both by weight, is about as follows: Natural vs. Portland Cement Mortar. It is sometimes a question whether portland or natural cement should be used. If a quick-setting cement is required, then natural cement is to be preferred, since as a rule the natural cements are quicker-setting, although there are many and marked exceptions to this rule. Other things being the same, a slow setting cement is preferable, since it is not so likely to set before reaching its place in the wall. This is an important item, since with a quick-setting cement any slight delay may necessitate the throwing away of a hoxful of mortar or the removal of a stone to scrape out the partially-set mortar.
Generally, however, this question should be decided upon economical grounds, which makes it a question of relative strength and relative prices. The tensile strength of natural and land cement mortars is shown in Fig. 8, page 123. The cost of mortar of various proportions of sand may be computed as in the preceding section. Assuming portland cement to cost $1.80 per barrel, natural 75 cents per barrel, and sand 75 cents per cubic yard, and using Table 22, page 120, the cost of the materials in a cubic yard of mortar is as in Fig. 9.
By plotting the strength of portland and natural cement mortar 6 months old and the cost of a yard of mortar as given in Fig. 9, Fig. 10 is obtained, which shows the relation between the strength at 6 months and the cost of the mortar made of the two kinds of cement. The curves lie so close together that the diagram is not very significant, but it will serve to illustrate an interesting, if not valuable, method of investigating the relative economy of natural and portland cement. Notice
that for any tensile strength under about 300 lb. per sq. in. (the strength of neat natural ce ment) either natural or portland cement may be used, but that the former is a little cheaper. In other words, Fig. 10 shows that if a strength of about 300 lb. per sq. in. at 6 months is sufficient, natural cement is the cheaper. A considerable change in prices does not materially alter the re sult, and hence the conclusion may be drawn that if a strength of 250 to 350 lb. per sq. in. at 6 months is sufficient, natural cement is more economical than portland.
For other ages it should be remembered that as the age increases natural cement is relatively stronger than portland (§ 253).
However, in this connection it should not be forgotten that other considerations than strength and cost may govern the choice of a cement; for example, uniformity of product, rapidity of set, and soundness are of equal or greater importance than strength and cost. Portland cement is more uniform in quality than natural cement, and for this reason is usually selected in preference to natural cement. The rapid development of the portland cement industry in recent years in this country has greatly increased the use of port land cement relative to that of natural cement.
Mortar made of two brands of portland or natural cement will differ considerably in economic value, and hence to be of the highest value the above comparison should be made between the most economical poitland and the most economical natural cement as determined by the method described in 5 180. Prices vary with locality, and hence there may be places in which the above investiga tion as to the relative economy of natural and portland cement will lead to a considerable saving in construction work requiring a large amount of cement. Such an investigation is less important now than formerly, owing to the decrease in price and increase in strength of portland cement.