Experiments upon mixtures of lime and cement show that 10 to 15 per cent of lime (measured as unslaked line) may be substi tuted for an equal weight of cement in a 1 to 3 cement mortar with out sensibly decreasing the strength of the mortar. In some instances when not more than 10 per cent of line is used the strength is increased and the mortar made more dense. As the proportion of lime is increased the strength of the mortar is lessened. I'or mortars leaner than 1 to 3 of Portland cement the use of a small amount of lime is usually an advantage.
With some natural cements, lime may be used to replaee cement to the extent of 25 to 30 per cent of the weight of the cement with out appreciable loss of strength in the mortar. Cement so treated become- slower in action and is longer in gaining strength than when used without lime. Mixtures of this kind with either Portland or natural cement are frequently used in mortar for ordinary building operations. Hydrated lime is sometimes added to cement for the purpose of rendering the mortar less permeable where water-tight work is needed. and is also sometimes added to Portland cement concrete in small quantity to make the concrete flow more readily in filling the forms.
36. Strength of Cement Mortar.—The strength of cement mortar is dependent upon the quality and proportions of cement and sand; the quantity of water used in gaging; the method of mixing and thoroughness of working; the temperature and moisture conditions under which it is kept during hardening; the age of the mortar.
The effect upon tensile strength of varying proportions of cement and sand is shown in Fig. 2, which gives the relative strengths for an average Portland cement, or cement paste, and mortars tivith standard sand, for a period of one year after mixing. Individual cements may vary quite widely from the curves shown. Some gain strength more slowly at first and continue to gain for a longer period. Others have greater early strength and show more loss of strength during the period of retrogression.
Nearly all Portland cements after gaining strength rapidly for a time reach a maximum and then lose strength for a period. This loss of strength is usually regained later. It seldom occurs in less than three months or more than one year after the mortar is mixed. Cement which gains strength very rapidly and has high early strength is apt to suffer greater loss of strength later than cements of more moderate action, and less likely to regain fully the losses. Mortars usually show less of the effects of retrogression than cement paste, and frequently continue to gain strength for much longer periods.
Fig. 3 shows average values for good grades of natural cement.
These cements vary more widely than Portlands. They gain strength much more slowly and continue to gain for a longer period. Character of Sand.—Coarse, well-graded sand usually gives higher strength in cement mortar than standard Ottawa sand while fine or poorly graded sand may fall below the strength shown by standard sand. Sands showing less than 75 per cent of the strength given by standard ;anti are Door materials and are sometimes rejected by specifications for masonry materials.
Fincncsj of emu nt.—The fineness of the cement has an important influence upon the strength of mortar. Table V shows the results of a series of tests made upon Portland cement by \Ir. Richard K. Meade) In making these tests a bag of cement was selected and divided into five parts, and each of these ground to a different degree of fineness.
The strength of neat cement is decreased by fine grinding, while the strength of sand mortar is increased by find grinding. The same strength may be reacher] in sand mortar by using less cement when the cement is finely ground than when it is coarsely ground.
In the table it is shown that the strength of 1 to 4 mortar with cement 90 per cent fine is stronger than 1 to 3 mortar with cement 80 per cent fine.
The desirability of fine grinding depends upon the relative costs of cement ground to different degrees of fineness. Fine grinding increases the rapidity of setting very rapidly, and many Portland cements if ground so that 95 per cent passes the No. 200 sieve become so quick setting that they could not be used for ordinary work. In order to secure greater fineness, the methods of manufacture would need considerable modification.
Effect of Consistency upon Strength.—The amount of water used in mixing mortar necessarily depends upon the requirements of the use to be made of the mortar. The mortar used in concrete is usually much softer than that employed in masonry construction, or than the consistency used in testing.
For well-compacted mortar, strength decreases as the quantity of water used in mixing increases. The extent of this effect varies with the character of the sand, being less for coarse than for fine sand. This difference is very considerable in short time tests, but disappears to considerable extent as the age of the mortar increases. When tested after seven and twenty-eight days, mortar of standard consistency may have nearly double the strength of that mixed with 50 per cent more water.
Cement mortar hardens more rapidly and attains greater strength if kept moist during setting and the first period of hardening than if it be exposed at that time to dry air,