Rctcmpering.—Masons frequently mix mortar in considerable quantities, and if the mass becomes stiffened before being used, add more water and work again to plastic condition. After the second tempering the cement is much less active than at first, and remains a longer time in a workable condition. This practice is not approved by engineers and is not permitted in good engineering construction, although there is some dispute as to the extent of the injurious effects.
Cement when retempered becomes very slow in action, both in setting and hardening. The quicker-setting cements are usually more affected than the slow setting. The strength during the earlier periods of hardening is lessened, although the final strength may not be impaired. Portland cement may ordinarily be used for two, or sometimes three hours after mixing without appreciably affecting its action. When retempered after a longer period it will usually become slower in action, but may in some cases gain as much strength in thirty to sixty days.
Continuous working materially improves time strength of mortar, and when allowed to stand after mixing it should be frequently worked.
Grout.—Mortar when made thin, so that it can be poured into cracks or small openings, is known as grout. Mixtures of cement and sand used in this manner are difficult to handle without sepa ration of the materials. They should be used only under excep tional circumstances and when stiffer mortar cannot be applied.
34. Yield of Mortar.—The volume of mortar formed by mixing given quantities of cement and sand depends mainly upon the den sities of the materials. It is affected by the method of preparing the mortar, the uniformity of the mixture, and the degree of com pactness. The net volume of materials entering into the com position of mortar is readily found from their weights and densities, but it represents only approximately the resulting volume. An accurate knowledge of the yield of any particular mixture is to be obtained only by experimenting upon the materials to be employed.
The amount of cement paste made by a given weight of cement powder varies with the specific gravity of the cement and the amount of water necessary in gaging. The lighter cements require more water and yield less paste for a given volume of cement than the heavier ones. To form a cubic foot of plastic paste requires usually
from 80 to 95 pounds of natural cement, while from 95 to 101 pounds of Portland cement are necessary.
Table IV gives approximate quantities of materials ordinarily required for 1 cubic yard of compact plastic mortar. A barrel of cement is taken as 4 cubic feet, corresponding to a weight of 9.4 pounds per cubic foot for Portland cement and 70 pounds for natural cement. The sand is dry and measured loose.
The differences in quantities are mainly due to variations in the fineness of the sand, in the amount of moisture contained by the sand, and in the compactness given to the mortar. Less materials are required when using fine than when using coarse sand; more materials are required when the sand is moist than when it is dry. The compactness of the mortar is affected by the quantity of water used in mixing and the method of placing the mortar.
35. Mixtures of Lime and Cement.—The addition of slaked or hydrated lime to cement mortar causes the mortar to work more smoothly, and makes it easier and more economical to handle in masonry construction.
A lean cement mortar may be improved in density and strength by the addition of a small quantity of lime paste. Lime in larger quantities, or lime added to rich mortar, diminishes the strength of the mortar hut may sometimes be economical, through cheapen ing the mortar and improving its working qualities, when high strength is not of special importance.
Lime may he used with cement either by mixing lime paste with cement mortar, or by mixing dry hydrated lime with cement before mixing the mortar. Lime must always be thoroughly slaked before mixing with cement, as unhydrated lime in cement mortar is always a detriment. It is also essential that the mixture be very uniform, and that the mortar be worked to an even color. For this reason, the use of dry hydrated lime is to be preferred over lime paste.
In proportioning lime to cement. the method of measurement is important. Hydrated lime from nearly pure limestone contains about 75 per cent of quicklime and ordinary- lime paste contains about 10 per cent of lime by weight. About 25 pounds of quick lime are required to make a cubic foot of lime paste.