Clean and sharp sand usually gives higher strength in mortar than that containing admixtures of clay or earth, or that composed of rounded grains, coarse sand usually giving greater strength than that which is very fine. It is often difficult, however, to judge of the quality of sand without experimenting with it. In some cases a small amount of fine clay appears to increase the strength of mor tar, while a judicious mixture in the sand of grains of various sizes may be of value in reducing the volume of interstices. Mortar composed of sand and cement usually possesses greater ability to adhere to other surfaces when coarse sand is used that when the sand is fine.
Effect of quantity of water used in mixing mortar is one of the most important elements; the less the quantity, pro vided there be sufficient to thoroughly dampen the mass of cement, the quicker the set. With some Portland eements, changing the quantity of water used in mixing from 20 to 25 per cent of the weight doubles or even triples the time required for the mortar to set.
When the quantity of water used in mixing is sufficient to reduce the mortar to a soft condition, the hardening as well as the setting becomes slow, and the strength during the early period is less than when a less quantity of water is used. This difference disappears to a considerable extent with time, and the mortar mixed wet may eventually gain as much strength as though mixed with less water.
Cement mortar kept under water hardens more rapidly in the early period than that exposed to the air. Nearly any cement mortar will harden more rapidly and gain greater strength if kept moist during the operation of setting and the first period of harden ing than if it be exposed at that time to dry air. Sudden drying out about the time of completing setting causes a considerable loss of strength in cement mortar, and frequently the mortar so treated is filled with drying cracks. This result is usually more marked when the mortar has been mixed quite wet.
Effect of Temperature.—The temperature of the water used in mixing and that of the air in which the mortar is placed during setting has an important bearing upon the time required for setting; the higher the temperature, within certain limits, the more rapid the set. Some cements which require several hours to set when mixed with water at temperature of F. will set in a few minutes
if the temperature of the water be increased to SO° F. Below a certain inferior limit, ordinarily from 30° to 40° F., the mortar sets with extreme slowness or not at all, while at a certain upper limit, in some cements between 100° and 140° F., a change suddenly occurs from very rapid to very slow rate of set, which then decreases as the temperature increases until the cement ceases to set.
The temperature of the air or water in which the mortar is immersed while hardening has a very important effect upon the gain in strength. Heat accelerates the action, while at temperatures near the freezing-point of water the gain in strength is very slow.
13. Portland Cement.—The term Portland cement is used to 'esignate material formed by burning to incipient fusion a finely ground mixture of definite proportions of limestone and argillaceous materials, and grinding the clinker so formed to fine powder. Several classes of materials are used for this purpose. Hard limestone or chalk, consisting of nearly pure carbonate of lime, is frequently employed, mixed with clay or shale to furnish the hydraulic ingredi ents. In the Lehigh District in Pennsylvania cenvant rock, eon silting of limestone containing silica and alumina in sufficient quanti ties to make natural cement when burned alone, is mixed with nearly pure limestone to obtain the proper Portland cement composition. In the Michigan district marl and clay excavated in soft and wet condition are used. In a few instances limestone is mixed with blast-furnace slag for the production of Portland cement. This is quite distinct from the manufacture of slag cement (so called) in which the materials are not burned together.
To make good Portland cement it is always necessary that the ingredients be very carefully proportioned and that the mixture be very homogeneous. This requires the pulverization of the mate rials and their uniform incorporation into the mixture before burning.
The burning of Portland cement requires high heat to insure complete combination of the lime with the silica and alumina. In underburned cement, a part of the lime may be left as caustic lime, uncombined with the clay. This is apt to produce unsound cement, which may swell and crack after being used.