Whenever a quick-setting cement is desired, which shall attain a greater strength than the natural cements, a mixture of Portland and natural cement may be used. "Such mortar sets about as quickly as if made with natural cement alone, and acquires great subsequent strength, due to the Portland cement contained in it. The strength of the mixed mortar is almost exactly a mean between that of the two mortars separate."
An economical and strong mortar for use in dry places may be made by mixing Rosendale cement with lime mortar, in the proportion of 1 part of cement to 4 parts of lime mortar. The lime mortar should be well worked and the lime thoroughly slaked before the cement is added, and only a small quantity of the cement and lime mortar should be made up at a time. Such a mortar has a strength which is about a mean between that of lime mortar and Rosendale cement mortar, and which is amply sufficient for ordinary brickwork. Portland cement, mixed with lime mortar in the above proportion, gives no better results than good Rosendale cement. It is better to use a small proportion of lime with cement mortar than to use too large a proportion of sand, as the latter makes the mortar porous and liable to disintegrate rapidly. In England a mixture of Portland cement and lime mortar appears to be much used. Lime should not be added to cement mortar when it is to be used in wet places.
132. Grout is a very thin liquid mortar sometimes poured over courses of masonry or brickwork in order that it may penetrate into empty joints left in consequence of bad workmanship. It is also sometimes necessary to use it in deep and narrow joints between large stones. Its use is not generally recommended by writers on mortars, and the writer believes that it should not be used in stonework where it can be avoided. For brickwork, however, the author feels convinced that walls grouted with a moderately thin mortar every course makes a solid job. If the bricks are well wet before laying, and every joint slushed full of stiff mortar, it is impossible to get anything stronger; but in most localities it is difficult to get such work without keeping an inspector constantly on the ground, and when the walls are grouted the joints are sure to be filled. In his own practice the author always specifies grouting for all brick footings and foundation walls. Many of the largest buildings in New York City have grouted walls.
The following memoranda, made up from data given by Prof. Baker, will be found useful in estimating the amounts of materials required in making any given quantity of mortar:
Lime Mortar. - A barrel of lime weighs about 230 pounds; a bushel of lime, 75 pounds. One barrel (or three bushels) of lime and 1 yard of sand will make 1 yard of 1 to 3 lime mortar, and will lay about 80 cubic feet of rough brickwork or common rubble.
Cement Mortar. - 1.8 barrels, or 540 pounds, of natural cement and .94 cubic yards of sand will make 1 cubic yard of 1 to 3 mortar; two barrels, or 675 pounds, of Portland cement and .94 cubic yard of sand will also make 1 cubic yard of 1 to 3 mortar; 1.7 barrels, or 525 pounds, of Portland cement and .98 cubic yard of sand will make 1 cubic yard of 1 to 4 mortar; 1 cubic yard of mortar will lay from 67 to 80 cubic feet of rough rubble or brickwork, from 90 to 108 cubic feet of brickwork with 3/8 to ¼-inch joints, and from 324 to 378 cubic feet of stone ashlar.
A cubic foot of common brickwork contains about eighteen bricks.