This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Walls are usually built of brick or stone, which are sometimes, though rarely, laid up dry, but usually with mortar filling all the joints. The object of mortar is threefold:
1. To keep out wet and changes of temperature by filling all the crevices and joints.
2. To cement the whole into one mass, keeping the several parts from separating, and,
3. To form a sort of cushion, to distribute the crushing evenly, taking up any inequalities of the brick or stone, in their beds, which might fracture each other by bearing on one or two spots only.
To attain the first object, "grouting" is often resorted to. That is, the material is laid up with the joints only partly filled, and liquid cement-mortar is poured on till it runs into and fills all the joints. Theoretically this is often condemned, as it is apt to lead to careless and dirty work and the overlooking of the filling of some parts; but practically it makes the best work and is to be recommended, except, of course, in freezing weather, when as little water as possible should be used.
To attain the second object, of cementing the whole into one mass, it is necessary that the mortar should adhere firmly to all parts, and this necessitates soaking thoroughly the bricks or stones, as otherwise they will absorb the dampness from the mortar, which will crumble to dust and fail to set for want of water. Then, too, the brick and stone need washing, as any dust on them is apt to keep the mortar from clinching to them. In winter, of course, all wetting must be avoided, and as the mortar will not set so quickly, a little lime is added, to keep it warm and prevent freezing.
To attain the third object, the mortar joint must be made thick enough to take up any inequalities of the brick or stone. It is, therefore, impossible to set any standard for joints, as the more irregular the beds of the brick or stone, the larger should be the joint. For general brickwork it will do to assume that the joints shall not average over one-quarter of an inch above irregularities. Specify, therefore, that, say, eight courses of brick laid up "in the wall" shall not exceed by more than two inches in height eight courses of brick laid up "dry." For front work it is usual to gauge the brick, to get them of exactly even width, and to lay them up with one-eighth inch joints, using, as a rule,"putty" mortar. "While this makes the prettiest wall, it is the weakest, as the mortar has little strength, and the joint being so small it is impossible to bond the facing back, except every five or six courses in height. "Putty" mortar is made of lime, water and white bad, care being taken to avoid all sand or grit in the mortar or on the beds.
The best mortar consists of English Portland cement and sharp, clean, coarse sand. The less sand the stronger the mortar.
Sand for all mortars should be free from earth, salt, or other impurities. It should be carefully screened, and for very important work should be washed. The coarser and sharper the sand the better the cement will stick to it. English Portland cement will stand as much as three or four parts of sand. Next to English come the German Portland cements, which are nearly as good. Then the American Portland, and lastly the Rosendale and Virginia cements. Good qualities of Rosendale cements will stand as much as two-and-a-half of sand. Of limes, the French lime of Teil is the strongest and most expensive. Good, hard-burned lime makes a fairly good mortar. It should be thoroughly slacked, as otherwise, if it should absorb any dampness afterwards, it will begin to burn and swell again. At least forty-eight hours should be allowed the lime for slacking, and it is very desirable to strain it to avoid unslacked lumps. Lime will take more sand than cement, and can be mixed with from two to four of sand, much depending on the quality of the sand, and particularly on the "fatness" of the lime. It is better to use plenty of sand (with lime) rather than too little; it is a matter, however, for practical judgment and experiment, and while the specification should call for but two parts of sand to one of lime, the architect should feel at liberty to allow more sand if thought desirable. Lime and Rosendale cement are often mixed in equal proportions, and from three to five parts of sand added; that is, one of lime, one of cement, and three to live of sand. It is advisable to specify that all parts shall be actually measured in barrels, to avoid such tricks, for instance, as hiring a decrepit laborer to shovel cement or lime, while two or three of the strongest laborers are shovelling sand, it being called one of cement to two or three of sand. A little lime should be added, even to the very best mortars, in winter, to prevent their freezing.
When a wall has been frozen, it should be taken down and re-built. Never build on ice, but use salt, if necessary, to thaw it; sweep off the salt-water, which is apt to rot the mortar, and then take off a few courses of brick before continuing the work. Protect walls from rain and frost in winter by using boards and tarpaulins. Some writers claim that it docs no harm for a wall to freeze; this may be so, provided all parts freeze together and are kept frozen until set, and that they do not alternately freeze and thaw, which latter will undoubtedly rot the mortar.
Plaster-of-Paris makes a good mortar, but is expensive and cannot stand dampness. Cements or limes that will set under water are called hydraulic.
Quickness of setting is a very desirable point in cements. All cement-mortars, therefore, must be used perfectly fresh; any that has begun to set, or has frozen, should be condemned, though many contractors have a trick of cutting it up and using it over with fresh mortar. To keep dampness out of cellar-walls the outside should be plastered with a mortar of some good hydraulic cement, with not more than one part of sand to one part of cement; this cement should be scratched, roughened, and then the cement covered outside with a heavy coat of asphalt, put on hot and with the trowel. In brick walls, the coat of cement can be omitted and the joints raked out, the asphalt being applied directly against the brick. This asphalt should be made to form a tight joint, with the slate or asphalt damp-course, which is built through bottom of wall, to stop the rise of dampness from capillary attraction.