This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The wisdom of allowing no more than four volumes of sand to be used with cement is manifest, and it is certainly better that even a smaller proportion of sand should be used, or that a certain amount of thoroughly slaked lime should be added in order that the mortar may be more dense. The writer usually specifies cement-mortar to be a 1 to 2 mixture, and never goes beyond 1 to 3.
Excellent mortar can be made from hydraulic lime, such as the well-known Lias limes, mixed with sand in the proportion of 1 to 2. The lump or "shell" lime may be used, but the ground lime is much to he preferred, especially where a mortar-mill is not available. The ground lime can be distinguished from Portland cement by its yellow colour.
Selenitic limes are also preferable to common lime, but are not very largely used.
As sand (or some substitute for sand) forms the greater part of nearly all mortars, its importance cannot be denied. Certainly pure sand is inert, but much "sand" used in buildings is mixed with clay, iron and other salts, and organic impurities, and is detrimental to the lime or cement with which it is used. In one case al>out a thousand concrete blocks, in which sand containing iron-pyrites had been used, were quite worthless, as the pyrites destroyed the setting properties of the cement. The salt in sea-sand, when this is made into mortar or plaster, attracts moisture, causing dampness and often leading to efflorescence. The clay in loamy pit-sand may lessen the strength of cement-mortar as much as 50 per cent. Soot in mortar or plaster will cause stains in paint and wall-paper. Organic matter, such as dung in road-scrapings, may lead to the colonization of the house-walls with innumerable micro-organisms, which may be quite harmless or quite otherwise.
Sand from quarries, quickly-flowing streams, and little-frequented roads macadamized with coarse-grained stone, is usually suitable for mortar. Pit-sand is good, if reasonably free from clay and other impurities.
"Sand" from sluggish streams and ditches, from roads macadamized with hard, fine-grained limestone and "granite", and from foundries, had better be rejected; so also must street-sweepings.
The principal substitutes for sand are ashes or "breeze", brick-dust, and burnt clay-ballast. Ashes yield mortars of a somewhat weak and porous character, and may interfere with the proper setting of cement if they contain coal-dust or other impurities. Brick-dust and clay-ballast make good mortar, if they are properly burnt, hard and clean.
When a mortar-mill is not used, all grit and lumps should be carefully screened from the sand and lime before these are mixed, as they would tend to crack the bricks and stones if used in the mortar.
In making mortar, a little sand more or less does not matter very much when ordinary lime is the matrix, but even in this case the measurement of the lime and sand should be carried out with some approximation to accuracy. When, however, hydraulic lime and cement are used, the careful measurement of these and the sand, in suitable boxes or frames, must be insisted on.
Water used in mortar should be "fresh" and clean.
The proper use of mortar now calls for notice. It is in vain to have good mortar if it is not properly used. The one flagrant defect in brickwork is usually that the vertical joints are not flushed with mortar. The bed-joints are almost invariably entirely filled, but the ends of the bricks receive the merest scraping on the front edge, while in thick walls the sides of the fillingln bricks may receive none at all. Only the closest supervision of a resolute clerk-of-works tan prevent the "brickies" from scamping their work in this Mortar made from cement or hydraulic lime must be mixed in small quantities and used fresh. Mortar which has once "set" to any appreciable extent cannot be remixed without loss of ultimate strength.
The thickness of mortar-joints in brickwork depends on the regularity of the bricks, the fineness of the mortar, and the care of the workman. In good work the joints are usually about ¼ of an inch thick, certainly not more than 3/8. In stone walls there is more variation than in brick, from the thick joints of rubble and flint work to the ashlar joints scarcely thicker than a penny.
As water is absolutely essential not only for the initiation but also for the continuation and completion of the chemical processes involved in the setting and hardening of hydraulic limes and cements, it is imperative that the moisture should not be abstracted from the mortar too soon. Hence the necessity of protecting stucco from brilliant sunshine, or of repeatedly spraying it with water; hence also the necessity of dipping bricks in water immediately before using them, and of sprinkling a dry course of bricks with water before the bed 0f mortar is spread above it to receive the next course. With lime-mortar also, a moderate use of water in the same way is advantageous, although the lack of it has not so marked an effect as with cement and hydraulic lime.
The method of finishing: the joints externally, although apparently a small matter, is by no means unimportant. The joints may be finished as the walling proceeds, or may be left rough to be raked out and finished at some subsequent period. To distinguish the two operations, the former is sometimes spoken of as "jointing", while the latter is always known as "pointing". "Jointing" ought always to be adopted unless the mortar is of wretched quality or likely to be damaged by frost The most common forms of mortar-joints are shown in Fig. 63. a is the flat or flush joint; this joint is often finished by having a jointing-iron run along it while the mortar is wet, leaving an impression as at B, C, or D, according to the shape of the jointer.
E is known as the weather joint, and is made by pressing the upper part of the mortar into the joint with the trowel. F is the struck joint, and is like the last, except that the mortar overhangs the lower course a little. The cut or mason's joint is similar to the last, but with the lower edge of the projecting mortar neatly cut with the trowel to a straight line; this is the best form of joint, as it protects the wall and does not form a ledge for holding water, h is the mason's V-joint.