This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Selenitic mortar is generally made by mixing selenitic cement and sand. It was at one time made by mixing a small proportion of calcined sulphate with ordinary lime and sand. The licences now issued by the patentees render it necessary that selenitic cement should be used. The proportion of sulphate required to develop the characteristics of the material is added to the cement before it is sold, and the process of mixing the mortar is carried on under the following rules: - 1 bush. (1.28 cub. ft.) of prepared selenitic lime requires about 6 gal. of water (2 full-sized pails). If prepared in a mortar-mill: (1) Pour into the pan of the edge-runner 4 full-sized pails of water; (2) gradually add to the water in the pan 2 bush prepared selenitic lime, and grind to the consistency of creamy paste, and in no case should it be thinner; (3) throw into the pan 10 or 12 bush. clean sharp sand, burnt clay, ballast, or broken bricks, which must be well ground till thoroughly incorporated; if necessary, water can be added to this in grinding, which is preferable to adding an excess of water to the prepared lime before adding the sand. When the mortar-mill cannot be used, an ordinary plasterers' tub (containing about 30-40 gal.) or trough, with outlet or sluice, may be substituted.
If prepared in a plasterers' tub: (1) Pour into the tub 4 full-sized pails of water; (2) gradually add to the water in the tub 2 bush, prepared selenitic lime, which must be kept well stirred until thoroughly mixed with the water to the consistency of creamy paste, and in no case should it be thinner; (3) measure out 10-12 bush. clean sharp sand or burnt clay ballast, and form a ring, into which pour the selenitic lime from the tub, adding water as necessary; this should be turned over 2 or 3 times, and well mixed with the larry or mortar hook. Both the above mixtures are suitable for bricklayers' mortar or for first coat of plastering on brickwork. A box measuring inside 13i in. by 13 1/8 in. by 13 1/8 in. would contain about 1 bush, and would be useful for measuring the lime, and should be kept dry for that purpose; and a box without a bottom, measuring inside 36 in. by 18 in. by 18 in. would contain about 5 1/4 bush., and would be very useful for measuring the sand. Increase or decrease the quantities given proportionately with the requirements. The prepared selenitic lime must be kept perfectly dry until made into mortar for use.
It is of the utmost importance that the mode here indicated of preparing the mortar, concrete, etc, should be observed, viz. first well stirring the prepared selenitic cement in the water before mixing it with the sand, ballast, or other ingredient, otherwise the cement will slake and spoil.
A few years ago persons using selenitic mortar were permitted to add the sulphate for themselves, and where selenitic cement is not procurable the process might still be useful. It is conducted as follows: - 3 pints plaster of Paris are stirred in 2 gal. water; after the mixture is complete, it is poured into the pan of a mortar-mill; 4 gal. water are added, and the mill revolved 3 or 4 times, so as to ensure thorough mixing: 1 bush, finely-ground unslaked lime is added; the mixture is continued till the whole becomes a creamy paste, and then 5 bush, sand are gradually introduced, the whole being thoroughly mixed. No more is mixed than will be required during the day. If the water gets heated or sets too rapidly, a little more plaster of Paris should be added, but not more than 1/2 pint extra per bushel of lime. When the lime used in this last-described process is deficient in hydraulic properties, a proportion of selenitic clay should be added so as to bring the total amount of clay in the prepared lime up to about 20 per cent.
It will be seen that the addition of the plaster of Paris, clay, etc, requires considerable skill and judgment, and the simpler process is to use the selenitic cement, in which the necessary additions have already been carefully made.
Bad lime is much improved by mixing Portland cement with it. Gillmore says that lime paste may be added to a cement paste in much larger quantities than is usually practised in important works without any considerable loss of tensile strength or hardness. There is no material diminution of strength until the volume of lime paste becomes nearly equal to that of the cement paste, and it may be used within that limit without apprehension under the most unfavourable circumstances in which mortars can be placed. The following was used in the outer wall of the Albert Hall: - 1 Portland cement, 1 grey lime (Burham), 6 clean pit sand. The lime was slaked for 24 hours, then mixed with sand for 10 minutes; the cement was added, and the whole ground for 1 minute. Such a mixture must be used at once.
"Grout" is very thin liquid mortar, sometimes poured over courses of masonry or brickwork, in order that it may penetrate into empty joints left by bad workmanship or owing to the uneven character of the building material. It may also be necessary in deep narrow joints between large stones. It is deficient in strength, and should be avoided when possible.
Fat lime mortars, unless improved by adding pozzuolana and similar substances, are so wanting in strength that any precautions in using them are of little avail. In using hydraulic limes and cements it should be remembered that the presence of moisture favours the continuance of the formation of the silicates, etc, commenced in the kiln, and that the setting action of mortars so composed is prematurely stopped if they are allowed to dry too quickly. It is, therefore, of the utmost importance, especially in hot weather, that the bricks or stones to be imbedded in the mortar should be thoroughly soaked, so that they cannot absorb the moisture from the mortar, as well as to remove the dust from their surfaces, which would otherwise prevent the mortar from adhering. Mortar should be used as stiff as it can be spread; the joints should be all well filled. Grout should never be used, except where, from the position of the joint, it cannot be filled by mortar of proper consistence. In frosty weather, the freezing and expansion of the water in the mortar disintegrates it and destroys any work in which it may be laid.
Mortar should always be placed for the use of the builder on a small platform or "banker," or in a tub, to keep it from the dirt.
Masonry - Brickwork.