It was pointed out many years agol that the bad qualities of rich limes "may be in some degree corrected by the use of a comparatively small quantity of the coarsest sugar dissolved in the water with which they are worked up," and that sugar was extensively used in the East for common mortars made of calcined shells, which when well prepared "resist the action of the weather for centuries." A recent discussion on the subject has led to experiments being made to ascertain the effect of sugar on Portland cement; and it was found that the addition of from 1/8 per cent to 2 per cent of pure sugar to Dyckerhoff's German Portland cement increased its strength after three months considerably. The sugar is said to "retard the setting," and thus permit the chemical changes in the cement to take place more perfectly. More than 2 per cent of sugar made the cement useless.2
The licenses 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, which are taken from the circular of the patentees : -
N.B. - One bushel3 of prepared selenitic lime requires about six gallons of water (two full-sized pails).
1st, Pour into the pan of tbe edge-runner four full-sized pails of water.
2d, Gradually add to the water in the pan 2 bushels of prepared selenitic lime, and grind to the consistency of creamy paste, and in no case should it be thinner.
3d, Throw into the pan 10 or 12 bushels of 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 plasterer's tub (containing about 30 or 40 gallons) or trough, with outlet or sluice, may be substituted.
1 Vicat on Cements (Smith), published in 1837. 2 Engineering, 1888, p. 102. 3 A striked bushel = 1.28 cubic foot (see page 158).
1st, Pour into the tub 4 full-sized paila of water.
2d, Gradually add to the water in the tub 2 bushels of 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.
3d, Measure out 10 or 12 bushels of 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 two or three times, and well mixed with the larry or mortar hook.
N.B. - The Selenitic Cement Company recommend that the workman intrusted with the making up of the selenitic mortar be supplied with suitable measures for his lime and sand, to ensure that the proportions stated in the circulars be adhered to. The want of this frequently leads to unsatisfactory results.
A box measuring inside 131/8 inches by 131/8 inches by 131/8 inches would contain about 1 bushel, 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 inches by 18 inches by 18 inches would contain about 51/4 bushels, 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.
N.B. - 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 of lime for themselves, and where selenitic cement is not procurable the process might still be useful.
It is conducted as follows :
Three pints of plaster of Paris are stirred in 2 gallons of water. After the mixture is complete it is poured into the pan of a mortar mill; then 4 gallons of water are added, and the mill revolved three or four times, so as to ensure thorough mixing.
A bushel of finely-ground unslaked lime is now added; the mixing is continued till the whole becomes a creamy paste, and then 5 bushels of sand are gradually introduced, the whole being thoroughly mixed.
No more is mixed than will be required during the day.
If the mortar gets heated or sets too slowly, 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. Any lime requiring more than 71/2 per cent of plaster of Paris added to stop slaking with heat will require selenitic clay.
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 by the patentees.
The following Table, from the patentees' circular, show3 the strength of selenitic cement mortar with different proportions of sand as compared with mortars made with other cements.