The quicklime and sand having been procured, and their proportions decided, the preparation of the ingredients commences.
In a short time - varying according to the nature of the lime - it will be found thoroughly slaked to a dry powder.
In nearly all limes, however, there will be found overburnt refractory particles, and these should be carefully removed by screening - especially in the case of hydraulic limes; for if they get into the mortar and are used, they may slake at some future time, and by their expansion destroy the work.
The fat limes may be slaked in any convenient quantity, whether required for immediate use or not. Plenty of water may be used in slaking without fear of injuring them, and they will be found ready for use in two or three hours.
Hydraulic limes should be left (after being wetted and covered up) for a period varying from twelve to forty-eight hours, according to the extent of the hydraulic properties they possess; the greater these are, the longer will they be in slaking. Care should be taken not to use too much water, as it absorbs the heat and checks the slaking process. Only so much should be slaked at once as can be worked off within the next eight or ten days.
With strong hydraulic limes, or with others that are known to contain overburnt particles, it is advisable to slake the lime separately, and to screen out all dangerous lumps, etc., before adding the sand, or the safest plan is to have the lime ground before using it.
When lime is purchased ready ground there is sometimes danger of its having become air-slaked, by which wear and tear of machinery in grinding is saved at the expense of loss of energy on the part of the lime.
At the same time, if unadulterated and fresh, ground lime is likely to be of good quality for the reasons stated at p. 155.
The quantity of water required for slaking varies with the pureness and freshness of the lime, and is generally between one-third and one-half of its bulk.
A pure lime requires more water than one with hydraulic properties, as it evolves more heat and expands more in slaking.
A recently-burnt lime requires more water than one that has been allowed to get stale.
The great object in mixing is to thoroughly incorporate the ingredients, so that no two grains of dry sand should lie together without an intervening layer or film of lime or cement.
On extensive works a mortar-mill is universally adopted for mixing the ingredients, and, indeed, is absolutely necessary for the intimate incorporation of large quantities.
A few different forms of mortar-mill are shown and described at page 223 et seq.
The heap of slaked lime covered with sand, above described, (p. 202) is roughly turned over and shovelled into the revolving pan of the mortar-mill, enough water being added to bring the mixture to the consistency of thick honey.
When the ingredients are thoroughly mixed and ground together, the mortar is shovelled out of the pan on to a "banker" or platform to keep it from the dirty ground, whence it is taken away by the labourers in their hods.
A good deal has been said regarding the number of revolutions that should be given to the pan. Nothing seems to have been settled upon this point except that the mortar should be thoroughly mixed, yet not kept so long in the mill as to be ground to pap. About twenty minutes is a good time for running each charge of about 2/3 of a cubic yard.
On very small works the mixing is effected by hand or in a pug-mill. It is evident, however, that such a mixture must be very incomplete unless a great deal of time is devoted to it.
Before hydraulic lime is mixed in this manner it is absolutely necessary that it should first be ground to a fine powder, and with any description of lime the smallest refractory unslaked particles should be carefully screened out.
Mortar, when made with cement, should be mixed dry, the ingredients being carefully turned over together two or three times before the water is added. By this process a very thorough incorporation of the materials can be effected.
Quantities mixed. - If a hydraulic mortar is allowed to commence to set and is then disturbed, it is greatly injured. Care should be taken, therefore, to mix it only so long as is required for thorough reduction and incorporation of the ingredients, and only to prepare so much as can be used within a few hours. With fat limes it matters little whether large or small quantities of mortar are made at once, because they set very slowly.
Very quick-setting cements must he used immediately they are mixed.
The bulk of mortar produced in proportion to that of the ingredients differs greatly according to the nature of the lime or cement and the quantity and description of the sand added to it.
The more hydraulic limes produce a smaller amount of mortar because they expand less in slaking.
The following Table shows the bulk of mortar found by experiment to be produced from a few of the most common ingredients in ordinary use. It must be regarded only as a guide to the approximate quantities. The actual bulk would vary according to the freshness of the lime and the coarseness of the sand.