It was the opinion of the ancients, and is still received among our modern builders, that the hardest lime-stone fur nishes the best lime for mortar ; but the experiments of Dr. Higgins and Mr. Smeaton have proved this to be a mis take, and that the softest chalk lime, if thoroughly burned, is equally durable with the hardest stone lime, or even marble : but though stone and chalk lime are equally good under this condition, there is it very important praeti•al diffiTence between them ; as the chalk lime absorbs carbonic acid with much greater avidity; and if it be only partially calcined, will, on the application of water, fall into a coarse powder, which stone lime will not do.
For making mortar, the lime should be immediately used from the kiln ; and in slacking it, no more water should be allowed than what is just sufficient: and fur this purpose Dr. Higgins recommends lime-water.
The sand made use of should be perfectly clean ; if there is any mixture of clay or mud, it should be divested of either, or both, by washing it in running water. Mr. Smeaton has fully shown by experiments, that mortar, though of the best quality, when mixed with a small proportion of unburnt clay, never acquires that hardness, which, without this addi tion, it speedily would have attained. If sea-sand be used, it requires to he well washed with fresh water, to dissolve the salt with which it is mixed, otherwise the cement into which it enters, never becomes thoroughly dry and hard. The sharper and coarser the sand is, the stronger is the mortar; also a less proportion of lime is necessary. It is therefore more profitable to use the largest proportion of sand, as this ingredient is the cheapest its the composition.
The best proportion of lime and sand in the composition of mortar is yet a desideratum.
It may be affirmed, in general, that no more lime is required to a given quantity- of sand, than what is just sufficient to surround the particles, or to use the least lime, so as to preserve the necessary degree of plasticity. Mortar in which sand predominates, requires less water in preparing, and therefore sets sooner : it is harder, and less liable to crack in drying; for this reason, that lime shrinks greatly in dry ing, while sand retains its original magnitude. We are informed by Vitruvius, lib. ii., chap. v., that the Roman builders allowed three parts of pit sand, or two of river or sea sand, to one of lime; but Pliny, Hist. Kat. lib. xxxvi., prescribes four parts of coarse sharp pit sand, and only one of lime. The general proportion given by our London
builders, is 11-, cwt., or thirty-seven bushels of lime, and 21, loads of sand ; but it' proper care were taken in the burning of the lime, the quality of the sand, and in temper ing the materials, a much greater quantity of sand might be admitted.
Mr. Smeaton observes, that there is scarcely any mortar but which, if the lime be well burned, and the composition well beaten in the raking, will require two measures of sand to one of unslacked lime; and it is singular, that the more the mortar is wrought or beat, a greater proportion of sand may be admitted. Ile found that by good beating, the same quantity of lime would take its one measure of tetras, and three of clean sand, which seems to be the greatest useful proportion.
Dr. Higgins found that a certain proportion of coarse and fine sand improved the composition of mortar; the best proportion of ingredients, according to experiments made by him, is as f4illows, by measure : Lime, newly slacked, one part; fine sand, three parts ; coarse sand, four parts. He also tInind that in addition of one-fourth part of the quantity of lime, of burnt bone-ashes, improved the mortar, by giving it tenacity, and rendering it less liable to crack in dry ing.
The mortar should be made under ground, then covered ap, kept there for a considerable length of time, the longer the better ; and when it is to be used, it should be beat up afresh. This makes it set sooner, renders it less liable to crack, and harder when dry.
The stony consistence which it acquires in drying, is owing to the absorption of carbonic acid, and a combination of part of the water with the lime : and hence it is that nine that has been long kept after burning is unfit for the purpose of mortar, for in the course of keeping, so much carbonic acid has been imbibed as to have little better effect, in a composition of sand and water, than chalk or lime-stone reduced to a powder from the crude state, would have in place of it.
Grout is a cement, containing a larger proportion of water than is employed in comin4)11 mortar, so as to make it suffi ciently fluid to penetrate the narrow irregular interstices of rough stone walls. Grout should be trade of mortar dint has been long kept and thoroughly beat, as it will then concrete in the space of a day : NN hereas, if this precaution be neglected, it will be a long time beIbre it sets, and may even retnse setting for ever. See GROUT.