Chap. V.

MORTAR AND CEMENTS.

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black colour: it is composed of silica and alumina, with a small quantity of othermatters; a portion analysed by M. Berthier contained silica 44'5, alumina 15, lime 8*8,magnesia 4 7, oxide of iron 12, soda 4, potash 1-4, water 9*2, in 100 parts, and the onlypreparation to which it was subjected was grinding and sifting; when reduced to a finepowder, it is beaten with a due proportion of lime to a proper consistency.

Puzzolana was obtained by the Romans near Babe, and, mixed with lime and smallstones, Vitruvius tells us it acquired great hardness from the moisture it absorbed, andthat it would resist the dashing of the waves and the action of sea-water. Acids will,however, act upon it, though some specimens are not at all affected by them ; others,when washed with sulphuric acid, become covered with an aluminous efflorescence; puz-zolana thrown into limpid lime-water decomposes it more or less, but a sufficient quan-tity restores it to a state of purity.

We find the mortars or cements used by the Romans in this island composed of chalklime, sand, pounded brick or tile dust, occasionally mixed with ashes of wood or charcoal,the residue of the hearth where the lime was burnt, and the carbonic acid of the chalk drivenoff; their mortar is remarkably hard, generally of a pinkish tint, and often when brokenexhibits cavities which contain crystals of the carbonate of lime. This kind of mortar isperhaps the most ancient of the artificial puzzolanas known, and we find it through-out Europe in every place occupied by the Romans; its weight and durability haveoccasioned it to be considered as more excellent in quality than that made at the presentday ; but this apparent advantage is entirely owing to time, which has permitted it to ab-sorb from the atmosphere a greater quantity of carbonic acid, and to become in consequencemore solid, and capable of bearing greater weight.

Numerous experiments were made during the last century' by the French chemists uponthe ochreous clays, in order to form a substance that would answ’er the same purpose asthe Italian puzzolanas ; and M. Bruyere has shown us that by a mixture of powdered clayand lime, in the proportion of three of the first to one of the latter, an active cement isformed, which sets under water in a few hours, but never arrives at a very great degree ofhardness : all kinds of clay composed of silica and alumina, and a little oxide of iron, andno carbonate of lime, if soft and fine in their textures, will produce, by proper treatment,a very active and energetic puzzolana.

A clay which does not effervesce in nitric acid, and when burnt has a brick-red colour, ifafter it is pulverised, it is thinly spread in a layer on a plate of iron, and heated to redness,allow'ed to remain about twenty minutes, taking care continually to stir or move it with arod, so that all the particles may be properly and thoroughly calcined, forms an excellentartificial puzzolana, which, mixed into a stiff paste with half its weight of lime, becomesvery hard in water.

Pipe clay, which contains a considerable quantity of silica, but no carbonate of lime, afterburning, will set, with the same proportion of lime as in the preceding example, into a veryhard cement, after it has been immersed five or six months.

Clay calcined at too great an heat loses its quality as a puzzolana, and it never shouldbe so great as is required to burn a sound brick; the artificial puzzolanas, made by theancients by pulverising old tiles, bricks, and the residue of their potteries, shows us thatthose materials in which there had been a deficiency of burning were employed in preference.

Sand varies in the size of its particles, and where rubble-work is employed, after thecoarse stones are filled in with the finer sorts, the interstices are run in with some cementingsubstance ; to estimate the exact amount of all these spaces, a measure is made of each ofthe varieties, and afterwards of the quantity of water to fill them up. In pebbles of abouthalf an inch in diameter, the void is equal to half the measure which contains them ; ingravels, five-twelfths; common sand, two-fifths; fine sand, one-third; and very fine sand,two-sevenths.

To ascertain what proportion of each, when mixed, would approximate a solid mass, orfill up all the voids, requires more consideration, though this is commonly done by firstfilling a measure with the larger stones, and then by degrees adding the finer; and whenall in the proportions of their bulk have been calculated, they may be put together, and ifthe measure, when duly shaken, is no more than full, we are sure the interstices are filled up.

Tlie sand usually preferred for making mortar has a sharp grit, and is obtained fromrivers; the proportions with which it is compounded with the lime varies in differentdistricts according to the quality. Mortar for ordinary constructions is composed of onepart of stone lime and three of sand mixed together ; the lime, being in a perfectly drystate, is thrown into a basin formed by heaping up the sand around it; water is thensprinkled or thrown on to slack the lime, and it is immediately covered over with sand ;after remaining some time in this state, and the whole of the lime has been reduced topowder, it is turned together, then passed through a wire screen, where all the core, or thatportion of the lime which has not slaked, is taken out. The whole has then more waterpoured upon it, and being triturated or larryed, is rendered fit for the workman.

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