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CEM

^unt - Ual cements are found in Russia, France , and otherles> an d indeed tlie substance so extensively used innot^H an< l very improperly termed Roman cement, iscai„ ; 51 m °re than a natural cement, resulting from a slightc ei) t p 0n °‘ a calcareous mineral, containing about 31 per°f 0 °chreous clay, and a few hundredths of carbonate

M],

ma gnesiaea the

and manganese. It may be observed, that

27 ^ “‘ B proportion of clay in calcareous minerals exceedsCulci • P er cent > R is seldom converted into lime byW hic ,; t,on . hut they then furnish a kind of natural cement,tvat er Can be use< i pulverizing it, and kneading it with

for ]!> ere are s °roe natural cements which do not set in waterSoljjip 11 ^ days ’ ^ ut R iese are novv rarely used ; those whichp 0We / fffiickly, being generally preferred. The adhesiveand wp°t Sotne cements in the open air, is very remarkable;by p ' have ourselves seen 33 bricks stuck to one anotherside °J nan cement, and projecting at right angles from the•pj a wall.

of p 1Jr aa g<llaceous limestones, and the artificial mixturesstituf e 1 Une an< ^ cRuy; * n tRe proportions requisite to con-Ua( llr ^] '3'firaulic lime by the ordinary calcination, become" ‘ or artificial cements, when they have been subjected

Merely

(j ;i | c 10 a simple incandescence, kept up for some minutes,follow aieo «s cements may be classed according to the threeevents’* <bv ’ s ' ons > namely, simple calcareous cements , water1st S \J na ^ la i and mastics.

U)oft a ’ lm ple calcareous cements include those kinds of° f Ce r hi ? h are employed in land-building, and consistC;d c ' aan( l) and fresh water.

' a S> wlM° US eart l* s are converted into quick-lime by burn-? 0 'vd er c *, Reing wetted with water falls into an impalpableh is ijp great extrication of heat : and if in this stateaec orne ^ sa nd and water, the mass will concrete anda<!Cor din» St0n .y su bstance, which will be more or less perfect°f hio>„ v *° * ta treatment, or to the quality and quantities^j>redients.

^ e priy e ^ car honated lime has been thoroughly burned, it isac id. jj. °f Rs W'ater, and all, or nearly all, of its carbonicci . °f the water, during the process of calcination,Lir ne _ ri . le< ^ °ff in the form of steam.

' v R<:n f u j S j 0ne loses about ■* of its weight by burning, andln ° r e th an ^ ^ urne d, falls freely, and will produce something^'suve ti '° u ^ e the quantity of powder, or slacked lime, inthe burnt lime-stone consisted of.a<! 'd witij Une ’ b >' ' Je i n g exposed to the air, absorbs carbonic^C d sreat ? r or less rapidity, as its texture is less or

1<lr tl le U * Q ’ an< l this, by continued exposure, becomes unfitf'ade of °! n P° s >tion of mortar; hence it is that quick-limebet ' v ®en *i la 'k’ cannot be kept for the same length of time^°tie. le burning and slacking, as that made from

V^k, an d limestone, with respect to their use inr Pur e L , a y R e divided into two kinds—simple lime-stone,c °!' ta ins f r onate of lime, and argillo-ferruginous lime, which^inatjQ, ° ni 'S'V t0 tV °f clay, and oxide of iron, previous tol) b l ^ s tin„ ’. R‘ ere are no external marks by which these canl) 66 ' 1 the col e< ^- ^ rora ea ch other, but whatever may havet^^es w ,° Ur * n ^ le crude state, the former, when calcined,ute > and the latter more or less of an ochreyI,,. 0 'Qorta^ ' v ’ b ' te Rinds are more abundant, and when madec >, Con ; v 'H admit of a greater portion of sand than thefo^P° s ition e 3 Uent ’y are niore generally employed in thea U kind 0 mortar ; Rot the brown lime is by far the besti tlt fcn tly s j ac k cement. If white, brown, and shell lime,° a a tifl‘ n ea> . b e separately beat up with a little water" aste > R will be found that the white lime, whether

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made from chalk, lime-stone, or marble, will not acquire anydegree of hardness; the brown lime will become considerablyindurated; and the shell lime will be concreted into a firmcement, which, though it will fall to pieces in water, is wellqualified for interior finishings, where it can be kept dry.

It was the opinion of the ancients, and is still receivedamong our modern builders, that the hardest lime-stone fur-nishes the best lime for mortar; but the experiments ofDr. Higgins and Mr. Smeaton have proved this to he a mis-take, and that the softest chalk lime, if thoroughly burned,is equally durable with the hardest stone lime, or evenmarble: but though stone and chalk lime are equally goodunder this condition, there is a very important practicaldifference between them ; as the chalk lime absorbs carbonicacid with much greater avidity; and if it be only partiallycalcined, will, on the application of water, fall into a coarsepowder, wffiich stone lime will not do.

For making mortar, the lime should be immediately usedfrom the kiln; and in slacking it, no more water should beallowed than what is just sufficient: and for this purposeDr. Ilijrsins recommends lime-water.

The sand made use of should be perfectly clean; if thereis any mixture of clay or mud, it should be divested of either,or both, by washing it in running water. Mr. Smeaton hasfully shown hy experiments, that mortar, though of the bestquality, when mixed with a small proportion of unburntclay, never acquires that hardness, which, without this addi-tion, it speedily would have attained. If sea-sand be used,it requires to be well washed with fresh water, to dissolvethe salt with which it is mixed, otherwise the cement intowhich it enters, never becomes thoroughly dry and hard.The sharper and coarser the sand is, the stronger is themortar; also a less proportion of lime is necessary. It istherefore more profitable to use the largest proportion ofsand, as this ingredient is the cheapest in the composition.

The best proportion of lime and sand in the compositionof mortar is yet a desideratum.

It may be affirmed, in general, that no more lime isrequired to a given quantity of sand, than what is justsufficient to surround the particles, or to use the least lime,so as to preserve the necessary degree of plasticity. Mortarin which sand predominates, requires less water in preparing,and therefore sets sooner: it is harder, and less liable to crackin drying; for this reason, that lime shrinks greatly in dry-ing, while sand retains its original magnitude. We areinformed by Vitruvius , lib. ii. chap, v., that the Romanbuilders allowed three parts of pit sand, or two of river orsea sand, to one of lime; but Pliny , Hist. Nat. lib. xxxvi.,prescribes four parts of coarse sharp pit sand, and only oneof lime. The general proportion given by our London builders, is 1^ cwt., or thirty-seven bushels of lime, and 2£loads of sand; but if proper care were taken in theburning of the lime, the quality of the sand, and in temper-ing the materials, a much greater quantity of sand mightbe admitted.

Mr. Smeaton observes, that there is scarcely any mortarbut which, if the lime be well burned, and the compositionwell beaten in the making, will require two measuresof sand to one of unslacked lime; and it is singular, thatthe more the mortar is wrought or beat, a greater proportionof sand may be admitted. He found that by good beating,the same quantity of lime would take in one measure ofterras, and three of clean sand, which seems to be thegreatest useful proportion.

Dr. Higgins found that a certain proportion of coarse andfine sand improved the composition of mortar; the bestproportion of ingredients, according to experiments made