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lime, cement, water, sand, mass, proportion, mortar, hard, firm and particles

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QUICK-LIME, such lime as is in the caustic or most active state, and which possesses the greatest power of opera ting upon diGrent substances with which it may conic in contact. It is quite the opposite, in its qualities and proper ties, to that which has fallen down into a powdery state, in consequence of being saturated with water and carbonic acid gas, or fixed air, or which is slaked and become effete.

According to Dr. Anderson, lime is in the best and most fit state fur the purpose of cement, when most perfectly caustic, or in the most crystallizing condition. It is remarked, that the powder of lime, when reduced by means of water into a fluid, or thin paste-like form, and then suffered to become dry, concretes into a coherent mass, which fixes to stones and other rough bodies in a very firm manner, and in this way becomes a proper cement for building any sort of walls. And that, after this pasty material has one become firmly dry, it is quite indissoluble in water, and incapable of ever being softened again by the moisture of the atmosphere or other similar causes. Hence it excels many other sorts of cements.

When composed for the purpose of building walls, &,e.; it is usually denominated mortar ; but when formed as an appli cation in the way of a smooth coating upon any plain sur face without intermixture with stony matters, it is commonly termed plaster.

When made from the lime of the purer sort of lime-stone, it is found to be more soft and crumbly, and to acquire a less degree of hardness, and to be broken with much less force, than where the lime-stone, from which it is made, contains a large proportion of sand ; in which ease it becomes much more hard, firm and durable.

It has, however, been discovered, that the purest lime may be rendered a firm cement by adding a proper proportion of clean hard sand to it ; hence the practice of blending sand with lime, when intended for mortar, has become so universal. This is fully shown to have been very early the case, by the oldest lime-built walls which are now to be met with.

It nevertheless still remains a desideratum to ascertain the due proportion of sand necessary, as both writers and prac tical masons greatly disagree in opinion on this matter, as well as in their directions about the mode of mixing the mate rials, and of applying the cement ; some of the more modern, especially ascribing extraordinary of cts to a small variation in these particulars, while others deny that these circumstances have any sensible effect on the durability and firmness of the cement.

It is conceived that these different and contradictory opinions arise from the imperfect knowledge of the nature of quielt-lime, and the variations it may admit of ; for these variations are so very great, as to render it impossible to afford any generid rules that can possibly apply in all cases. It is, therefore, conceived to behove those who are desirous of acquiring any consistent and satisfitetory know ledge on this head, to endeavour to ascertain, in the first place, the circumstances which render calcareous substances at all capable of becoming a cement, and then to trace the several changes that may be produced upon it by extraneous causes.

having explained the circumstances which cause the differences in limestone, and pointed out the different con stituent principles of it, as well as various other peculiarities ; it is known, that lime which has in any way absorbed its full quantity of air from the atmosphere, ant become mild, is altogether unfit for becoming a cement, and that, of course, a great change may be produced upon the quality of any lime, by having allowed less or more of it to be in this state before it is worked up into mortar. And farther, that if a large

quantity of water be put to fresh-slaked quick-lime, and beat up with it into a thin sort of paste, the water dissolves a small portion of the lime, which, as it gradually absorbs its air, is converted into crystals ; between the particles of which crystals, that part of the lime which was not dissolved, and the other extraneous matters which may have been mixed with it, are entangled, so as to form a firm coherent mass of the whole. And that the pasty substance formed in this manner, is the well-known article mortar ; and this hetero geneous, imperfectly semi-crystallized mass, constitutes the common cement employed in building ordinary walls, or other erections. These circumstances, therefore, being known, it is thought that it will not be difficult to comprehend what are the particulars necessary to firm the most perfect cement of this nature. That since lime becomes a cement only in con sequence of a certain degree of crystallization taking place in the whole mass, it is sufficiently obvious that the firmness and perfection of that cement must depend upon the perfec tion of the crystals, and the hardness of the matters that are entangled among them ; for if the crystals are ever so perfect and hard of themselves, if they be separated from each other by any brittle incoherent medium, it is evident that the whole mass must remain in some degree brittle and incoherent. That as water can only dissolve a very small proportion of lime, even when in its most perfect saline or caustic state, or while it remains deprived of its carbonic acid gas, and, as happens in other similar cases, no more of the lime can be reduced to a crystalline mass than has been actually dissolved in the water ; it follows, of course, that if mortar be made of pure lime and water alone, a very small proportion only can be dissolved by that small quantity of water that is added to it ; and as this small proportion alone can afterwards be crystallized, all the remaining undissolved particles of the lime will be entangled among the few crys tals that arc formed. And as the undissolved lime in this mass will in time absorb its air, and be converted into 71Zild calcareous earth, without having had a sufficiency of water to allow it to crystallize, it must concrete into a friable mass, exactly resembling chalk ; this kind of mortar, therefore, when as dry as it can be made, and in its highest degree of perfection, will always be soft., and easily crumbled into pow der. But if, instead of forming the mortar of pure lime alone, a large proportion of sand be added to it, the water will, in this case, dissolve as much of the lime as in the former ; and the particles of hard sand, like sticks or threads, when ma king sugar-candy or other crystals, while surrounded by the watery solution, will help to forward the crystallization, and render it more perfect than it otherwise would have been, so as firmly to cement the particles of sand to each other. And as the granules of sand are perfectly hard of themselves, so as not to admit of being broken down like the particles of chalk, it necessarily follows, that the cement made of these mate rials must be much more perfect, in every respect, than the former.

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