Bourne Archive: Muspratt: Cement
http://boar.org.uk/aaiwxw3Muspratt1Cement.htm Latest edit 17 May 2011
The Bourne Archive
Extracts Concerning Building Cements, 1.
This is part of an
article on chemical bonding materials under the heading ‘Cement’. It is
presented here in three web pages, respectively dealing; 1, with fairly simple
lime burning kilns; 2,
with a large lime kiln in
Vol. 1. pp. 453-4
BUILDING CEMENTS. – Having thus briefly enumerated the principal cementing materials employed for miscellaneous purposes, attention will now be directed to the more important part of the subject which relates to architecture. In this department, the substances which are commonly known by the term cement, whether hydraulic or otherwise, deserve particular notice as they contribute to the solidity and durability of the building in a remarkable manner. And being so important, it would be well if the rising architects of the present age, devoted more to their studies to the consideration of this subject.
How well do the enduring architectural remains of Egypt, Greece, and Rome, as also many of the edifices of he early and middle ages of our own era, testify to the quality of the binding medium employed, having withstood the assaults of time, whilst numerous others of later date have mouldered away, in consequence of this material being imperfect! It has been already stated that the action of hardening, as manifested by some bodies, is due to chemical as well as a mechanical agency; such is especially the case with mortars, and hence it will be necessary to dwell somewhat in detail upon this subject, commencing with ordinary or common mortar. But, as introductory to this, it may be proper, in the first place, to say a few words upon the principal component in cement – the lime, and to give a short description of the manner in which it is prepared.
LIME. – The great source of this base is the various chalk and limestone deposits found in the geological formations of every country; but besides these, very large beds of lime salts exist in many other states, and indeed it is met with more or less in all soils, in the ashes of most plants, and also in he bones of animals combined with various acids.
limestones, and other calcareous rocks, never exist in a pure state; for, besides the carbonate of lime, which is the principal, and nearly the entire ingredient, other substances such as magnesia, clay ferruginous and bituminous matters, are contained in them, and from which they obtain their specific names. Only calcareous spar, and a few other minerals, are entirely composed of pure carbonate of lime. Besides the designations magnesian, argillaceous, et cetera, limestones are often named from the peculiarity of their molecular arrangement. The mineralogists give the appellation of compact, pulverulent, chalky, lamellar, saccharoid, granular, concreted, oolitic, et cetera, to different limestones, according to their respective species.
Ordinary lime may be prepared from most of these; but the facility of so preparing it is greater in some cases than in others, and the lime itself manifests different characteristics. All that is required is to expel the carbonic acid; and heat is the best agent for effecting this. Kilns of various construction are employed, wherein limestones are burned by the agency of peat, wood, or coal, according as the facilities of the locality offer the one or the other in more or less abundance. The proper form of the kiln is a matter of much interest to lime-burners, as a great economy of fuel may be effected by having it of a certain shape; besides that, when properly constructed, it burns much better.
The more common form of kilns exhibits an elliptical section, the upper end being wider than the lower one, wherein is the eye or draft hole. This shape is more advantageous than that of an inverted cone, as some kilns are occasionally constructed, for the former concentrates the heat more towards the top, and therefore the material in this part is acted upon more than it would be if the top or mouth were wider than any other part. Greater facilities are offered for drawing off the burned lime, and the kiln itself is less injured, when the form is elliptical or oval.
But where large supplies of lime are required, these comparatively rude forms are laid aside altogether, and a more scientific construction is adopted. One of the most effectual of these is reported by Mr. LOUDON to be that invented by Mr. BOOKER of Dublin, but since considerably improved. It is composed of two long narrow truncated cones, placed base to base. The height of the kiln is from twenty to thirty feet, and the diameter in the middle is seven feet, and it is contracted to three at the top and bottom.
Fig. 297 shows a vertical section of this kiln, and Fig. 298 a transverse vertical cut, showing the position of the shed placed over it. In the first of these, a is the side opening to the back of the fuel chamber, which is about two feet square, with iron bars across. On each side of this chamber is an aperture, by which the air is carried to the back, and the entrance of these to the fire is shown at a. The top of the kiln is arched over, the arch springing from b. The kiln is fed through the apertures in the arch, and which have cast-iron covers, c c c, with lids turning on a pivot appended to them, by means of which the draught is regulated. In Fig. 298 the fuel chamber is shown at d, and e e are the air-flues between the double doors of the chamber, A is the space where the loading carts stand; c, the cast-iron cover of the feeding aperture, and h, the cover of the chimney of the kiln shed, . This shed over the mouth of the kiln is very beneficial in keeping the materials dry, and heating them more or less before they are admitted into the shaft. C represents the door by which the limestones are conveyed into BB. The kiln is usually built on the face of a steep bank, and is constructed of fire-brick or fire-stone.