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cements are slow-setting, and these do not contain 

 in..:,- ilian 22 per cent, of clay. They set under 

 \\.itiT wlirn half i hdr weight consists of clay. The 

 proportion of sand used with Roman cement should 

 nut Hindi exceed that of the cement. When em- 

 ]ilit\vd for external coatings of buildings it is apt 

 ;<> I'tlloresce and become unsightly. 



I'lirthtml < ' mcnt. This is considered by far the 

 most important of the stone cements. It is an 

 aitiiirial product, named from its resemblance to 

 1'oiil.iml Stone, but is much more largely used 

 tlian KOI 1 1, i ii cement. In the manufacture of Port- 

 Lux 1 cement on the banks of the Thames and the 

 M cil way by the wet process, three parts of white 

 dmlk are mixed with one part of clay or mud from 

 the lower reaches of these rivers. The two sub- 

 stances, along with water, are placed in a 'wash 

 mill ' in which strong revolving knives or cutters 

 reduce the whole to a creamy ' slurry ' or slip. 

 The slurry then passes by gravitation to backs or 

 reservoirs. There it is allowed to settle for some 

 weeks, when the superfluous water is removed by 

 decantation. The mixture is next dried on heated 

 iron plates or on the floor of a heated chamber, 

 and then burned in kilns. Finally it is ground to 

 a fine powder. Modifications of the wet process 

 by which the large reservoirs are dispensed with 

 have been introduced in recent years. In other 

 parts of the country Portland cement is manu- 

 factured by the dry process from the hard lime- 

 stones of other formations than the chalk, along 

 with clay or shale. These limestones are crushed 

 small, mixed in the proper proportion with clay 

 or shale, then roughly burned, and ground to 

 powder. This powder slightly moistened is passed 

 through a pug-mill, ana then made into bricks, 

 which are afterwards burned in kilns and reduced to 

 powder. 



Since Portland cement is hardly ever employed 

 in the pure or neat state, its strength is perhaps 

 best tested when it is mixed with an equal weight 

 of sand. The best cement so mixed and moulded 

 in the state of a stiff mortar, into any convenient 

 shape, when tested after the lapse of seven days, 

 during six of which it is customary to keep it 

 immersed in water, exceeds in tensile strength 

 200 Ib. per square inch, and in crushing strength, 

 tested by 1 ! indi cubes, 1000 Ib. for the same area. 

 Its strength in the unmixed state is much greater. 

 Much of the Portland cement made is, however, 

 little more than half as strong as the best kind. 

 Roman cement of good quality mixed to the same 

 extent with sand as the above, and tested under the 

 same conditions, has on an average a tensile strength 

 of 30 Ib., and a crushing strength of 200 Ib. in each 

 case per square inch. Portland cement is slow in 

 setting compared with most varieties of Roman 

 cement. Both Portland and Roman cement form 

 hydraulic mortars that is, they set under water. 

 No mortar will do this which contains less than 

 10 per cent, of silica. 



Till close on 1840 Portland cement was hardly 

 known, but the use of it has extended rapidly, 

 especially in recent years. Its most important 

 application is in the construction of docks and 

 harl>ours, many of which are partly or wholly 

 built of it, mixed with sand and broken stones, in 

 the form of a concrete. In this state, or simply 

 mixed with sand, it is also much employed for 

 other purposes where strength and durability are 

 required. Owing to the nature of some of the 

 extensive engineering works in which Portland 

 cement is largely used, it is plainly of great con- 

 sequence that its properties should be thoroughly 

 understood. Numerous failures with it have taken 

 place. The chemical investigation into the case 

 of the Aberdeen docks in 1887 distinctly showed 

 the deleterious action of sea- water upon this sub- 



stance. But it is also known that object* made of 

 unmixed Portland cement from the works of gome 

 of the best makers will sometimes keep good for 

 nearly twenty years, and then crumble to niece* 

 even when not exposed out of doors at all. Of 

 course explanations of these failures are forth- 

 coming. They are generally attributed to carelewt- 

 ness in the manufacture of the cement, or in the 

 selection of the materials for it. But if they occur, 

 as they have done, with cements that have stood 

 very well the ordinary mechanical tests, how can 

 any cement of this kind be entirely depended upon 

 for durability? Twenty, thirty, or even fifty .M-ais 

 is far too short a time to test the lasting property 

 of a building material of this nature. The uxe of 

 Portland cement in pavements and for architectural 

 ornaments is not attended with much risk, and 

 for such purposes it is very suitable. The capital 

 employed in the manufacture in Great Britain is 

 probably near two millions sterling. For American 

 cements, see ROSENDALE. 



Scott's Selenic Cement consists of burnt limestone 

 mixed with about 5 per cent, of sulphate of lime 

 in the form of plaster of Paris, and ground to 

 powder. The presence of the sulphate arrests the 

 slaking action of the lime, causes the cement to 

 set more quickly, and admits of more sand being 

 used with it than ordinary lime does. This cement 

 has been a good deal used for plastering, and to 

 some extent also for mortar. 



Plaster of Paris (see ALABASTER and GYPSUM). 

 This material is used for cementing marble and 

 alabaster in much the same way as mortar is in 

 brick-work. It is also employed for uniting the 

 separately moulded pieces of any larg^e object cast 

 in the same material. Sometimes it is selected 

 for fixing metal mounts to glass. 



Keenes Cement is made by saturating plaster of 

 Paris in small lumps with alum and recalcining 

 it. It then forms a nard plaster for the. projecting 

 portions of halls and rooms, such as pilasters, 

 columns, and skirtings. It is capable of taking 

 a high polish. 



Parian or Keating's Cement somewhat resembles 

 Keene's. In its manufacture borax as well as 

 alum is added to the plaster of Paris. 



Martin's Cement is another kind, with plaster of 

 Paris for its basis, but instead of borax, carbonate 

 of potash is added, and sometimes hydrochloric 

 acid as well. With the exception of Scott's, these 

 plaster of Paris cements are only used in plastering 

 or other internal work not for mortars. 



Mastic Cement, consisting of a mixture of burnt 

 clay or limestone in a powdered state, with boiled 

 oil and litharge, was more in use formerly than 

 now ; but though expensive, it is an excellent 

 material for preventing the admission of rain-water 

 at certain joints about buildings, such as where 

 wood and stone work come together at windows. 

 It was also used for covering external mouldings. 



Rust or Iron Cement. Joints in iron- work, such 

 as those for hot-water pipes, are filled up with a 

 cement of iron borings or turnings, mixed with 

 at least 2 per cent, of sal-ammoniac. Sometimes 

 sulphur in powder is added. The iron oxidises 

 ana forms a firm joint. 



Sulphur Cement. For jointing earthenware 

 pipes, and occasionally for fixing bars of iron 

 into stone, a cement is made of sulphur, resin, and 

 brick-dust. It is a cheap but not a strong cement 

 where metal is concerned. 



Water-glass Cements. For furnaces one kind 

 consists of burnt and unburnt fireclay made 

 plastic with silicate of soda or water-glass. 

 Another cement, capable of standing a high neat, 

 is formed of asbestos powder made into a paste 

 with silicate of soda. The same silicate mixed 

 with ground glass makes an acid-proof cement. 



