Page:The American Cyclopædia (1879) Volume IV.djvu/192

 184: CEMENTS lime, SiO 3, 3CaO, and A1,0 S , 3CaO, which on the addition of water become hydrates, SiOs, 3CaO, 6HO, and A1 S O S, 3CaO, 6HO. But if the calcination has been effected with intense heat, so as to produce partial vitrification, the reaction will become more complex, silicate of alumina and silicate of lime being formed first ; after which, on the addition of water, de- composition takes place with the formation of hydrosilicate and hydroaluminate of lime, each containing three equivalents of water instead of six. From the fact that increased hydraulicity is conferred on some earths which are used in making hydraulic cement by mix- ing them with lime that has been slaked a long time and exposed to the air, and therefore partially reconverted to a carbonate, it would appear that double decomposition is often fa- vorable to the silicification of the lime ; or in other words, that on quitting its union with carbonic acid it more readily combines, in con- sequence of being in a nascent state, with the silica, which on leaving its union with an alkali is also in a nascent condition. What in- fluence oxide of iron may have on the harden- ing of cements is still a matter of discussion. Experience sometimes seems to favor the idea that it is beneficial in ultimate hardening ; but very hard cements have been made which con- tained scarcely a trace of iron, and the subject must be considered as involved in doubt. The hydraulic cements principally employed in Eu- rope are the Eoman, Portland, Medina, Mul- grave, and those made on the continent from pozzuolana, trass, santorin, and Teil lime. Modern Roman cement, so called because of its similarity to the old Roman cement, is princi- pally made in England and France. The Eng- lish article is made from nodules, or septaria, found in the Kimmeridge and London clays. The first modern Roman cement was made by Mr. Parker of London, who patented his pro- cess in 1796 ; it consisted in calcining the sep- taria stones nearly to the point of vitrification, and then crushing and grinding them to powder without admixture of any other material, the product being therefore a natural cement. Ro- man cement is also made at Boulogne from nodules of septaria lying in the same geological formation, and does not essentially differ in quality from that made in England. Roman cement is now made in England from a clay shale found above the chalk formation in the isle of Sheppey and the isle of Wight. In Germany Roman cement is also made from limestones of a similar character. All the Ro- man cements contain a notable quantity of oxide of iron, as will be seen in the following table of analyses by Michaelis : CONSTITUENTS. 1. 2_ a 4. Lime B848 65-50 47-88 66'88 Magnesia 6-00 1-78 24-26 2-25 Silicic acid 28-88 25-00 Ml 28-66 Alumina 6-40 6-96 1-60 724 Oxide of iron... 4-80 9-68 jo-so 7-97 No. 1 is Roman cement from the Rudersdorf limestone ; 2, from limestone shale from the isle of Sheppey ; 3, from limestone forming the under bed of the lead ores at Tarnowitz ; and No. 4 is from Hausbergen limestone. Portland cement, an artificial compound, so named from its resemblance to Portland stone, was first manufactured by Joseph Aspdin of Leeds, Eng- land. His method was, according to his let- ters patent, to pulverize limestone and burn it in a kiln ; then to add an equal weight of clay and thoroughly knead the mixture with water to a plastic mass, which was dried, broken in pieces, and again burnt to expel all the car- bonic acid. Since that time, however, great improvements have been made in its manufac- ture, both in England and on the continent. To obtain the best results, it has been found that the lime should be in a state of carbonate when it is mixed with the clay, and there- fore, on account of greater facility and cheap- ness, chalk is employed instead of limestone. Two methods are practised, one in England and the other in Germany, which are some- what different. In England the wet method is followed, which consists in mixing the ma- terials together with water and grinding them to a pulp, which, after evaporation to the proper degree of stiffness, is made into balls or bricks, which are dried and then calcined to near the point of vitrification, after which they are ground in a mill. The proportions of the ingredients are from 65 to 75 per cent, of chalk to 25 or 35 per cent, of clay. In Ger- many the dry method is preferred, which con- sists in first drying the materials and reducing them to powder separately, after which they are mixed with water in the proper propor- tions, and made into bricks, which are then dried, calcined, and ground, as in the English method. The Germans also add a certain quantity of alkali, either soda or potash, de- pending upon the quantity which may already be contained in the clay, but enough to bring the amount to 3 or 4 per cent, of the whole, for the purpose of forming a soluble silicate to act upon the lime in setting; and sometimes a small amount of finely pulverized sand or quartz rock is added. In making mortar of Portland cement Mr. Lipowitz, a German manufacturer, recommends the use of about 100 parts of ce- ment to 44 parts of water, but the propor- tions are subject to variation. Analyses of four varieties of Portland cement made in Eng- land and Germany are given by Michaelis : CONSTITUENTS. 1. 2. 8. 4. Lime 69-06 62-8J CO-88 56-06 Silicic acid 24-07 28-22 26-98 22-92 6-92 6-27 T-IVI 8-00 Oxide of iron 8-41 2-00 2-46 6-46 0-62 1-14 0-28 0-77 Potash 0-78 ( 1 0- I 0-94 1-18 Soda 0-87 1-2 ' 0-80 1-70 ftiilpjmt* nf Hm, . . . 2-85 ' 1-80 1-62 1-75 Clay sand 1-47 2-64 1-04 2-27 Total... 100-20 99-55 99-84 99-06