The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

THE GEOLOGY OF CEMENTS 187 from the London clay septaria, and was called Roman cement from the erroneous tradition that the Romans made cement from them. They consist of nodules of calcareous clay traversed by septa of calcite; their usual composition is about silica 18 per cent., alumina 3 to 5 per cent., iron oxide, 5 per cent., lime 30 per cent., and carbon dioxide 31 per cent. Hydraulic cement passes by the reduction in uncombined lime into Portland cement, which is made from a mixture of finely ground limestone and clay; the mixture is heated to a little below its fusion point—usually to between 2600° and 3000° F.—when the materials by diffusion form new com- pounds. As the most important constituent in Portland cement is the tricalcic silicate, 3Ca0, SiO,, its ideal composi- tion is lime 73-6 per cent., silica 26:4 per cent. The constitu- ents have to be mixed in precise proportions, and therefore soft pure materials are desirable, such as chalk and river clay. The third group, including the Roman Pozzolana cement, are made from volcanic tuff such as the Trass of the Rhine. The glass in these tuffs is unstable and combines with lime to form silicates of lime and alumina without the use of heat. Blast furnace slag, which is also a silicate glass, and various organic materials, such as diatom earth, which consist of unstable amorphous silica, make similar cement when ground with lime. The three types of hydraulic cement depend upon analo- gous chemical processes. The modern interpretation of the constitution of Portland cement was founded by H. Le Chatelier (1883, etc, and his Constitution of Hydraulic Mortars, New York, 1905). He regarded Portland cement as consisting of crystals of tricalcic silicate in a crystalline ground mass. Later researches show that the constitution is more complex, and is dependent upon the interaction of several lime silicates and lime aluminates, each of which has a definite composition, specific gravity, and optical properties. These artificial mineral species at a high tem- perature, below that of fusion, combine in solid solution and form what Tornebohm called alite and celite. Alite is a solid solution of tricalcic aluminate (3Ca0, AlOg) in tricalcic silicate (3Ca0, Si0,), and celite of dicalcic aluminate (2Ca0, Al, Oy) in dicalcic silicate (2Ca0, SiO). According to. Cl. Richardson (Eng. News., liii, 1905,

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