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An hydraulic lime is one which will set and harden under water. This property is common to several well-known materials used for constructional purposes, notably Portland cement. The power of setting and hardening under water in the case of hydraulic lime is due to the formation during the burning of the lime of certain chemical compounds of silica, alumina and lime, of which tri-calcic silicate (3CaO.Si02) is the chief. Other compounds such as di-calcic aluminate (2Ca0.A1203) and certain ferrites of limes are also formed, but these act only as fluxes and aid in the
combination of lime and silica, and in themselves do not increase the hydraulic properties of the lime.
Henry Pooley, Junr.
A reply to J. Singleton-Green, Esq., B.SC., A.M.I.Struct.E. Sir,-If the discussion in your paper upon Concrete Roads is to be continued for the benefit of those really interested in this important branch of engineering it appears necessary to focus it upon those lines which lead to useful information.
With the gradual raising of concrete engineering into a definite branch of science, the need for accuracy in every detail and, therefore, for testing at each successive step, has brought about the introduction of quite a large number of appliances used in controlling operations. Such checking of operations is required not only in connection with the primary materials-cement, sand, the larger aggregates and water-but the secondary adjuncts, including reinforcements, waterproofing or other ingredients, and also the mixtures at various stages, as well as the completed work, whether this be in the form of mass construction or moulded units.