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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.
VII-GENERAL DISCUSSION. Qualities of the Aggregates Nothing was brought out in the tests to show marked differences in strengths because of the differences in the nature of the surface of the grains and the effect of smoothness or of differences in the porosity of the grain on the adhesion of the cement to the grain. There was some indication that the smooth and non-porous grain of the Ottawa sand gave slightly lower strength results. J. Singleton-Green