CEMENT AGAIN.

Otago Daily Times, Issue 7777, 22 January 1887, Page 4

CEMENT AGAIN.

The following extract from Scientific' Notes in '7 the Australasian of January lwUl beY found - interesting:—Some time ago I met a friend who V was not scientific, and who. had gone in for the . cement trade, and dropped a lot of mone Jr. therein. I asked him how'lle.was getting on, and he exclaimed, "Oh, we're, making soma splendid cement; but, -it, it wont stick." I am reminded of this fact because M^THichel, a French chemist, has been investigating the reason-why cement "sticks." or, sets at all. I; . shall communicate the sameVresultjsomeday toVmy friend, but, meantime, I hastentb,, give it to. sV my readers.. The setting of cement is; depend- - ent, first of all, upon the addition^, of.water,' which should be as limited and uniform as pos- ~ sible. Next, on the density-.and intimate mix- ; ture of the materials; and, lastly, on the formation of carbonate of ; lime. ' The thrise . principal constituents • of:'. cement:—■ silica, V lime, and alumina (or clay)'are all chemi- , cally active, but the proportion ,of lime heed " only be slight. It is the Alumina ;or clay' which determines the rapid/setting, but the, silica determines the permanent, .hardening. The chemical change which causes the]setting . only takes place. to a slight degree, and ie, principally restricted to the, surface!; Ohau'. average not more, than five: per cent, of the V materials become chemically combined. The alkalis seem to play no important:part in the setting of the cement. In Portland,cement, on the other hand, the hardening depends oh' the L V disruption of every grain after it has been baked and moulded. The grains are then covered with moisture and carbonate of lime,'and colloid (or '.',' liquid) silica flows, round them, and gives. them a certain crystalline structure. This process, however, depends on temperature; but silic*' seems to play tbe most important part in the process. ..*.. '-rV*V':. . VVV:*' ,", ..V