This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The materials are generally mixed in a dry state. The proportions decided upon are measured out either roughly by barrow-loads, or in a more precise manner by means of boxes made of sizes to suit the relative proportions of the ingredients to be used. Such boxes, in which the quantities to be mixed together can be accurately gauged, should always be used in mixing cement or other concretes intended for important work. The measured materials are then heaped up together, and turned over at least 2, better 3, times, so as to be most thoroughly incorporated. The dry mixture should then be sprinkled, not drenched, the water being added gradually through a "rose," no more being used than is necessary to mix the whole very thoroughly. If too much water be added, it is apt to wash the lime or cement away. The mixture should then again be burned over once or twice. When lime is used it should be in a fine powder. If a fat time (which is almost useless for concrete in most positions), it should be slaked and screened. If a hydraulic lime, it should be finely ground, or, in the absence of machinery or grinding, it should be carefully slaked, and all unslaked particles removed by passing at through a sieve or fine screen.
The lime is often used fresh from the kiln, piled on to he other ingredients during the mixing. This is apt to leave unslaked portions in the time, and is a dangerous practice. When Portland cement is used for concrete, it must the thoroughly cooled before mixing. Cements of the Roman class should be fresh.
When the mortar is prepared separately, and then added to the aggregate, it may bo mixed in mortar-mills, or by any other means available, the same precautions being taken as in mixing mortar for other purposes. The mortar should not bo too wet, but should, when added to the dry material, contain about as much moisture as coarse brown sugar. It can then be readily turned over and incorporated with the aggregate. The aggregate should be wet throughout, so that it may not suck the moisture out of the mortar.
Some engineers consider it important that the lime or cement and sand should be mixed dry with the aggregate ; others think that it is better to mix the mortar separately and then add it to the dry material. The relative advantages of these 2 methods depend opon circumstances. When the aggregate is in the form of sandy ballast or gravel, the second method could not be adopted without the expense of screening. The most ntimate mixture, and therefore (other conditions being the same) the best concrete, can probably be produced by mixing the matrix separately and adding it in a moist (not wet) state to the moistened aggregate. With quick-setting cements, this method seems to be open to the objection that the mortar will begin to set before being added to the aggregate, and that the setting process will be disturbed by the after process of mixing with ;he aggregate. As a rule, however, the second method is more expensive than that in which the dry materials are all mixed together; and when such is the case, it is not worth while to adopt it for ordinary concrete.