Portland cement is made by heating a mixture of limestone and clay (or similar materials) in a kiln to such a temperature that fusion takes place and clinkers are produced. These clinkers are ground to a fine powder, and are usually mixed with about three per cent of gypsum. The resulting powder consists chiefly of calcium and aluminum silicates. On mixing with water, these compounds combine with it, chemically, forming hydrated silicates of calcium and aluminum.
Lest it should be thought that an unnecessary incursion is being made into pure chemistry, it may be said that a thorough understanding of the statements contained above would enable the amateur to avoid more than half the failures which follow his attempts at concrete construction. If you contemplate working with Portland cement, even if only to the extent of closing a crack in your cellar floor, grasp the fundamental fact: The setting of concrete is not a process of "drying out"; it is the exact opposite - a process of hydration. Unless sufficient water is present concrete will never, can never, set really hard and solid. Moreover, the setting is, in reality, a very slow process, and concrete seldom reaches its maximum hardness in less than a year. It is, however, during the first few days that a sufficiency of moisture is so important, and after the concrete is mixed and poured into the forms steps must be taken to keep it damp. It should be protected with canvas, wet sand, or some other simple covering, and both concrete and covering should be sprinkled with water sufficiently often to keep the whole mass thoroughly damp for some days.
As regards the amount of water to be used in making up the mixture, this will vary considerably according to the condition of the sand and stone mixed with the cement. When the sand is very dry, more water will be needed than when it is wet. The exact amount of water required, therefore, is seldom given, and it is almost entirely a matter of personal judgment. This frequently presents difficulties to the amateur, and he nearly always errs on the side of insufficiency. He finds it difficult to believe that the sloppy mixture which he has prepared will ever set to the hard, stonelike mass which he aims to produce, and he adds a little more cement to "stiffen it up." This is often a fatal mistake and results in a soft concrete. The consistency which gives the best results is that of a jelly. The mixture is sometimes described as "quaky," which means that it will "shiver," just as a jelly does when it is jarred. It should be neither thin and watery, nor yet so stiff that it will not flow.
Concrete is a mixture of cement, sand, pebbles, or small stones, and water. All four constituents may be considered equally important. Cement is manufactured under strict chemical control, and if you buy any of the well-known brands you may have absolute confidence in its quality. The important part played by water in concrete construction has already been indicated, and on that point little more need be said, except as to the quality of the water. One authority on concrete has made the statement that water that is good enough for concrete is good enough to be drunk. This may be carrying things a little too far, but it is necessary that the water should be clean and free from suspended matter.
The importance of having good quality sand and stone is apparent. Concrete may be looked upon as a mixture of these two materials bound together by cement. It follows, therefore, that if the sand and stones are soft and weak, the whole mixture will be soft and weak. Do not use sand that powders easily when rubbed, or that contains appreciable amounts of clay or other impurities, and choose pebbles that are hard, clean and smooth, or broken stone from granite, trap-rock, or other hard rocks. If sand or stones of this description are not available, make the best of poor material by washing it thoroughly before use, so as to remove the clay and other foreign matter.
The proportions of sand, stone, and cement recommended vary considerably according to the work to be done. An average mixture which will be found suitable for most work around the home consists of one part of cement, two of sand, and four of gravel or stone. This is known as a 1:2:4 mixture, and the proportions are given according to volume - not weight. For foundations a 1:2½ : 5 mixture is frequently used, and in the construction of roadways a 1: 1½ :3 or even 1:1:1½ mixture is preferred. The general rule, in this connection, is that the greater the proportion of cement the more water-proof and dense is the concrete, and, up to a certain limit, the greater the proportion of sand and stone, the stronger is the concrete. There must be sufficient cement to produce a thorough coating of the sand and stone, as there will not be otherwise a perfect bond between the different particles constituting the mixture. For the same reason, the concrete must be very thoroughly mixed before use. For facing walls, filling cracks and making repairs, generally, a mixture of cement and sand, only, is used, usually in the proportion of 1:2.1