Sand and gravel are the main constituents in most mortars and concretes. These aggregates must be able to tolerate chlorides and alkaline conditions as well as humidity and frost. They must also have the required mechanical strength. Irregular-shaped gravel and sand is best, whilst river sand with rounded grains is far inferior because it offers less bonding surface to the cement. It should be as free of sulphur and earth impurities as possible; mica content can greatly weaken gravels since it provides a slip surface. Inland sand is considered to be best. It is possible to use coastal sand but contact with salt water means that it will contain chloride that corrodes steel. This can be washed out with fresh water.
Sand should not be too fine, except for final coats of wall rendering, which need to be very smooth. The basic principle influencing the choice of sand and aggregates for mixing in concrete is that the grains should be of varied sizes. In this way, the smaller ones fill up the spaces between the larger ones, and far less cement is thus needed to cover all of the grains in the mixture. Therefore, a correct mix of grain sizes in a concrete mix has a big effect on how strong it will be, given a fixed amount of cement.
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