The way that concrete is mixed is very important in determining the quality of the final product after it is poured. The exact materials used and the proportion of those materials are both very important. Knowing what ratio of sand to cement to use depends partly on what the concrete is being used for.
A pretty standard recipe for mixing concrete calls for one part of cement to every two parts of sand. The result of this recipe, assuming that four parts of gravel are also included in the mix, is a C20-rated concrete. Concrete is rated on a number system that indicates how strong the mix is after it has cured for approximately a month. C20 concrete would be considered a medium-strength concrete mix. Higher numbers indicate a stronger concrete.
Increasing the Strength
Adjusting the strength of concrete based on the ratio of sand to cement is easy to do. The closer you bring the ratio to an even one-to-one ratio of sand to cement, the stronger you will make the concrete. On the other hand, if you push the ratio in the other direction you will get a product that has slightly less strength.
While concrete mixes all contain Portland cement, the actual type of sand used varies. In addition to the ratio, the type of sand that you use in the concrete mix also affects the strength of the concrete. Using unwashed beach sand, for instance, is likely to create a concrete mixture that is not as strong as products made with sand that has been cleaned in order to produce a consistent quality.
It is important to realize that the ratio of sand to cement is not the only factor that affects the strength of concrete. Concrete is typically made from four ingredients, not just two. In addition to the cement-to-sand ratio, the ratio of gravel to sand must also be considered. Finally, the amount of water added to the mixture is also important in the strength of the final product.