Moisture is cement's worst enemy. A nominal mix is typically made using cement with a specific gravity of 3.15. Any changes to this number will have an impact on the mix design. It has the potential to enhance the water-cement ratio, hence affecting the workability and strength of concrete. Changes can also be made by adding more solid material (such as sand or gravel) to reduce the amount of cement used in the mix.
For example, if the cement content of the mix is reduced, then more air is allowed into the concrete while still maintaining the required slump. This increases the volume fraction of aggregate in the mix, which improves the overall strength and durability of the concrete.
If, however, the cement content of the mix is increased, then less air is needed in the concrete because there is more cement in the mix that can absorb more air. This reduces the volume fraction of aggregate in the mix, which decreases the strength and durability of the concrete.
The specific gravity of cement affects the mix design for two reasons: first, it determines how much water is needed and second, it influences the amount of coarse aggregates (such as gravel) required in the mix.
For example, if the cement's specific gravity is lower than 1.4, then more water is needed to produce the same-sized piece of concrete.
The following criteria influence the selection of concrete mix design:
Using too much cement on concrete can have a number of negative consequences. If too much water is added to the mix, the workability of the concrete may worsen, and some of the aggregates will not effectively link to the cement. If too much is utilized in comparison to the aggregate, the structural integrity of the finished product would most certainly suffer.
Cement reacts with water to form calcium hydroxide, which causes the mortar or concrete to harden. This reaction needs moisture to occur, so if too much water is removed from the mixture, the cement will not be able to fully cure. Curing occurs when the water content is reduced to below about 15 percent by volume. As long as there is some water present, the cement paste will continue to react with itself and other ingredients to form a strong binding material. However, if the mixture is completely dried out, it will no longer cure and its strength will be compromised.
Adding too much cement relative to the amount of water results in a concrete that is harder but less elastic. The excess cement acts as a filler, preventing the smaller particles from coming into contact with each other. This reduces the overall density of the concrete, which means it won't support itself as well as a concrete with a higher density. Concrete that is too soft due to an excessive amount of water can be tempered with sand or steel chips to make it more resistant to stress.