Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixture eventually hardens into a stone-like material.
Cement and water are the two ingredients that do all the chemical reaction stuff. The gravel and sand give strength.
Related topics: [What is cement] [Reinforcing in concrete] [Making a concrete slab]
If you mix gravel, sand and water together, nothing will happen.
If you mix cement, dry gravel and dry sand together, nothing will happen.
It is only when the cement is mixed with the water that something happens!
For ordinary “around the house” type concrete jobs, the gravel (stone) is usually a mixture of stone sizes up to 20mm (3/4″) diameter. Some building supply stores sell bags of gravel and sand (or similar type of aggregate) already mixed. When making a concrete mix, the sand or fine stone is needed to fill the gaps in between the bigger stones, so you have a good binding of the ingredients.
Did you know? Some building supply stores sell bags of gravel and sand (or similar type of aggregate) already mixed. The name of the mix varies from country to country. In some places it is called ‘builders mix’, other places it is called ‘ballast’ and some places it is just called ‘aggregate’.
The portions of each ingredient (cement, sand, gravel and water) will determine how strong the concrete will be. For example, if you use more water than is recommended, or less cement or sand than is recommended, then the concrete will not be as strong as it could be.
Tip: How clean should the water and ingredients be?
Rule of thumb: if the water is good enough to drink, then it is good enough to be used for making concrete! The cement and sand and gravel should also be clean.