What is CONCRETE? Concrete is made from four basic ingredients: coarse aggregate (gravel), sand, portland cement, and water. Those are the basic ingredients, but as part of the mixing process, there is also some air, either intentional (entrained air) or just caught up in the mixing (entrapped air). All of these ingredients are mixed together then put into place before the cement hydrates and before you know it the cement turns to stone.
PORTLAND CEMENT is the magic powder that turns the concrete mixture into stone. Portland cement was invented in 1824 by Joseph Aspdin. He called it portland cement because it looked like stone that was quarried on the Isle of Portland near the coast of England.
Portland cement starts as rock and soil quarried from the earth - mostly limestone and clay. This stone is crushed to about 3/4 inch in diameter. The limestone and clay are put into a rotating kiln, which is heated to about 3000° F with coal or natural gas. The rock melts and comes out the other end as little glassy balls, about the size of marbles, called clinker.
The clinker is ground to a fine gray powder - this is portland cement. It is shipped to the ready-mix plant in special train cars or trucks, or it can be put into bags. Traditionally, cement has been placed into bags or sacks weighing about 94-pounds, which contain 1 cubic foot of cement powder. Cement is to concrete as flour is to bread. But cement alone is NOT concrete, any more than flour is bread. It's the production process that helps turn cement into concrete.
Water is the essential ingredient that starts the reaction of the cement powder. Concrete doesn't get hard because it "dries" it gets hard because there is a chemical reaction between the cement powder and the water called "hydration". The chemicals in the cement react with the water to grow into crystals that form a structure around the sand and gravel and become concrete (liquid stone).
Instead of thinking of concrete as bread, we like to think of it as a very hard fruitcake—lots of chunky stuff held together by the cake. That "chunky stuff" is the aggregate that helps to hold everything together, like the portland cement, and water that has hardened. Aggregate is best if it has a gradation and is not all one size, that way the particles fit together better, forming a matrix that is glued together by the cement-water mixture (the paste).From Concrete Construction Magazine - February 2012