The aggregate are a sharp-edged rocks or gravel. The sand fills in the spaces between the gravel.
Cement when mixed with water forms the bond between the aggregate and sand.
Cement is a mixture of CaO, Al2O3 and SiO2. The exact formula depends on the application. Some mixtures will harden or cure faster but result in a weaker concrete. Faster curing generates more heat which may be a problem for large structures such as dams. The workability is measured with a slump test where a cylinder of concrete 12 inches high is allowed to settle or slump.