What is Concrete?
Concrete is a mixture of two components: Aggregates (rock and sand) and paste. The paste is a mixture of portland cement and water. Reaction between the cement and water, binds the aggregates in a rocklike mass called concrete.
The formula %'s of the above mentioned products (or mix design ) varies depending on the end use and workability of the concrete required for the job.
Myth: Concrete does not dry and harden as one would think. Instead it is a chemical reaction between the Portland cement and water in the mix, hydration , that binds the sand and aggregate together.
The strength of the concrete can be manipulated by the ratio of portland cement and water. Normally the dryer the mix (less water) the higher the strength. The sand and aggregate need to be relatively clean. The presence of dusts, silts, clays, etc… can be a detriment to strength, by discouraging a clean bond with the aggregate surface. The water content in the sand and aggregate also will effect the overall water/Portland cement ratio, and ultimately the strength of the concrete.
Additives are used in mix designs frequently to: adjust setting time or hardening, reduce water demands, increase strength, increase flowability/workability (getting it out of the ready-mix truck and placed with ease), intentionally entrained air (serves to reduce cracking in freeze/thaw cycles) etc…