Concrete has two basic components: aggregates and paste. Aggregates come in two sizes. “Fine” aggregates or sand are smaller that ¼ inch in diameter. “Coarse” aggregates are larger than ¼ inch and are usually gravel or crushed stone. Paste is composed of cement, water, and sometimes, quality enhancing admixtures. It functions first, to coat and lubricate the individual grains of sand, thereby making it “workable.” Secondly, paste is the “glue” which binds the entire mixture together. When sand is added to paste, it is known as mortar. A chemical reaction, called hydration, between cement and water gives mortar its cementing property.
Here is a good way of understanding the process of making concrete:
Portland cement + water = cement paste
Cement paste + sand = mortar
Coarse aggregate + mortar = concrete
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Contrary to popular belief, concrete does not set and harden through a drying-out process. Through favorable temperature and moisture conditions the hydration process transforms the cement into a mineral “glue” which acts to bind the sand and coarse aggregates together. The hardening process is known as curing. With proper curing, hydration takes place very rapidly at first, and then more slowly for a long time.