Portland Cement:

Clinker Compounds:

-C3A

-C3S

-C2S

-C4AF

-Gypsum (CSH2) is added to the clinker.

Water is added to make hydrated paste.

Hydrated Paste Conceptual Reactions (equations not balanced):
1)C3A + CSH2 + H2O à Ettringite (C6AS3H32)

2)C3S + H2O à C3S2H3 (glue) + CH

3)C2S + H2O à C3S2H3 (glue) + CH

4)Ettringite + C3A + H2O à Monosulfates (3C4ASH18)

5)C4AF + H2O à Hydrogarnets

Above is a very basic outline of the chemical compounds and reaction of portland cement. The clinker compounds are formed from heating raw materials in a kiln. When gypsum and water are added, the chemical compounds react with each other.

One of the first new crystal structures to form is ettringite; it forms almost instantly when water is added to portland cement. Ettringite is a long, large crystal that is only stable in a sulfate saturated environment. The formation of ettringite eventually uses up all the sulfate in the environment causing it to form into three monosulfate crystals. The three monosulfates usually do not form into anything else.

The three monosulfates are two and a half times smaller than the one ettringite. It is, therefore, this linear process of large to small crystals that shrinkage-compensating concretes uses to expand and then contract the concrete. They basically provide an environment to stabilize the ettringite while the glue is forming. As the ettringite and glue form together, the overall crystal structure of the concrete builds up and thus expands. If timed and proportioned properly, the ettringite will run out of sulfates at about seven days from cement finishing and form into smaller monosulfates thus causing the concrete to contract to its original size.