Aggregate: The inert filler material that makes up the bulk of concrete. Usually sand, gravel, and rocks. Fibers and reinforcing bars are not considered aggregate.
Bleeding: An undesirable process of mix water separating from the fresh cement paste or concrete while it is being placed or consolidated.
Cement: This word is used colloquially to mean several very different things: the dry unreacted powder that comes in a sack, the sticky fluid stuff formed just after water is added, and the rocklike substance that forms later on. As noted above, people also tend to use it to refer to concrete. Obviously this won’t work for people who want to have technical discussions. For our purposes, the word cement used by itself refers to the dry unreacted powder.
Cement paste: Cement (see above) that has been mixed with water. Usually the term implies that it has already become hard (see Fresh).
Concrete: A mixture of sand, gravel, and rocks held together by cement paste. The world’s most widely-used man-made material.
Curing/Hardening: Essentially interchangeable terms that mean the process of continued strength gain after the cement paste has set due to chemical reactions between cement and water.
Fresh: Refers to cement paste or concrete that has been recently mixed and is still fluid. This is what those big trucks with the rotating container on the back are full of. (These are often called “cement mixers” but now you know why they should be called “concrete mixers”).
Hardened: Refers to cement paste or concrete that has gained enough strength to bear some load.
Heat of hydration: Like most spontaneous chemical reactions, the hydration reactions between cement and water are exothermic, meaning that they release heat. Large volumes of concrete can warm up considerably during the first few days after mixing when hydration is rapid. This is generally a bad thing, for reasons that will be discussed.
Hydration: The chemical reactions between cement and water. Hydration is what causes cement paste to first set and then harden.
Hydration products: The new solid phases that are formed by hydration.
Mature: Refers to cement paste or concrete that has reached close to its full strength and is reacting very slowly, if at all. An age of 28 days is a very rough rule of thumb for reaching maturity.
Mortar: A mixture of cement paste and sand used in thin layers to hold together bricks or stones. Technically, mortar is just a specific type of concrete with a small maximum aggregate size.
Placing: The process of transferring fresh concrete from the mixer to the formwork that defines its final location and shape
Segregation: An undesirable process of the aggregate particles becoming unevenly distributed within the fresh cement paste while the concrete is being placed or consolidated.
Set: The transition from fresh cement paste to hardened cement paste. The terms “initial set” and “final set” refer to specific times when the paste becomes no longer workable and completely rigid, respectively. “Setting” is the process by which this occurs.
Young: Refers to cement paste or concrete that has recently set and is now actively hardening. What constitutes “young” in terms of time is variable; the term implies that the paste has undergone a only fraction of its full reaction and is thus weak and vulnerable to damage. This could be anywhere from a few hours to weeks depending on the mix design and the temperature.