In many of our articles, we have discussed various aspects of cement chemistry: from the simplistic water (H2O) molecule to remarkable ettringite—the longest molecule of them all (3CaO·Al2O3·3CaSO4·32H2O)—in concrete-related terminology, that is. We thought of now discussing portland cement minerals beginning with tricalcium silicate, as well as dicalcium silicate and tricalcium aluminate. Concrete's birth involves portland cement clinker (after it is processed), the cement's union with aggregates and water that results in its hardening and strength development, and its slow but progressive aging during which time it fulfills its service goal(s).
Tricalcium silicate (3CaO·SiO2) (abbreviated as C3S in cement parlance, and also known as alite, and “tricalcium” because it contains three calcium molecules) is the major component of portland cement and constitutes 50% to 65% of the cement. It serves as an early- and late-strength developer. It is made through solid solution transformations during the cement burning process, and typically forms as rectangles with two opposite corners truncated. When the truncated rectangle is almost equiaxial it sometimes looks like a hexagon, and can contain small, spherical inclusions of periclase (magnesium oxide, MgO) and, more rarely dicalcium silicate.
Dicalcium silicate (2CaO·SiO2) (abbreviated as C2S in cement parlance, also known as belite or felite, and “dicalcium” because it contains two calcium molecules) is the second major component of portland cement and constitutes 20% to 45% of the cement. It serves as a late-strength developer. It is made through solid solution and solution transformations during the cement burning process and typically forms as spherical crystals. There is also a variant wherein potassium infiltrates the crystal, replaces calcium, and the formula changes from C2S to KC23S12, which is more water reactive than its cousin C2S but not as reactive as its more powerful uncle, C3S. The changed crystal looks like a ball of yarn.
Finally, a third prominent mineral, tricalcium aluminate (3CaO·Al2O3) (abbreviated C3A in cement parlance, and “tricalcium” because like tricalcium silicate it contains three calcium molecules) is usually third in importance, depending on circumstance and how you view it, and constitutes 5% to 12% of the cement. It serves as an early-strength developer. It usually occurs in the “melt” portion of the cement clinker. There is also a variant wherein sodium infiltrates the crystal, replaces some calcium, and the formula changes from C3A to NC8A3. C3A has a cubic crystal habit; the sodium version is orthorhombic and occurs as slender prisms or laths. C3A hydrates rapidly and viciously, forming calcium aluminate hydrates, which if allowed to run amok can cause premature concrete stiffening in a ready-mix truck along with the development of heat. Gypsum (CaSO4·2H2O) is used to control its temper and does so by releasing SO4 (sulfate radical) so the marriage forms a long, slender, and weak ettringite prodigy that easily mixes through.
These are the main cement contributors to concrete setting, hardening, and strength development. Each plays a role in every concrete's manifest destiny.
Minor to trace minerals include: (a) tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3) (abbreviated C4AF in cement parlance, and known as brownmillerite, celite, and “tetra” because there are four calcium molecules) that doesn't do much hydrating; (b) aphthitalite (Na,K(SO4); (c) periclase (MgO); and (d) free lime (CaO, uncombined lime). Aphthitalite is important—it houses most of the alkalies (Na, K). Periclase and free lime also are important because they possess a potential for causing deleterious expansion. A 6% limit is imposed on periclase, and an autoclave test is used to limit expansions caused by the two when they hydrate.
As typical for most materials, there are variants and intricate details that get in the way of general understandings; those we have tried to avoid to keep your attention on major aspects.
So there you have it: mineral names, chemical formulas, and their abbreviations—the stuff that makes up cement clinker. You take that stuff, combine it with gypsum (CaSO4·2H2O) until it has face-powder fineness, and it becomes portland cement. Commercially it's sold in 94-pound bags or in bulk, where it becomes the most fundamental part of the construction material's giant contribution to the world—portland cement-based concrete.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for more than 48 years.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago.