We all have passions—portland cement's passion is water. With water, it becomes another material that possesses a myriad of physical and chemical properties. It thirsts for water and can consume what little there is in ambient air but only when relative humidity is about 78% or higher. Below that level it lolls in quiescence. In an abundance of water, the passion quickly satisfies—almost—because as long as there are residual cement particles and moisture available, some attraction remains. The process usually is called hydration. Simple to say, but a complexity intermixed in an enigma, the minute details of which scientists have sought to unravel for nearly two centuries. In addition, and rarely mentioned, is hydrolysis, similar to hydration but having a different kind of birth.
We wrote about hydration in a previous column, “The Miracle of Cement Hydration” (see November 2006 Concrete Construction). The article examined basic hydration products, heat that develops, and varieties of premature stiffening caused by the cement-water mixture alone or when admixtures are incompatible, but not about concrete's physical properties (early and late) that result as a consequence of hydration reactions. What are these consequences? To some, its effects on concrete placement, consolidation, and finishing—to others, its effects on varieties of strength, shrinkage, expansion, permeability, porosity, modulus of elasticity, creep, paste-aggregate bond, chemical and physical durability of all kinds, and wear and abrasion resistance.
Another way to appreciate hydration is to imagine becoming a part of the hydration process and observing. Let's place ourselves in a void filled with water between cement particles before they react with water, and follow events that lead to setting and strength development. We would feel the wetness of water around us and hang suspended with companion cement particles that want to migrate downward as gravity tugs. Upon contact with the cement, the water becomes a solution enriched in calcium, sodium, potassium, aluminum, and sulfate ions. When these ions are released, the process generates heat of solution.
Then a disturbance begins: the precipitation of calcium hydroxide (Ca(OH)2) (abbreviated CH, mineral name portlandite). Rapidly more calcium (Ca++) and hydroxyl (OH-) ions floating in the solution progressively cling and build an ordered structure. For portlandite, the structure is hexagonal, to which additional calcium and hydroxyl ions adhere. Very, very early, long, slender, hexagonal prisms form because growth is ferociously fast on ends and very, very slow on the sides. The shape elongates, as crystallographers would say, because of the growth on ends (basal faces).
Under ideal conditions that include a very high water-cement ratio, this rapid elongation dramatically slows very early while growth continues on prism (side) faces, so it gets fatter. The end shapes are squat tablets like aspirin but with hexagonal sides. We conclude the shape is a result of initially high calcium ion concentrations that prompt growth on basal faces. The concentration subsequently drops, prompting growth on prism faces.
Admixtures directly influence calcium ion concentration. Accelerators like calcium chloride keep calcium concentrations high so portlandite forms as chisel-like crystals having hexagonal handles with angled ends, akin to what crystallographers call scalenohedron ends. Quartz crystals can have similar shapes, sometimes with a second intersecting face on either the left or right side of the scalenohedral face, which leads to right- or left-handed quartz and, now, maybe right- or left-handed portlandite. Retarders rapidly suppress calcium ion concentrations so portlandite forms as thin, hexagonal platelets because of greater growth on prism faces than on ends.
This is due to hydrolysis—not hydration—because there is no water (H2O) in the portlandite structure (Ca(OH)2) as there is for the calcium silicate compounds that form calcium silicate hydrates—3CaO·2SiO2·XH2O. Along with calcium aluminates, their progenies, and marriages with the sulfate clan is for another column.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for more than 52 years.William Hime was a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA more than 58 years ago.