When we discussed hydration last month, it dealt mostly with calcium hydroxide (Ca(OH) or CH), which seemed like a good way to introduce the subject—after all, calcium hydroxide constitutes about 20% to 25% of the portland cement paste. We left you floating in calcium (Ca), sodium (Na), potassium (K), aluminum (Al), and sulfate (SO4) in an interstice between sand particles, along with portland cement particles, watching calcium hydroxide crystallizing nearby as we slowly drifted downward because of gravity.
Although we didn't initially notice, tectoids (cigar-shaped particles) secured themselves to basal faces of the calcium hydroxide crystals, and were engulfed by subsequent calcium hydroxide molecules as they attached themselves. The tectoids are calcium silicate hydrates (3CaO·2SiO2·XH2O, C-S-H) that could be said to strengthen the weak basal-face cleavage of the calcium hydroxide—like reinforcement. During this initial stage of action, rapidly developing on cement particles is acicular ettringite (3CaO·Al2O3·3CaSO4·32H2O)—like “spikes” on a mine floating in water.
Meanwhile, action becomes more noticeable on portland cement particle surfaces as the calcium silicates (3CaO·SiO2, C3S, and 2CaO·SiO2, C2S) reach out to catch and ingest water molecules and begin to form a microstructure of calcium silicate hydrates. That, along with calcium hydroxide, form on aggregate surfaces. As these processes proceed, our progressively slow downward drift stops because the hydrates have linked, forming an initial microstructure, albeit poor but enough to cause slight rigidity. This results in initial set as the microstructure fills with cement hydration products—CH and C-S-Hand attachment to aggregate becomes serious.
Before that rigidity happens, we feel an upward movement of water—bleed water—as it rises to provide space for the solids moving downward, until some semblance of rigidity occurs and all movement of solids cease. What is remarkable is that all of these reactions have been accommodated. There is no noticeable pushing or shoving. The calcium hydroxide engulfs whatever is in its way, and the calcium silicate hydrates form on surfaces and stop growing when water-filled space is depleted.
The more water initially used, the greater water volume between aggregate particles and the greater the dilution of cement hydration products. This dilution results in more porous and permeable pastes, lower strengths, and greater drying shrinkage.
The calcium silicate hydrates are poorly crystalline; yet have a definable structure consisting of submicro sheets spaced about 0.002 microns (there are 25,400 microns per inch) apart, between which water is sandwiched. This “adsorbed” water is held by surface-energy forces. The original water-filled space unfilled by hydration products become capillary pores having sizes of about 0.025 microns. It is the capillary pores that control permeability. As they plug with products from continued cement hydration, they become discontinuous and permeability slows dramatically. That never happens at greater than about 0.70 water-cement (w/c) ratios.
The result of these reactions is formation of paste consisting of residual and relict portland cement particles, calcium hydroxide, calcium silicate hydrates, and ettringite—each created by the flat out passion by the cement for water. If there is not enough sulfate, the acicular ettringite converts to the platy, lower sulfate hydrate, calcium monosulfoaluminate.
As we are drifting along, we begin moseying upward while watching an occasional upward rush of water, like a gulf stream. The now upward movement of water—bleed water—passes around aggregate particles, the smaller ones that is, but finds shelter and gets trapped under the larger ones. The lingering water creates water gain, resulting in high w/c ratios along the undersides of aggregate particles. If carried to extreme, continuing differential settlement of solids results in water-filled voids below aggregates—now, that's a high w/c ratio!
Well, we have been under water for quite awhile and need a respite, let alone a breath of air. So let's migrate to one of those upward moving bleed water streams and get to the surface before finishing densifies the surface and blocks our passage. We do, dodging some aggregates in our path and, along with extremely fine fines from the aggregates, exit like a burp.
William Hime was a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 58 years ago.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for 52 years.