The click word is “greening.” The word's significance perhaps is summarized best as anything that helps our world become physically better and healthier. In our industry, it includes cutting back carbon dioxide emissions by using less energy consumptive materials—that is, by maximizing use of waste products.
Many articles have been written on how our industry can contribute. This can occur in a number of ways: (a) use substitutes and replacements for portland cement with waste products such as coal fly ash, ground-granulated blast furnace slag cement, rice hull ash, silica fume, pozzolans of all types, and ground limestone; (b) reduce the “paste” fraction by minimizing amounts of these materials using well-graded aggregates; and (c) use water-reducing admixtures to lower w/cms and enhance concrete properties. All of this has been proven!
But how about throwing into the foray entrained air and maximizing, rather than minimizing, the amount of water as long as desired concrete properties are maintained? Air occupies space, which reduces the amount of concrete solids and that reduces cementitious materials contents. Batch water initially occupies interstitial spaces that, in turn, replace fine and very fine cementitious materials solids.
Another aspect is the tie up of water, now progressively becoming a precious commodity, because it's bound up by cement hydration products—at least until it's released by carbonation. Usually it is said the lower the w/cm the better, but how low a w/cm is needed for a particular concrete use? A potential conflict is: (a) although using low w/cms to increase strengths, decrease permeability and drying shrinkage, and increase durability, it means increased cementitious materials solids contents by comparison to the same mixture at higher w/cm contents; and (b) increased water creates over yielding that decreases cementitious materials solids contents by comparison to the same mixture at lower water contents.
A general formula for enhancing the greening of our concrete industry includes maximizing replacement of portland cement with complementary cementitious materials, optimizing grading of aggregates, maximizing use of entrained air, and using higher w/cms—and all commensurate with concrete performance requirements. To fine tune that even further, since portland cement replacements are done on a weight basis, the lower the specific gravity of fly ash, for example, the greater its volume (like a pound of feathers versus a pound of gold)—so why not maximize use of lower specific gravity fly ashes along with the magnificence of the others?
The Biosphere Project of several years ago is an example of concrete's effects on ecological systems. Just about everything was monitored and controlled during the “closed circuit” ecological system's projected life so all forms of life sustainability could be evaluated. Among these was a check on atmospheric gases, including carbon dioxide. An unexpected problem encountered was fluctuations in carbon dioxide level. Vegetation, bacteria, and human breathing had been factored into the system, there were no leaks to the outside, everything expected to happen was happening—but not necessarily as projected. Someone eventually realized that contributing to the carbon dioxide dilemma was concrete exposed to the Biosphere atmosphere. The concrete was consuming carbon dioxide, both carbonating and releasing water.
This is not the only time in our Earth's history when varying geological environments have changed its character. Our Earth has been constantly changing: during the Carboniferous Period, vegetation ran amuck helped by a carbon dioxide-rich atmosphere and when great coal deposits were formed; by the extensive red-rock beds (red due to iron oxides) formed during oxygen-rich chapters in the Cretaceous and Jurassic Periods, also a time of rapid polarity changes; and the more recent Pleistocene Epoch, during glacial and interglacial periods when the Earth was cooled and heated a number of times, and on an almost periodic basis. We are now on the shoulder of one of these periods. Changes to our Earth will continue even without our help.
We need to remember we have this vast and glorious world to live in, and to use what we need. But we also have an obligation to keep it going like it was, at least to avoid accelerating worldly changes, and make our Earth sustainable and livable as long as it will allow. Our debt to our world is for its use and our commitment to greening is our obligation to the future.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago.
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.