High-Alkali Cements for 21st Century Concretes


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Title: High-Alkali Cements for 21st Century Concretes

Author(s): J. Davidovits

Publication: Special Publication

Volume: 144


Appears on pages(s): 383-398

Keywords: alkali-aggregate reactions; alkalies; blast furnace slag; blended cements; carbon dioxide; cements; emissions; fly ash; pozzolans; spectroscopy; Manufacturing

Date: 3/1/1994

Recent literature suggests that there is considerable potential for reduction in the emission of CO 2 to the environment through the manufacture of new types of cement that do not rely on the calcination of limestone (and accompanying release of CO 2). The 1988 1-billion metric t worldwide production of cement accounted for 1 billion metric t of CO 2 release, i.e., 5 percent of the 1988 world CO 2 emission (human activity only). This is equivalent to the CO 2 emission of all Japanese activity. The use of lesser amounts of calcium-based cements could be achieved through their partial replacement by alkali-activated alumino-silicate materials, which do not release large quantities of CO 2 in their manufacture. The fostering of low-CO 2 high-alkali-based cements will mean a dramatic change in the research and development presently carried out in the USA and other countries. Alkalies are generally thought of as the cause of deleterious alkali-aggregate reaction. As a result, the tendency has been to avoid any addition of alkali portland cement products, often requiring cement manufacturers to supply low-alkali cements. The use of MANSMR spectrography for the determination of composition of alkali-activated cements, in combination with ASTM C 227 bar expansion, allows the prediction of potential for alkali-aggregate reaction. A preliminary study involving Al and Si MANSMR spectroscopy revealed that the alkali-activated alumino-silicate cements are the synthetic analogues of natural pozzolans that are known to effectively suppress alkali-aggregate reaction. These cements, even with alkali contents as high as 9.2 percent, do not generate any deleterious alkali-aggregate reaction, according to the ASTM C 227 bar expansion test. Industrial experience based on the use of alkali-activated slags in Eastern Europe since 1964, associated with the commercially produced alkali-activated cements in the US since 1988, suggest that high-alkali cements will ultimately improve the concrete used in buildings and highways, and also serve global need by reducing emission of CO 2 and reducing energy consumption during cement manufacturing. In terms of a 5 percent growth scenario, the predicted business as usual (BaU) world cement production for the year 2015 equals 3500 million metric t. Based on an amount of blended portland cement production on the order of 1850 million metric t (1000 portland + 560 slag + 290 fly ash) in the 21st century, the need for novel alkali-activated cementitious materials could be in the range of 1650 million metric t.