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From about 1940 alkalies have been known as a potentially important species in concrete, despite being a minor cement constituent. The hydration reaction of silica in aggregates in concrete in the presence of alkalies, which may cause deleterious expansion, is chemically similar to the process which makes finely ground cementitious components, such as blast-furnace slag, and mineral admixtures, such as fly ashes, natural pozzolana, and silica fume, able to prevent expansive reactions.
The chemistry of this process is briefly reviewed, and the impact of elevated temperatures, which occur more often in field concrete than in laboratory concrete and mortars, is discussed.
The advantageous effects of alkalies on the rheology of concrete containing mineral admixtures are discussed with reference to recent research regarding the electrochemistry of cement paste. The corresponding effect in densifying the microstructure of the cement paste in hardened concrete is also discussed.
Updating of the application of silicate chemistry is proposed as a basis for development of testing tailored to serve effectively the increased uses of the mineral admixtures.
alkalies, concrete, cement, silica, chemistry, rheology, hydration, expansion, cracking, corrosion, testing, pH
Principal, G.M. Idorn Consult ApS, Nærum,
Professor, Materials Research Laboratory, The Pennsylvania State University, College Park, PA