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The setting behavior of geopolymer pastes as a function of time was studied using two methods: penetration resistance and ultrasonic shear wave reflection. Several starting materials were included—metakaolin, Class C and Class F fly ashes, and slag—and chemical parameters known to affect set were varied. The geopolymers showed a wide range of set times compared to a reference Portland cement paste: some were much more rapid, some were similar, and some were much slower. Although most geopolymers formed gels that were both solid and had measurable strength, some, initially, formed a soft gel that had no measurable strength. Therefore, to fully characterize setting behavior, it is necessary to use both types of tests. It was seen that setting behavior was sensitive to chemical parameters, with setting delayed somewhat with increasing silica/alumina and increasing water/alkali, and accelerated substantially with calcium hydroxide substitution.

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