The early age ambient temperature hydration of a hybrid cement formulation containing very high volumes of coal fly ash (~80% by dry mass) and activated by Na2SO4 is presented. The Na2SO4 salt acts as a safe and convenient in situ source of alkali to activate fly ash glassy phases without undesirable effects on cement clinker hydration. Comparison to a reference paste with gypsum instead of sodium sulfate revealed that Na2SO4 reduced setting times, shortened the induction period, and increased early alite hydration and compressive strength development, but also restricted ettringite formation. When replacing the active fly ash component for milled sand of a similar particle size, the Na2SO4-activated pastes set even quicker, no ettringite was observed, and early strengths were considerably reduced. Possible reaction mechanisms in the hybrid pastes are discussed.
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