Very High Volume Fly Ash Cements. Early Age Hydration Study Using Na 2 SO 4 as an Activator

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 Na 2 SO 4 is presented. The Na 2 SO 4 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 Na 2 SO 4 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 Na 2 SO 4 ‐activated pastes set even quicker, no ettringite was observed, and early strengths were considerably reduced. Possible reaction mechanisms in the hybrid pastes are discussed.