On the hydration of Belite-Ye'elimite-Ferrite (BYF) cement pastes: Effect of the water-to-cement ratio and presence of fly ash

This work reports the evolution of the phase assemblage, pore solution, porosity, and mechanical resistance of Belite-Ye'elimite-Ferrite (BYF) cement pastes. Different water-to-cement (w/c) ratios were considered and the addition of fly ash (FA) evaluated. The initial 1-day hydration of BYF cement paste was characterized by the ye'elimite hydration accompanied by a pronounced alkalinity increase controlled mainly by the molar ratio of calcium sulfate to ye'elimite. Throughout the 1-year test, the phase assemblages of 0.50 and 0.67 w/c samples were similar, composed of ettringite, AFm, stratlingite, silicious hydrogarnet, and C-S-H. The hydration of the 0.40 w/c cement paste was almost complete within a month. Additions of 25 wt% FA increased the fraction of stratlingite and ettringite without long-term strength losses compared to plain BYF cement paste with the same water-to-binder ratio. Results from different characterization techniques for both liquid and solid phases at early-age (first 24 h) were complemented by thermodynamic calculations.