How Competitive Hydration between C₃A and C₃S in the Fast Dissolution Stage Affects the Hydration of Low-Heat Portland Cement Blended Pastes

Abstract

Xinjiang has a temperature difference of 20°C between day and night, interannual temperature difference of 50°C, the control of the temperature cracks in mass concrete becomes difficult. Although low-heat portland (P·LH) cement concrete containing fly ash (FA) can reduce hydration heat, the mismatch between its mechanical and thermal properties is still not fully resolved. An attempt to explore the effect of FA on the competitive hydration reaction of C3A and C3S at super-early ages, and how this effect changes the physical and chemical properties of hardened cement pastes by electrical resistivity and zeta potential and common microstructure test methods, and to provide a theoretical basis for optimization of mechanical and thermal properties of P·LH cement concrete with FA. Results showed that in the fast dissolution stage, a numerous {Ca6[Al(OH)6]224H2O}6+ ions and a very small amount of H3SiO4−/H2SiO42− ions were formed, and incorporating 20%, 30%, 40% FA primarily inhibited the Al(OH)63− hydration and the {Ca6[Al(OH)6]224H2O}6+ generation, resulting in the generation time of AFt in comparison with pure cement, respectively, delayed by 44%, 140%, and 524%. This phenomenon decreased the nucleation and crystallization growth rate of AFt and Ca(OH)2 in the induction stage, and reduced the consumption of gypsum in the acceleration stage. Hence, monosulfoaluminate did not exist for pastes with more than 30% FA, and the number of particles involved in the reaction for the P·LH cement stone with 20%, 30%, and 40% FA decreased by 4.0%, 46.5%, and 52.0% in the deceleration stage compared with the pure cement. The research results can provide a theoretical reference for modifying chemical properties of FA to meet the matching of mechanical strength and thermal of P·LH cement.