Hydration and microstructure of ternary high-ferrite Portland cement blends incorporating a large amount of limestone powder and fly ash

Abstract

This paper studied the mechanical properties, hydration process, and microstructure of ternary high-ferrite Portland cement (HFC) blends with replacement at levels up to 50 % with limestone powder (LP) and fly ash (FA). Two mechanisms, synergy and negative effects between LP and FA are observed in ternary HFC-LP-FA blends. The synergistic effects between LP and FA in ternary HFC blends mainly exhibit improving flexural strengths, improving compressive strengths before 28 days, decreasing drying shrinkage and cumulative heat, accelerating the crystal nucleation and growth process (NG), stabilizing the ettringite (AFt) phase, prolonging polymerization of alumino-silicate chains, and refining the pore size distribution, etc. However, the synergistic effect between LP and FA in ternary HFC blends cannot compensate for the dilution effect caused by a large amount of LP and FA replacement. The adverse effects between LP and FA in ternary HFC blends are emerged and mainly exhibited decreasing in compressive strengths after 28 days and increasing total porosity percentages. LP plays a predominant role at an early age (before 28 days) in HFC blends and increases the cumulative heat; Specifically the cumulative heat of LP25 (25 indicates percentage replacement of HFC) before 10 h is 2.2 J/g higher compared to HFC. In addition, a rapid increase of bound water is emerged from 3 days to 28 days, for instance the bound water content in LP25 increases by 55.9 % from 3 days to 28 days. It is attributed to the nucleation effect of LP providing more nucleation sites and accelerating NG process. LP in HFC blends leads to the formation of monocarbonate and AFt phase trends to stabilization. FA plays a predominant role at the later hydration age (from 28 days to 90 days) in HFC blends attributing its pozzolanic reaction. Furthermore, the synergistic effects between FA and LP in ternary HFC blends are mainly occurred at a later age.