Hydration heat, strength and microstructure characteristics of UHPC containing blast furnace slag

Ultra-high performance concrete (UHPC) is an innovative cement-based composite with high mechanical performance under tensile and compressive loads, extremely low permeability, and excellent durability. Because of these features, UHPC has the potential to contribute to the development of new architectural perspectives and structural systems with prolonged service life; therefore, it is anticipated that the use of UHPC in cast-in-situ applications will increase in the near future. As a result of its high Portland cement dosage, the hydration heat of UHPC can be relatively high compared to that of conventional concrete. Thus, ground granulated blast furnace slag (GGBS) can be used in UHPC formulation for reducing Portland cement dosage thereby limiting hydration heat while also addressing ecological and engineering concerns. In the scope of this study, the effects of GGBS replacement (0%, 30%, and 60%) on the hydration heat, strength, and microstructural characteristics of UHPC were studied. Results showed that GGBS-bearing UHPCs are more sensitive to ambient temperature in respect to cumulative heat. 60% GGBS replacement reduced cumulative heat release by 36% and 28% at 20 °C and 35 °C respectively. So, the benefit of GGBS on reducing hydration heat is less pronounced in hot weather. Performance differences in strength depending on the replacement ratio were only noticeable on the first day of curing. Prolonged curing time and fiber inclusion eliminated strength differences. Microstructural investigations indicated that Ca(OH)₂ can be lowered up to 0.5%, and the Ca/Si ratio of the C–S–H phase was reduced below the value of 1.0 after 90 days of curing as a result of GGBS replacement.