Effect of content and fineness of slag as high volume cement replacement on strength and durability of ultra-high performance mortar

Replacement of cement by slag in ultra-high performance concrete (UHPC) makes it a green material for building and construction application. However, slag hydrates slower which delay strength gain in UHPC which can be addressed by using slag of high fineness. In this study, cement is replaced by slag at 20% and 60% by weight. Slag of three different fineness is used: 400±4 m²/kg, 556±5 m²/kg and 750± 5 m²/kg at constant water-binder ratio of 0.25 for all mixes. Compressive and flexural strength were measured for mechanical performance while water penetration test and chloride migration test were conducted to enumerate durability.Results show that early and late compressive strength and flexural strength are improved significantly by incorporation of slag with fineness 556 ± 5 m²/kg compared to reference and slag of 400 m²/kg fineness. However, at similar fineness 20% slag incorporation is found to produce higher early strength compared to 60% slag although 90 day strength for both replacement percentages are similar. On durability aspects it is found that slag with fineness of 556 m²/kg and 750 m²/kg demonstrates significantly lower penetration depth and very high resistance to chloride migration at 28 day and 90 day age irrespective of slag replacement percentage. The study suggests that slag of high fineness (about 556 m²/kg based on the study) and 20-60% cement replacement can significantly accelerate hydration and improve long term durability of UHPC mortar.