Mixture design and early age investigations of more sustainable UHPC

This paper deals with the possibility to make the Ultra-High Performance Concrete (UHPC) as an environmentally friendly, through cement replacement with Blast Furnace Slag (BFS) and considering its cost and its early age performance. Thereby, a preliminary investigation is carried out, to design an UHPC with local materials and where Silica Fume (SF) is partially replaced with crushed quartz. The results showed that, from a mixture standpoint: i) the use of high energy mixer is necessary to ensure a homogeneous and workable mixture; ii) the polycarboxylate superplasticizer with chemical structure of acrylic polymer (ACP1) and saturation dosage of 1.8% gives better workability for UHPC, compared to other six superplasticizers; iii) despite its negative effect on slump flow, Grey Silica Fume (silicium) ensures better compressive strength at early age, than Light one (zirconium). Results showed also that for 30% of cement replacement, the nucleation effect of BFS particles prevails and induces an acceleration of setting and hydration reaction of cement. For BFS contents of 50 and 80%, the dilution effect is dominating, which decelerates the setting and hydration reaction and decreases the peak of heat flow. BFS addition also affects the strength development. At 3 days, the strengths of the blended mixtures of UHPC with 50 and 80% of BFS were decreased by 26 and 66% respectively, while in contrast, there was a slight increase in the strength (+3%) with 30% of BFS compared to the reference mixture (UHPC1). For UHPC containing 80% of BFS, the use of chemical activator of potassium hydroxide, with a concentration of [KOH]3, causes slag's dissolution and reaction, which in turn improves the compressive strength by 42% and 11%, at 3 and 7 days, respectively.