Abstract
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.
Highlights
Introduction and scientific backgroundDeveloped in last decade, Ultra High Performance Concrete (UHPC) represents a revolutionary concrete type, characterized by the absence of coarse aggregates, a dense and homogeneous microstructure [1,2,3,4,5,6] and a self-compactness [7,8,9]
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 ensures better compressive strength at early age, than Light one
This original paper dealt with the mixture design and early age investigations of more sustainable ultra-high performance concrete (UHPC)
Summary
Ultra High Performance Concrete (UHPC) represents a revolutionary concrete type, characterized by the absence of coarse aggregates, a dense and homogeneous microstructure [1,2,3,4,5,6] and a self-compactness [7,8,9]. The variation of BFS amount is an essential item that affects the setting and hydration behavior in addition to strength of UHPC blended mixtures, for high BFS content This is attributed to low slag activity at early ages with its latent reaction and dilution impact related to the reduction of cement amount. Even if the early age properties of mortars or ordinary and high performance concretes are largely studied, those of UHPC should be more investigated, in the presence of BFS and a chemical activator The influence of the latter on the workability of UHPC, on its setting and hydration kinetic is worthwhile to investigate and needs further deepening. The effect of chemical activation of BFS on the hydration mechanism and the compressive strength of UHPC at an early age will be deepened
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