Effect of polycarboxylate and crushed quartz in UHPC: Microstructural investigation

•We investigated the development of extrudable formulations of ultra high performance concretes.•A part of silica fume of classical composition is replaced by crushed quartz.•The macroscopic variations-already published-are highlighted here at the microscopic scale.•Increasing the polycarboxylate content from 1.8% to 2% accelerate the belite reactivity. The context of this study was to find an extrudable formulation of Ultra High Performance Concretes usable in an aggressive environment. In the five mixtures of this study, the only variable parameters are the polycarboxylate (PCE) dosage and the crushed quartz to micro silica ratio. This paper is devoted to a systematic microstructural study using Scanning Electronic Microscopy (SEM), with the idea to link the microstructure with the macroscopic results. Through this study, the effect of the polycarboxylate on the hydration of the type of cement used here is pointed out. From 1.8% to 2% polycarboxylate content, we observe a change in the hydration time for the two calcium silicates, the major phases of anhydrous cement. In the presence of microsilica and with 2wt.% PCE, we observe a higher consumption of dicalcium silicate, which should not have reacted with such a low water content (Water/Cement=0.16).