Why some carbonate fillers cause rapid increases of viscosity in dispersed cement-based materials: Fid-Bau Portal

Why some carbonate fillers cause rapid increases of viscosity in dispersed cement-based materials

Nehdi, M.

Fine carbonate fillers complement the deficiency in fine particles of the cement's particle size distribution, which can enhance both the flowability and stability of fresh concrete. They also fill in between the relatively coarser cement grains, reducing the room available for water and consequently the water demand. In conventional concrete mixtures, slight reductions in the setting time have been often reported when carbonate fillers were used, without significant effects on the workability. However, as the use of high-performance concrete (HPC) continues to rise, carbonate fillers are being added in low water/cement (w/c) ratio superplasticized-mixtures. Increasingly finer fillers are being specified because they enhance the packing density of the particulate system. In such conditions, rapid losses of workability have been reported. Yet, this did not seem to occur with all carbonate fillers. This paper investigates the potential causes of this phenomenon using a model system. Chemical oxide analysis was conducted on various carbonate fillers, which were then ground to sub-micron particle sizes in the presence of polyacrylate dispersants using an agitated ball mill-grinding machine. A rotational co-axial cylinders viscometer was used to study the rheology of the slurries thus obtained. Laser diffraction was employed to establish the particle size distribution of the input and output slurries. It was discovered that the magnesium (MgO) content of the carbonate filler can hinder the effect of the dispersant, which may combine with the acceleration of C₃S hydration due to calcite and possibly the formation of carboaluminates in Portland cement-based mixtures to cause rapid losses of workability.