Abstract
The main objective of this study has been to obtain a clearer knowledge about the effect of different crushed and natural mineral fillers on rheological parameters of matrix and concrete, in order to answer one of the most important questions when using manufactured sand – the question about the very high filler content.
The effect of 7 widely different fillers on rheology of filler modified paste (= matrix) and SCC was investigated at different w/b ratios (0.4, 0.5, 0.6 and 0.77). The fraction of solids was kept constant for most of the mixes. A total of 38 matrices were tested which were later “upscaled” to 22 SCC mixes. Two replacement levels of Vfiller/Vpowder = 0.20 and 0.33 were used in all matrix mixes (equal to the fly ash cement volume fraction in the reference with w/b = 0.4 without filler). Two different types of co-polymeric superplasticizers were used for the studies keeping the dosage constant at the level of 0.4% for the matrix mixes and adjusting the dosage according to the w/b ratio for the SCC mixes.
A Physica MCR300 Rheometer was used for the matrix flow tests (yield stress, plastic viscosity), static tests (yield stress, shear modulus) as well as for oscillatory tests (viscoelastic
properties). Fresh concrete properties such as slump-flow, density and air-void content were measured for the SCC mixes along with the rheological measurements being performed on a coaxial cylinder viscometer Viscometer 5 by ConTec. Within the project filler particle size distribution and specific surface area has been determined using four different characterization methods – sedimentation (Stoke’s law), laser diffraction, Blaine and BET.
The research carried out within this study is a strong foundation in order to fully understand the interaction between different types of mineral fillers and admixtures one hand and rheological properties of cement based particle suspensions such as matrix and concrete on the other. However, based on the results so far it’s still more new questions than answers.
First, it is rather clear that most of the available mineral particle characterization methods give very different results and it is still a challenge to get more understanding on this topic. Mineral composition of the fillers has been suggested as one of the possible reasons and further research directions have been proposed. The obtained results confirmed that it is possible to some extent relate the rheological differences of matrices to the specific surfaces of the fillers used to mix them. This seems to also be true for the structural regeneration and decomposition of the filler modified paste. However, the relation was rather limited and other relevant parameters for further studies have been proposed. No relation between the specific surface of the fillers and rheological properties of fresh SCC was found. The flowability of both matrix and concrete mixes was considerably improved when the mineral fillers replaced parts of the finer fly ash cement on volume basis. The natural filler gave the best flowability for both matrix and concrete as expected.