Investigation of the Relationship Between Formation Factor and Fresh Properties of Concrete
Isgor, O. Burkan
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Formation factor of fresh cementitious pastes was investigated experimentally as a function of time from initial mixing and mixture design properties such as supplementary cementitious material (SCM) replacement level, water-to-binder ratio (w/cm), and superplasticizer dosage. SCM types included fly ash, slag and silica fume. A total of 54 paste mixtures were studied. The formation factor of each fresh paste was determined at the 30th, 60th, and 90th minutes from initial mixing. It was shown that for a given type of paste mixture (e.g. OPC plus silica fume), formation factor decreases if porosity or w/cm ratio increases, and this relationship can be well formulized by a power function. Although both paste and pore solution resistivity decrease with time in fresh cement paste mixtures until initial setting, their ratio (formation factor) remains relatively constant because it is only indicative of physical formation of solid particles in the pore solution. Formation factor of fresh cement paste is strongly correlated to its porosity through Archie's law, which implies that formation factor decreases if porosity increases. This decrease of formation factor is attributed to the smaller solid particles fraction (i.e., 1-φ) with high resistivity (i.e., lower amount of non-conductive component compared to conductive component). The tortuosity of paste affects the formation factor even at a constant porosity. Smaller size, angular shape, and more even distribution of particles increase the tortuosity of the paste. Slag and fly ash particles considerably decrease tortuosity; whereas silica fume incorporated pastes have almost the same tortuosity as OPC pastes. Superplasticizer addition significantly increases tortuosity through a better distribution of solid particles.