Effects of superplasticiser on rheology and cohesiveness of CSF cement paste

Superplasticiser (SP) is now an indispensable ingredient for the production of concrete. However, its proper use is a great concern because its over-dosage could lead to drastic reduction in the cohesiveness of concrete. Furthermore, experience has indicated that the SP demand, saturation dosage and dosage causing segregation are highly dependent on the powder content and therefore the effects of SP are not simple functions of dosage. This study aims to evaluate the effects of SP dosage on the fresh properties of cement paste containing condensed silica fume (CSF). A number of cement paste samples with different SP dosages and CSF contents were tested. The results showed that the addition of SP would improve the packing density, flowability and rheology but impair the cohesiveness of cement paste. It would also render the cement paste shear thickening. To cater for the effects of powder fineness, it is proposed that the SP dosage should be measured in terms of the quantity of SP per surface area of the powder content.