Investigation of the Suitability of Water Demand Studies as an Indicator of Incompatibility Between Binders and Superplasticizers

Use of superplasticizers (SP) in concrete production has advantages that are manifold, leading to improved performance in many characteristics including strength and durability. The use of SP ensures maximum workability with reduced water demand, heat of hydration, segregation, and bleeding. However, since most of the concrete mixtures used today included many additional ingredients especially in the binder phase, it is essential that the compatibility between ingredients and chemical admixtures is well established beforehand. In this study, the possibility of using a simple test such as the water demand variation with SP dosage, using Vicat’s apparatus, as an indicatory test to identify any incompatibility is explored. The experimental program consisted of studies on pastes composed of 2 brands of Portland Pozzolana Cement (PPC) with two major families of superplasticizers, viz., Sulphonated Naphthalene Formaldehyde (SNF), and Polycarboxylate Ether (PCE). Both SPs showed different trends in characteristics like water demand, workability, fluidity of paste, mixing time at the same SP dosage (solid content). PCE showed a trend of decrease in water demand whereas in case of SNF water demand decreased initially and later increased at slightly higher dosages of SP indicating that overdosing greatly affects the cement–superplasticizer interaction in SNF. The effect of the addition of more pozzolanic materials to fly ash based PPC, for increasing the sustainability potential was analyzed by replacing PPC with 5% and 10% of fly ash and Coconut Shell Ash (CSA). It was found that at different SP dosages, the water demand decreased considerably at initial dosages compared to the reference mix. The results from studies with SNF concluded that incompatibility exists irrespective of the cement brand. To understand the relation of incompatibility with a change in mixing method, hand mix was replaced by cake mixer, also different mixing pans were used and the graphs were compared. The experiment clarifies that although the trend of water demand–SP dosage graph (in percentage) remains the same, workability and water demand changed with the change in mixing. Though validation of these results has not been done with standard tests such as Marsh cone and mini-slump tests, the study leads to the conclusion that the water demand studies could be considered an indicative qualifier to identify compatibility issues and can be used complementing the standard tests which would provide quantitative evidence of the SP saturation dosage.