Effect of polycarboxylate superplasticiser on the adsorption and rheology of self-compacting concrete paste

Abstract

Self-compacting concrete (SCC) is a concrete which can compact by its own weight without the need of manual intervention. In order to achieve the desired workability without increasing the content of water within self-compacting concrete systems, superplasticisers (SP) are generally used as admixtures and added into the cement systems. These chemical admixtures are adsorbed onto the surface of cement particles and disperse agglomerated particles which release entrapped waters, resulting in an increase in fluidity. These admixtures however react differently depending on cement compositions and can develop compatibility issues with the cement used. It is more rational to investigate the flow behaviour of self-compacting concrete on paste scale since, SP-cement interactions within concrete systems occur normally at paste level. The aim of this research was to investigate the effectiveness of a modified superplasticiser obtained by blending two different polycarboxylate SPs at different proportions, on the rheology of three CEM II cements with distinct chemical and physical characteristics. An Anton Paar MCR51 rotational rheometer with roughened parallel plates was used to determine the rheological properties of cement pastes and a mini-slump test was used to determine the cement workability by considering their yield stress developments. The total organic carbon (TOC) approach was used to determine the adsorption of SP on cement particles using the depletion method. The effectiveness of the modified SP products was determined by comparing their adsorption behaviour and induced rheological performances in cement paste systems. It was found that SPs with greater molecular weight and longer side chains easily adsorb on cement particles, thus improving the repulsion phenomenon within the cement system that in its turn improves the fluidity of the overall systems. It is thus possible to tailor rheological performances of SCC using modified SP based on SPs with different molecular structures. The desired fluidity would therefore be achieved by fine-tuning the proportion of SP with molecular weight and longer side chains within the SP blend.