The effect of the kind of calcium sulfate in cements on the dispersing ability of poly β-naphthalene sulfonate condensate superplasticizer

Many studies have been carried out to prevent the incompatibility problem between poly β-naphthalene sulfonate condensate superplasticizer (PNS) and cements. Several of them suggested that the effect of SO42− concentration on adsorption equilibrium might be more important for the performance of PNS than the effect of ionic strength on electrostatic repulsion. In this study, the influence mechanism of the kind of calcium sulfate in cements on the performance of PNS was analyzed in regards to SO42− concentration change with time elapse. It was found that SO42− concentration change was affected by two kinds of calcium sulfate, gypsum and hemihydrate. In case of using the cement containing mainly hemihydrate, SO42− concentration in the solution phase of cement paste initially after mixing is high; however, it decreases rapidly with time elapse. The adsorbed amount of PNS onto surface of cement particle is limited just after mixing because the SO42− concentration is high and increases with decreasing SO42− concentration. As a result, the flow loss of cement paste decreases.