Effect of borax on rheology of calcium sulphoaluminate cement paste in the presence of polycarboxylate superplasticizer

Abstract

Sulphoaluminate cement (SAC) is increasingly attracting the attention of academic and industry researchers, not only because of its fast setting and high early strength, but also due to the low CO2 emissions and low preparation temperature. The fast setting also results in poor workability. Generally, this problem can be improved by incorporation of retarders. However, it is often ignored that these retarders may obviously affect the plasticizing effect of superplasticizer by competitively adsorbing onto the surface of the cement particles. In this study, the effect of borax on rheology of SAC paste with polycarboxylate superplasticizer (PCE) has been investigated to gain deeper insight into competitive adsorption. The rheology was characterized with mini slump and rotor rheometer. Adsorption behavior was studied with total organic carbon analyzer and inductive coupled plasma emission spectrometer. The surface performance of cement grains was discussed with X-ray photoelectron spectrometer and scanning electron microscopy. Dispersion model was then proposed to illustrate the mechanism behind the difference in rheology. The results show that borax can be precipitated with Ca2+ in the immediate vicinity of cement particles to adsorb onto the surface of cement grains and form a calcium-based borate layer on the surface, which is responsible for the retarding effect and plasticizing effect of borax. Competitive adsorption would take place between PCE and borax, and this would tend to reduce adsorption amount of PCE and reduce the dispersion. The dispersion of PCE-borax system in SAC depends on the dosage of borax: a small amount of borax would increase the dispersion, mainly because the retarding effect and plasticizing effect of borax predominate; while a large amount of borax would reduce the dispersion, because the competitive adsorption effect becomes predominant. It is expected that these results would provide the useful experience for the application of PCE and borax in high fluidity SAC paste in real engineering practice.