Effect of superplasticisers on the hydration process, products and microstructure of tricalcium aluminate paste in the presence of gypsum

The effects of three commonly used superplasticisers (SPs) – sulfonated naphthalene formaldehyde (SNF), sulfonated acetone formaldehyde (SAF) and polycarboxylates (PCB) – on the hydration process, products and microstructure of tricalcium aluminate (C₃A) pastes in the presence of gypsum were systematically investigated using isothermal calorimetry, x-ray diffraction, environmental scanning electron microscopy, infrared spectroscopy and magic angle spinning–nuclear magnetic resonance spectroscopy. The results show that the hydration reaction rate of C₃A paste at the initial stage (0–2 h) was promoted by the incorporation of SPs, especially in the C₃A–SNF paste. Hydration was rapidly reduced at the early stage (2–12 h), but an acceleration in C₃A–SNF/PCB pastes occurred again at the late stage. The morphology of the C₃A–SNF/SAF paste changed from thin needle-shaped crystals at the initial stages to thick and coarse rod-like crystals at late stages. In the C₃A–PCB paste, very few crystals occurred at an early age, while a large amount of prismatic crystals were observed at the later age. Chemical environment studies revealed that the SPs did not visibly change the position or shape of the characteristic bands of aluminates, but only influenced their relative intensities. A predominant resonance, assigned to six-coordinated aluminium as [AlO₆] type, was observed for all the pastes with and without SP. The interaction mechanism between SP and C₃A was also explored.