Effect of superplasticisers and their mechanisms of action on magnesium oxysulfate cement properties

To improve the workability of magnesium oxysulfate (MOS) cement with a low water/cement ratio, polycarboxylate (PC) and naphthalene (NF) superplasticisers were used as modifiers and their effects on the physical properties of MOS cement were studied. Various approaches were used to analyse and explain the action mechanisms of the superplasticisers, including zeta potential measurements, scanning electron microscopy, X-ray diffraction, hydration analysis and mercury intrusion porosimetry. The results showed that the addition of superplasticisers decreased the compressive strength and water demand of the MOS cement and also retarded the hydration process, prolonged the setting time, changed the porosity and pore distribution, and decreased the relative content of the main hydration products in the cement. In the magnesium oxide–magnesium sulfate–water system studied, the hydrophobic interaction of the NF superplasticiser in the MOS cement was stronger than that of the PC superplasticiser. NF superplasticiser thus appears to be more suitable for MOS cement, improving its fluidity without significantly reducing its compressive strength. A dosage of 0·5% NF superplasticiser was found to give the best performance.