Effect of additive compounding on early properties of belite sulfoaluminate cement

Belite sulfoaluminate cement is attracting increasing attention and being considered as a potential alternative to ordinary Portland cement owing to lower preparation temperature and carbon dioxide (CO₂) emission. However, it still has disadvantages of poor workability and low early strength. This study investigated the effects of compound polycarboxylate superplasticiser (PCE) and early strength agent, calcium nitrite (Ca(NO₂)₂), on flowability and early strength of belite calcium sulfoaluminate (BCSA) cement. Variation patterns of setting time, fluidity, strength and polycarboxylate superplasticiser (PCE) adsorption of the paste were measured; the hydration products and microscopic morphology were also analysed by the heat of hydration, X-ray diffraction and scanning electron microscopy. Results showed that calcium nitrite effectively enhanced adsorption of PCE onto the surface of BCSA cement particles, leading to notable improvement in the fluidity of the paste (reaching up to 275 mm). In the initial hydration stage, 0.7% PCE compounded with 1.2% calcium nitrite inhibited the formation of ettringite (AFt), resulting in prolonged setting time. However, it deepened the degree of hydration of BCSA for 3 days and refined the hydration product, AFt crystals. Consequently, the compressive strength was increased to 95.75 MPa and 107.13 MPa for BCSA cement at 3 days and 28 days, respectively.