Effect of polycarboxylate superplasticizer and calcium nitrite on the fluidity and early strength of belite sulfoaluminate cement

Belite sulfoaluminate cement is attracting increasingly attention and considered as a prospective alternative to Ordinary Portland cement (OPC) owing to the lower preparation temperature and CO₂ emission. However, it still has disadvantages of poor workability and low early strength. Accordingly, this study investigated the effect of compound polycarboxylate superplasticizer (PCE) and early strength agent, calcium nitrite (Ca(NO₂)₂), on the flowability and early strength of BCSA cement. The variation patterns of setting time, fluidity, strength and PCE adsorption of the paste were measured. Furthermore, the hydration products and microscopic morphology were analyzed by the heat of hydration, X-ray diffraction analyzer and scanning electron microscope. The results showed that Ca(NO₂)₂ effectively enhanced the adsorption of PCE onto the surface of BCSA cement particles, leading to a 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% Ca(NO₂)₂ inhibited the formation of ettringite (AFt), resulting in prolonged setting time. However, it deepened the hydration degree of BCSA for 3d and refined the hydration product, AFt crystals. Consequently, the compressive strength has been increased to 95.75 MPa and 107.13 MPa for BCSA cement at 3d and 28d, respectively.