Preparation of iron-rich belite-sulfoaluminate cement utilizing iron tailings and fluorogypsum

In order to promote the development and application of low-carbon cement varieties and the recycling of industrial solid waste, this study used iron tailings and fluorogypsum to prepare iron-rich belite-sulfoaluminate cement (I-BCSA). The suitable conditions for the preparation of I-BCSA in this system were with an excessive addition of 6 wt% of SO₃ in the raw meal, at a calcination temperature of 1250 °C for 1.5 h, and an added-gypsum content of 15 wt%. Under this suitable preparation condition, the contents of ye'elimite, dicalcium silicate (C₂S) and tetracalcium aluminoferrite (C₄AF) in I-BCSA clinker were 29.17 %, 50.23 % and 16.71 %, respectively. In addition, the impurity ions (Mg, S and F ions) hindered the transformation of α'_H-C₂S into α'_L-C₂S during the cooling process of clinker by causing the distortion and the inside porosity decrease of α'_H-C₂S crystal (phase transition energy barrier: 254 eV → 553 eV). This resulted in 20.49 % of the C₂S in the I-BCSA clinker existing in the α'_H crystal form. The presence of C₄AF and α'_H-C₂S in clinker significantly improved the strength development of the product before 28 days. At a water/cement ratio of 0.35, the 3-day, 7-day, and 28-day compressive strengths of I-BCSA paste reached 43 MPa, 51 MPa, and 66 MPa, respectively, basically surpassed the strength development of commercial cement (including 42.5-grade of R-type OPC and CSA) exhibiting good early strength, under the same conditions. Moreover, the hydration products of I-BCSA were mainly ettringite, C-S-H gel, Fe(OH)₃ gel, Al(OH)₃ gel, C₃(A, F)·6H₂O and CaCO₃, with relative proportions of 50.13 %, 36.06 %, 4.57 %, 4.21 %, 2.54 % and 2.49 %, respectively. From the perspective of hydration products, the synergistic support of ettringite and C-S-H gel was more conducive to the strength development of paste than the support of ettringite or C-S-H gel alone. Finally, the long-term (after 28 days) performance of I-BCSA still needs to be further studied in the future to promote the practical application of I-BCSA.