Belite-calcium sulphoaluminate cement prepared by EMR and BS: Hydration characteristics and microstructure evolution behavior

Abstract

Belite-calcium sulphoaluminate (B-CSA) cement is regarded as one of the most promising alternatives to ordinary Portland cement (OPC). The production of B-CSA cement from industrial solid waste is a critical technology for maximising mineral resource utilisation. The morphology and microstructure of hydration products produced during the hydration process of B-CSA cement have a significant impact on the mechanical properties of the cement. In this paper, B-CSA cement prepared by electrolytic manganese residue (EMR) and barium slag (BS) was taken as the research object, and the changes of PH, turbidity and electrical conductivity in the early hydration process were monitored. Furthermore, the phase composition and microstructure of hydration products of cement paste were analysed using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TG), solid state MAS-NMR spectroscopy, nitrogen adsorption test, scanning electron microscope (SEM) and electron microprobe analyzer (EPMA). The results show that with the sharp change of pH value, turbidity and electrical conductivity in the early hydration process of B-CSA cement, a large amount of ettringite is formed during the rapid hydration of ye’elimite, the metastable AFt part is transformed into AFm and the hydration reaction of belite was inhibited. The hydration products are mainly ettringite, monosulfate and AH3, which are responsible for the strength development of B-CSA cement. The hydration rate of belite in the early stage is slow, and the C-S-H gel phase can be formed in an aluminum-rich environment. The microstructure evolution of cement paste was revealed by nitrogen adsorption tests and SEM. The crystal state of ettringite changes from columnar to fibrous to construct the hardened framework of disorderly distributed cement paste, and the gel phase fills in the void formed by the ettringite skeleton to form a dense structure and reduce the porosity. The interface between the unreacted belite phase and the AFt skeleton is clear, and the gel phase plays a role in connecting them. Mn2+ was enriched on the C-S-H gel phase to form the Ca-Si-Mn compound and solidified. Ba2+ preferentially forms BaSO4 with SO42− which precipitates at the interface between the ettringite phase and C-S-H gel phase. Based on the analysis of the test results, the hydration characteristics and microstructure evolution of B-CSA cement were determined.