The effect of boron oxide on the microstructure and hydration of calcium sulfoaluminate phase

Boron-rich waste causes numerous environmental problems when discharged directly into the environment. Here, various quantities of boron oxide (B₂O₃) were added to calcium sulfoaluminate (C₄A₃$) during the sintering process to demonstrate a potential use of boron-rich waste. The microstructure and hydration performance of C₄A₃$ with various B₂O₃ contents were investigated with scanning electron microscopy, x-ray diffraction, isothermal conduction calorimetry, thermogravimetric studies and compressive strength tests. B₂O₃-doped C₄A₃$ had a larger grain size than the pure phase; and were surrounded by amorphous phases. The presence of B₂O₃ was shown to promote the phase transition process through which C₄A₃$ changes from the orthorhombic to the cubic structure; and the substitution of Al³⁺ for B³⁺ in AlO₄ tetrahedra was surveyed by structural refinements. As the B₂O₃ content increased, the induction period of C₄A₃$ increased while the hydration rate decreased because of the amorphous phases around the C₄A₃$. However, the hydration degree of doped C₄A₃$ increased due to the slower reaction rate. Thus, when an appropriate amount of B₂O₃ was added to the C₄A₃$ during sintering, a significant improvement in the compressive strength of pastes was observed.