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 (C(4)A(3)$) during the sintering process to demonstrate a potential use of boron-rich waste. The microstructure and hydration performance of C(4)A(3)$ with various B2O3 contents were investigated with scanning electron microscopy, x-ray diffraction, isothermal conduction calorimetry, thermogravimetric studies and compressive strength tests. B2O3-doped C(4)A(3)$ 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 C(4)A(3)$ 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 C(4)A(3)$ increased while the hydration rate decreased because of the amorphous phases around the C(4)A(3)$. However, the hydration degree of doped C(4)A(3)$ increased due to the slower reaction rate. Thus, when an appropriate amount of B2O3 was added to the C(4)A(3)$ during sintering, a significant improvement in the compressive strength of pastes was observed.