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

Abstract

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.