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 (B _2 O _3 ) 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 B _2 O _3 contents were investigated with scanning electron microscopy, x-ray diffraction, isothermal conduction calorimetry, thermogravimetric studies and compressive strength tests. B _2 O _3 -doped C _4 A _3 $ had a larger grain size than the pure phase; and were surrounded by amorphous phases. The presence of B _2 O _3 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 Al ^3+ for B ^3+ in AlO _4 tetrahedra was surveyed by structural refinements. As the B _2 O _3 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 B _2 O _3 was added to the C _4 A _3 $ during sintering, a significant improvement in the compressive strength of pastes was observed.