The roles of water and polyelectrolytes in the phase transformation of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase of calcium carbonates and it is a precursor during the biomineralization of calcium carbonate in nature. Different ACC in the absence and presence of the additives of poly(sodium 4-styrene sulfonate) and poly(acrylic acid) were prepared and their transformation processes under controls of water were investigated. It was found that ACC powders generally contained about 15% water. Two different transformations via thermal and solution pathways were studied in the present work. We found that water was released from the ACC samples when the temperature was above 100 °C and the calcium carbonate began to crystallize at around 270–400 °C, which resulted the calcite crystals. The involvement of the polymer additives could inhibit this crystallization process and the crystallization temperature shifted to the higher values. During the solution transformation, the evolution steps of the ACC in different water–ethanol solution were monitored by FT-IR. In this case, water could accelerate the transformation and crystallization of ACC. An increasing of the water amount in the mixed solvents always led to the promotion of the transformation kinetics. Interestingly, the other metastable crystal phases of calcium carbonates, vaterite and aragonite, were readily to be induced during the solution transformation of ACC when the low water amounts were applied. However, water could still make them turned into the stable calcite phase with the experimental time prolonged. Since the additives influenced the transformation of ACC and inhibited the transformation of metastable crystal phase, they could be used to stabilize different phases of calcium carbonates in the solutions. The current experimental results confirmed that the controls of water amount and additives played important roles in the transformation of ACC.