Effect of carbon dioxide on self-setting apatite cement formation from tetracalcium phosphate and dicalcium phosphate dihydrate; ATR-IR and chemoinformatics analysis

Rapid self-setting apatite cement (SSAC) formation from tetracalcium phosphate (TeCP) and dicalcium phosphate dihydrate (DCPD) has been investigated by an attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy coupled with a principal component analysis (PCA). After TeCP and DCPD were kneaded with phosphoric acid, the peaks of ATR-IR spectra for the kneaded sample shift significantly in the ranges of 2250–2400 and 850–1150 cm⁻¹ due to the crystalline transformation into hydroxyapatite (HAp). The PCA results indicate that the loadings of principal components 1 and 2 (PC1 and PC2, respectively) are ascribed to CO₂ and phosphate group, respectively, in the transforming cement. The PC1 score initially increases to reach a maximum at around 1000 s and then decreases. In contrast, the PC2 score increases continuously, but its increment became lesser with time. Although the profiles of PC2 score against PC1 score are similar in shape, there are deviations among the profiles obtained through quadruplicate experiments. The scores are, therefore, time differentiated, and the relationship between the differentiated scores is analyzed. The time differentiation approach is found to be useful for understanding complicated chemical reactions. The PCA results suggest that SSAC formation can be divided into three major stages. In the first stage, CO₂ concentration in the transforming cement rapidly increases, which triggers HAp crystallization. In the second stage, CO₂ concentration still increases, but its increasing rate drastically decreases; HAp crystallization continues with increasing rate. In the last stage, CO₂ concentration decreases, and HAp crystallization significantly slows down.