Controlled morphologies and hydration process of hydratable alumina by using citric acid

Hydration mechanisms of hydratable alumina (HA) in citric acid at different pH are investigated in this study, aiming to improve rapid hydration process and massive hydration heat. By adding citric acid to adjust solution pHs to 2, 3, and 4, the crystal phase, morphologies, and microstructure of the HA hydrates are characterized by X-ray powder diffraction (XRD), differential scanning calorimetry (TG-DSC), and transmission electron microscopy (TEM), respectively. The results show that HA exhibits different hydration behaviors in different concentrations of acidic solutions. With decreasing pH of citric acid, the crystallinity of HA decreases whereas the dehydration process slows down. Excessive citric acid could displace hydroxyl which generates from pseudoboehmite gel. Meanwhile, the excess citrate ion can be absorbed on the surface of HA by forming numerous micelles, then compound to inhibit further hydration. Proper addition of citric acid helps to delay the hydration of HA and achieve better balance between bonding performance and hydration speed.