Effect of Molecular Structure of Organic Acids on the Crystal Habit of α-CaSO4·0.5H2O from Phosphogypsum

The organic acid crystal modifiers play an important role in the control of the crystal habit of α-hemihydrate gypsum (α-CaSO4·0.5H2O) from phosphogypsum, but the molecular structure characteristics of crystal modifiers have not been clarified, which makes it difficult to judge whether an organic acid has the ability to regulate the crystal habit of α-CaSO4·0.5H2O directly. In this work, the effect of organic acids with different molecular structures on the crystal habit of α-CaSO4·0.5H2O and its adsorption differences onto the α-CaSO4·0.5H2O surface were explored. The results show that the molecular structure characteristics of crystal modifiers contain two or more carboxylic groups (COOH) that are separated by two methylene or methine groups. Furthermore, organic acids with the regulation ability can adsorb on the surface of α-CaSO4·0.5H2O and change its growth habit. With the increase in the crystal modifier concentration, the α-CaSO4·0.5H2O crystals are shortened in length and enlarged in width, resulting in the decrease in aspect ratio and the increase in compressive strength. Conversely, when the adsorption of ineffective organic acids on the surface of α-CaSO4·0.5H2O was not detected, the α-CaSO4·0.5H2O crystals remained long hexagonal prisms. These results have guiding significance for the screening of novel organic acid crystal modifiers.