Effectiveness of Green Additives vs Poly(acrylic acid) in Inhibiting Calcium Sulfate Dihydrate Crystallization

The effects that 20 ppm poly(epoxysuccinic acid) (PESA), poly(aspartic acid) (PASP), and two poly(acrylic acid) (PAA) compounds with molecular weights of ∼2000 and ∼100 000 have on the crystallization of gypsum were evaluated at 21 °C by in situ UV-vis spectrophotometry. XRD and SEM were utilized for phase and morphological studies, while the way these additives are associated with the final gypsum crystals was evaluated by XPS. The comparison showed that PASP performed far better than the other antiscalants as it completely inhibited the formation of gypsum. In contrary, the presence of the low molecular weight PAA decreased the rate of crystallization far more than the larger molecular weight PAA. When the pH in the reacting solution was switched from ∼4 to ∼7, the efficiency of the low molecular weight PAA in inhibiting gypsum formation increased, while in the presence of the higher molecular weight PAA the opposite effect was observed.