Abstract
The effects of eight retarders, including tartaric acid, sodium tartrate, salicylic acid, melamine, sucrose, white cement, sodium triphosphate, and citric acid, on setting times and strength of building gypsum were investigated under different dosage. The retarding mechanism of that was analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), and the crystal morphology and characteristics of the samples mixed with citric acid and bone glue under different setting times were observed and compared. The results show that all eight kinds of retarders have a different degree of retarding effect on building gypsum, and at the same time, they all have different degree of influence on strength of gypsum, among which citric acid has the most significant influence. Due to the selective adsorption of citric acid, dihydrate gypsum crystals are changed from the original needle-like to the short column and the plate-like, the overlaps between the crystals are weakened, and the pore structure is deteriorated, and the strength are reduced. The addition of bone glue causes the crystals become coarse, the length become shorter, and the aspect ratio decreased, and it is mainly by delaying the dissolution of gypsum semihydrate and the growth of gypsum dihydrate nucleus to achieve retarding effect in building gypsum.
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