Which substance is added to prolong the thickening of gypsum?

Which substance is added to prolong the thickening of gypsum?

• A. Sodium citrate
• B. Sodium sulfate
• C. Salt
• D. Potassium sulfate

Answer: (A)

Prolonging thickening in gypsum is about slowing the setting reaction to keep the mix workable longer. Sodium citrate acts as a chemical retarder: it binds calcium ions released during the hydration of calcium sulfate hemihydrate, which slows the growth of the calcium sulfate dihydrate crystals. That interference delays gelation and extends the thickening (working) time of the mix. The other substances listed are not used for this purpose and can either have no reliable retarding effect or may even speed up the setting, so they don’t achieve the same extension of thickening.

Prolonging thickening in gypsum is about slowing the setting reaction to keep the mix workable longer. Sodium citrate acts as a chemical retarder: it binds calcium ions released during the hydration of calcium sulfate hemihydrate, which slows the growth of the calcium sulfate dihydrate crystals. That interference delays gelation and extends the thickening (working) time of the mix. The other substances listed are not used for this purpose and can either have no reliable retarding effect or may even speed up the setting, so they don’t achieve the same extension of thickening.